Fundamentals of Weed Science PDF

Fundamentals of Weed Science Robert L. Zimdahl Professor Emeritus Department of Bioagricultural Sciences and Pest Management Colorado State University Fort Collins, Colorado AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN F...

Fundamentals of Weed Science Robert L. Zimdahl Professor Emeritus Department of Bioagricultural Sciences and Pest Management Colorado State University Fort Collins, Colorado AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Academic Press is an imprint of Elsevier Academic Press is an imprint of Elsevier 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA 525 B Street, Suite 1900, San Diego, California 92101-4495, USA 84 Theobald’s Road, London WC1X 8RR, UK This book is printed on acid-free paper. Copyright © 2007, Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone: (+44) 1865 843830, fax: (+44) 1865 853333, E-mail: [email protected]. You may also complete your request online via the Elsevier homepage (http://elsevier.com), by selecting “Support & Contact” then “Copyright and Permission” and then “Obtaining Permissions.” Library of Congress Cataloging-in-Publication Data Zimdahl, Robert L. Fundamentals of weed science / Robert L. Zimdahl. p. cm. Includes bibliographical references and index. ISBN 978-0-12-372518-9 1. Weeds. 2. Weeds—Control. 3. Herbicides. I. Title. SB611.Z54 2007 632’.5—dc22 2007011591 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN: 978-0-12-372518-9 For information on all Academic Press publications visit our Web site at www.books.elsevier.com Printed in the United States of America 07 08 09 10 9 8 7 6 5 4 3 2 1 Working together to grow libraries in developing countries www.elsevier.com | www.bookaid.org | www.sabre.org  CONTENTS Preface xv Chapter 1 Introduction 1 Literature Cited 12 Chapter 2 Weeds—The Beginning 15 I. Definition of the Word Weed 16 II. Characteristics of Weeds 20 III. Harmful Aspects of Weeds 23 A. Plant Competition 23 B. Added Protection Costs 23 C. Reduced Quality of Farm Products 27 D. Reduced Quality of Animals 27 E. Increased Production and Processing Costs 30 F. Water Management 31 G. Human Health 32 H. Decreased Land Value and Reduced Crop Choice 32 I. Aesthetic Value 33 IV. Cost of Weeds 33 Things to Think About 38 Literature Cited 39 Chapter 3 Weed Classification 43 I. Phylogenetic Relationships 45 II. A Note About Names 46 iii iv Contents III. Classification Methods 48 A. Type of Plant 48 B. Habitat 48 Cropland 48 Rangeland 48 Forests 49 Aquatic 49 Environmental Weeds 50 Parasitic Weeds 51 C. Life History 53 Annuals 54 Biennials 55 Perennials 55 Things to Think About 57 Literature Cited 57 Chapter 4 Ethnobotany 59 I. Food for Humans 60 II. Feed for Animals 64 III. Medical Uses 66 IV. Agricultural Uses 68 V. Ornamental Uses 69 VI. Insect or Disease Traps 70 VII. Pollution Control 72 VIII. Other Uses 74 Things to Think About 76 Literature Cited 76 Chapter 5 Weed Reproduction and Dispersal 79 I. Seed Production 80 A. Seed Size 81 B. Seed Abundance 81 C. Seed Production 82 II. Seed Dispersal 88 A. Mechanical 88 B. Wind 90 C. Water 91 D. Human-Aided 93 E. Animal-Aided 96 F. Machinery 98 G. Mimicry 100 H. Other 100 I. Consequences of Weed Dispersal 100 Contents v III. Seed Germination—Dormancy 101 A. Causes 103 Light 104 Immature Embryo 105 Impermeable Seed Coat 106 Inhibitors 106 Oxygen 106 Temperature 106 After-Ripening Requirement 107 B. Classes of Dormancy 107 C. Consequences of Weed Seed Dormancy 109 IV. Vegetative or Asexual Reproduction 114 Things to Think About 118 Literature Cited 118 Chapter 6 Weed Ecology 123 I. Human Influences on Weed Ecology 125 II. The Weed-Crop Ecosystem 128 III. Environmental Interactions 130 A. Weeds and Climate 130 B. Edaphic Factors 132 C. Weeds and Biota 133 Similarity of Seed Size 133 Time of Seed Germination and Formation 133 Tillage, Rotation, and Harvest Practices 134 IV. Fundamental Ecological Concepts 136 A. Species 136 B. The Community 143 C. Ecological Succession 144 D. Interactions Between Weeds and Other Crop Pests 144 V. Plant Competition 146 A. Plant Competition Defined 148 B. Factors That Control the Degree of Competition 148 C. Competition for Nutrients 151 D. Competition for Water 156 E. Competition for Light 161 F. Factors for Which Plants Do Not Compete 163 VI. Plant Characteristics and Competitiveness 164 VII. The Relationship Between Weed Density and Crop Yield 165 VIII. Magnitude of Competitive Loss 169 IX. Duration of Competition 172 X. Economic Analyses 176 vi Contents XI. Mathematical Models of Competition 178 Things to Think About 180 Literature Cited 180 Chapter 7 Invasive Plants 187 I. The Definition of Invasive Species 187 II. The Identity of Invasive Plant Species 190 III. Why Do Invasions Occur? 197 IV. The Consequences of Plant Invasions 203 A. Tamarisk/Salt Cedar 207 B. Kudzu 208 C. Waterhyacinth 209 D. Purple Loosestrife 210 V. Management of Invasive Plant Species 214 Things to Think About 221 Literature Cited 221 Chapter 8 Allelopathy 227 I. Allelopathic Chemistry 233 II. Production of Allelochemicals 235 III. Allelopathy and Weed-Crop Ecology 237 A. Effects on Weed Species 237 B. Weed Interference 237 C. Weed Management 238 Things to Think About 243 Literature Cited 243 Chapter 9 The Significance of Plant Competition 247 I. General Considerations 247 II. Characteristics of Weeds 255 A. Competitive Ability 255 B. Reproductive Characteristics 256 C. Cultural Practices 257 Things to Think About 257 Literature Cited 257 Chapter 10 Methods of Weed Management and Control 259 I. The Definitions of Weed Prevention, Control, Eradication, and Management 260 A. Weed Prevention 260 B. Weed Control 261 C. Weed Eradication 261 Contents vii D. Weed Management 261 II. Weed Prevention 262 III. Mechanical Control 269 A. Hand-Pulling 270 B. Hand-Hoeing 270 C. Tillage 272 D. Mowing 285 E. Flooding, Salt Water, Draining, and Chaining 286 IV. Nonmechanical Methods 288 A. Heat 288 Flaming 288 Solarization and Heat 290 B. Mulching 292 C. Sound and Electricity 294 D. Light 295 V. Cultural Weed Control 295 A. Crop Competition 295 B. Planting Date and Population 300 C. Companion Cropping 302 D. Crop Rotations 306 E. Fertility Manipulation 309 VI. Herbicide-Resistant Crops 311 Things to Think About 317 Literature Cited 317 Chapter 11 Biological Weed Control 327 I. General 327 A. Definition 328 B. Advantages 328 C. Disadvantages 329 D. Use Considerations 331 II. Methods of Application 335 A. Classical, Inoculative, or Importation 336 B. Augmentative or Inundative 337 C. Conservation 338 D. Broad-Spectrum Control 338 III. Biological Control Agents 338 A. Classical or Inoculative Biological Control 339 Insects 339 B. Inundative or Augmentative 342 Fungi 342 C. Broad-Spectrum Control 346 Fish 346 Aquatic Mammals 348 Vertebrates 348 viii Contents IV. Integration of Techniques 350 Things to Think About 351 Literature Cited 352 Chapter 12 Introduction to Chemical Weed Control 357 I. History of Chemical Weed Control 358 A. The Blood, Sweat, and Tears Era 358 B. The Mechanical Era 358 C. The Chemical Era 359 II. Advantages of Herbicides 367 III. Disadvantages of Herbicides 374 A. Cost 374 B. Mammalian Toxicity 375 C. Environmental Persistence 375 D. Weed Resistance to Herbicides 375 E. Monoculture 381 F. Other 382 IV. Classification of Herbicides 383 A. Crop of Use 383 B. Observed Effect 388 C. Site of Uptake 388 D. Contact Versus Systemic Activity 388 E. Selectivity 389 F. Time of Application 389 G. Chemical Structure 390 H. Site/Mechanism of Action 390 Things to Think About 391 Literature Cited 391 Chapter 13 Properties and Uses of Herbicides 395 I. Introduction 395 II. Light-Dependent Herbicides 398 A. Inhibitors of Photosynthesis 398 Inhibitors of Photosynthesis at Photosystem II, Site A 399 Inhibitors of Photosynthesis at Photosystem II, Site A, but with Different Binding Behavior 401 Inhibitors of Photosynthesis at Photosystem II, Site B 402 Inhibitors of Photosynthesis at Photosystem I: Electron Divertors 403 Contents ix B. Inhibitors of Pigment Production 404 Inhibitors of Carotenoid Biosynthesis 404 Inhibitors of Phytoene Desaturase with Blockage of Carotenoid Biosynthesis 405 Inhibitors of 1-Deoxy-D-Xyulose 5-Phosphate Synthatase (DOXP Synthase) 405 C. Cell Membrane Disruptors and Inhibitors 406 Inhibitors of Protoporphyrinogen Oxidase (Protox) 406 III. Fatty Acid Biosynthesis Inhibitors 407 A. Inhibitors of Acetyl CoA Carboxylase— ACCase Inhibitors 408 Aryloxyphenoxypropionates 408 Cyclohexanediones 410 B. Inhibitors of Lipid Synthesis, Not ACCase Inhibition 410 C. Inhibition of Biosynthesis of Very Long Chain Fatty Acids 412 IV. Cell Growth Inhibition 414 A. Inhibition of Microtubule Assembly 414 Dinitroanilines 414 Other 416 B. Inhibition of Mitosis 416 C. Inhibition of Cell Wall Synthesis 416 V. Auxin-Like Action—Growth Regulators 417 A. Synthetic Auxins 417 Phenoxy Acids 418 Arylaliphatic or Benzoic Acids 421 Pyridinecarboxylic or Picolinic Acids 422 Other 423 B. Inhibitors of Indoleacetic Acid (IAA) Transport 423 VI. Amino Acid Biosynthesis Inhibitors 424 A. Inhibitors of Acetolactate Synthase (ALS)— Acetohydroxyacid Synthase (AHAS) 424 Sulfonylureas 424 Imidazolinones 426 Pyrimidinylthio-Benzoate 426 Sulfonylamino-Carbonyltriazolinone 426 Triazolopyrimidines 427 B. Inhibitors of 5-Enolpyruvyl-Shikimate-3- Phosphate Synthase (EPSP) 427 C. Inhibitors of Glutamine Synthetase (GS) 428 VII. Inhibitors of Respiration 428 A. Uncouplers of Oxidative Phosphorylation 429 Arsenites 429 Phenols 429 x Contents VIII. Unknown Mechanism of Action 430 IX. Summary 432 Things to Think About 434 Literature Cited 434 Chapter 14 Herbicides and Plants 437 I. Factors Affecting Herbicide Performance 438 II. General 438 A. Sprayer Calibration 438 B. Reaching the Target Plant 441 Drift 441 Volatility 444 III. Foliar Active Herbicides 445 A. Spray Retention 445 Leaf Properties 446 Other Factors 448 Characteristics of Spray Solution 448 B. Environmental Factors 449 Moisture 449 Temperature 450 Light 451 IV. Physiology of Herbicides in Plants 451 A. Foliar Absorption 451 Stomatal Penetration 452 Cuticular Penetration 452 Fate of Foliar Herbicides 452 Advantages and Disadvantages of Foliar Herbicides 453 B. Absorption from Soil 453 General 453 Advantages and Disadvantages of Soil-Applied Herbicides 454 Root Absorption 454 Influence of Soil pH 454 C. Shoot Versus Root Absorption 455 D. Absorption as a Determinant of Selectivity 456 E. Translocation 458 F. Translocation as a Determinant of Selectivity 458 G. Herbicide Metabolism in Plants 460 H. Metabolism as a Determinant of Selectivity 462 Things to Think About 466 Literature Cited 467 Contents xi Chapter 15 Herbicides and Soil 469 I. Soil 470 II. Factors Affecting Soil-Applied Herbicides 473 A. Physical Factors 473 Placement 474 Time of Application 474 Volatility 474 Adsorption 475 Leaching 477 Interactions with Soil Moisture 478 B. Chemical Factors 480 Microbial or Enzymatic Degradation 480 Chemical or Nonenzymatic Degradation 481 Photodegradation 482 III. Soil Persistence of Herbicides 482 Things to Think About 486 Literature Cited 486 Chapter 16 Herbicide Formulation 489 I. Introduction 489 II. Types of Herbicide Formulation 492 A. Liquid Formulations 492 Solution Concentrate 492 Emulsifiable Concentrate 493 Invert Emulsions 494 Flowable Concentrate 494 Encapsulated 495 B. Dry Formulations 495 Dusts and Dry Powders 495 Wettable Powders 495 Granules 496 Dry Flowable and Water Dispersible Granules 496 Water-Soluble Packets and Effervescent Tablets 497 Other 497 III. Surfactants and Adjuvants 498 Things to Think About 498 Literature Cited 498 Chapter 17 Herbicides and the Environment 501 I. Herbicide Performance 502 II. Ecological Changes 504 A. Effects of Herbicide Use 504 B. Herbicide Resistance 507 xii Contents C. Enhanced Soil Degradation 508 D. Influence of Herbicides on Soil 508 E. Herbicide-Disease Interactions 509 III. Environmental Contamination 511 A. Effects on Water 511 B. Human Effects 512 General 512 The Case of Flourides 515 2,4,5-T 516 Summary 517 C. Global Change 519 IV. Energy Relationships 520 V. Herbicide Safety 524 A. Perception of Risk 524 B. Rules for Safe Use of Herbicides 527 C. The LD50 of Some Herbicides 528 Things to Think About 532 Literature Cited 533 Supplementary Literature 534 Chapter 18 Pesticide Legislation and Registration 537 I. The Principles of Pesticide Registration 537 II. Federal Laws 539 III. Federal Insecticide, Fungicide, and Rodenticide Act 540 A. Amendments 541 IV. The Environmental Protection Agency (EPA) 543 V. Federal Environmental Pesticide Control Act 544 VI. Procedural Summary 545 VII. Tolerance Classes 547 A. Exempt 547 B. Zero 547 C. Finite 548 D. Nonfood Crop Registration 548 VIII. The Procedures for Pesticide Registration 548 Things to Think About 552 Literature Cited 552 Chapter 19 Weed Management Systems 553 I. Introduction 553 II. A Metaphor for Weed Management 556 Contents xiii III. The Logical Steps of Weed Management 557 A. Prevention 557 B. Mapping 558 C. Prioritization 558 D. Development of an Integrated Weed Management System 559 E. Implementation of Systematic Weed Management 561 F. Record Keeping and Evaluation 562 G. Persistence 562 IV. Weed Management Principles in Six Systems 563 A. Small Grain Crops 564 Prevention 564 Mechanical Methods 565 Cultural 566 Biological 568 Chemical 568 B. Corn and Row Crops 569 Prevention 570 Mechanical 570 Cultural 571 Biological 572 Chemical 572 Integrated Strategies 573 C. Turf 574 D. Pastures and Rangeland 576 E. Perennial Crops 578 Prevention 579 Cultural 579 Mechanical 580 Chemical 580 F. Aquatic Weed Sites 581 Classification of Weeds 581 Prevention 581 Mechanical 583 Biological 584 Chemical 585 G. Woody Plants 585 V. Molecular Biology in Weed Management 586 VI. Weed Management Decision Aids 588 VII. Summary 593 Things to Think About 593 Literature Cited 594 xiv Contents Chapter 20 Weed Science—The Future 599 I. Research Needs 600 A. Weed Biology 601 Weed Biology and Seed Dormancy 601 Weed-Crop Competition and Weed Ecology 602 B. Allelopathy 604 C. Biological Control 604 D. Weed Control and Bioeconomic Models 605 E. Bioeconomic Models 606 F. Herbicide Technology 606 Study of Plant Biochemistry and Physiology 608 Rate Reduction and Precise Application 609 Soil Persistence and Controlled Soil Life 609 Formulation Research 609 Perennial Weeds 610 Aquatic Weeds 610 Parasitic Weeds 610 Packaging and Labeling 611 The Agricultural Chemical Industry 611 G. Biotechnology and Herbicide-Tolerant Crops 613 H. Organic Agriculture 618 I. Engineering Research 619 J. Vegetation Management, Integration of Methods, and Remote Sensing 621 K. Other Research 623 II. Political Considerations 624 III. Conclusion 624 Things to Think About 626 Literature Cited 626 Appendix A List of Crops and Other Plants Cited in Text 629 Appendix B List of Weeds Cited in Text 635 Glossary of Terms Used in Weed Science 647 Index 655  PREFACE More than 30 years ago, the Monsanto Company distributed a picture that hangs in my study. It showed four books1 with several weed seedlings emerg- ing from each. It was a good picture that portrayed the beginnings of books relevant to weed science. Two (Ahlgren et al. and King) were textbooks, and two (Muenscher and Fernald) were plant identification books. They were the beginning of a now greatly expanded literature on weed science. Many, but not all, textbooks written for undergraduate weed science courses lack an ecological-management perspective on the rapidly developing science of weeds and their control. This book does not ignore the history of weed science and the development of chemical weed control, but it strives to include herbicides as one management technique among many rather than the primary method of choice to solve most weed problems. Science, of all kinds, is not in favor these days. Scientists, including weed scientists, eagerly accepted the credit when in 1945, after World War II, many advances in societal development were widely regarded as contributions of science. The public regarded these advances, which included herbicides and other pesticides, as desirable and benign. Now science is held responsible for many problems that have grown out of its linkage with technology. Herbicides are no longer regarded as benign but rather as threats to humans and the environment and are seen by many as undesirable scientific creations. The public’s attitude toward science and scientists has become a mingling of awe 1 The books pictured were Ahlgren, G.H., G.C. Klingman, and D.E. Wolf. 1951. Principles of Weed Control. J. Wiley & Sons, New York, 368 pp.; Fernald, M.L. 1970. Gray’s Manual of Botany. 8th ed. American Book Co. New York, 926 pp.; King, L.J. 1966. Weeds of the World: Biology and Control. Interscience Pub., Inc. New York, 526 pp.; and Muenscher, W.C. 1935. Weeds. The Macmillan Co. New York, 577 pp. xvi Preface and fear. The practice of science is constrained because while it claims to be an end in itself, it is publicly supported and tolerated because of its utility and its practical value, and it is feared because of its well-known undesirable con- sequences. Weed science is not atypical, and because of its close identification with chemical herbicides, it may be regarded with more apprehension than some other areas of agricultural science. The public’s lack of understanding or its misunderstanding of what weed scientists do will not lessen the need for what is done, and it increases the responsibility of weed scientists and educators to be clear about the problem of weeds and proposed solutions. The responsibility is not so much to educate the public about “what we do” as it is to engage in a conversation (a dialogue, not a monologue) with the public. It is an engagement in public scholarship, whereby original, peer-reviewed intellectual work is fully integrated with the social learning of the public ( Jordan et al., 2002).2 This book includes herbicides3 and their use as an important aspect of modern weed management, but it strives to place them in an ecological frame- work. Any book that purports to discuss the present state of the practice (and art) of weed management would be of little consequence and limited value to students and others who wish to know about weed management, as it is now practiced, if it omitted discussion of herbicides. Many weed scientists believe agriculture is a continuing struggle with weeds. That is, they believe that without good weed control, good, profitable agriculture is impossible and herbicides are an essential component of success. Each agricultural discipline sees itself as central to agriculture’s success and continued progress, and weed scientists are no exception. While not denying the importance of weed man- agement to successful agriculture, its role in the larger ecological context is emphasized. The role of culture, economics, and politics in weed management are mentioned but are not strong themes. This, the third edition, is not a complete revision of the original text, but it has been changed in several significant ways while maintaining an overall ecological framework. Some references in the first edition have been omitted, but 494 new references have been added, 294 of which are work published after 1999, and 89 of them are from the ecological literature. The literature review for this edition was completed in early 2006. The chapters are arranged in a logical progression. Chapter 1 addresses the question “Why should we study weeds?” The book guides discussions of why weeds are important. The second chapter pursues the discussion of the 2 Jordan, N., J. Gunsolus, R. Becker, and S. White. 2002. Public scholarship—linking weed science with public work. Weed Sci. 50:547–554. 3 Common names of herbicides will be used throughout the text except in some tables where they may be paired with one or more trade names. Preface xvii definition of weed that began in Chapter 1, and it presents the characteristics and harmful aspects of weeds. It concludes with a discussion of what weeds cost. Chapter 3 classifies weeds in several ways, and Chapter 4, unique among weed science texts, discusses the fact that not all plants that are weedy in some environments are all bad. Many plants have uses that are known to and studied by ethnobotanists. Weed reproduction and dispersal and the very important topics of seed germination and dormancy are presented in Chapter 5. Chapter 6 is important because it presents the fundamental ecological base of weed science, including plant competition and the interactions of weeds and other pests. Chapter 7, an extended discussion of the role and importance of invasive plant species, is new in this edition and also unique among weed science textbooks. It is followed by a discussion of allelopathy in Chapter 8—a subject included as a minor point in many weed science texts. Having established the ecological base of weed science, the significance of weed-crop competition is presented in Chapter 9. Chapter 10 begins with a consideration of weed management. For many this is the essence of weed science, the fundamental topic: How are weeds controlled? Weed problems are created, and those who wish to control them need to ask why the weed is there as well as how to manage or control it (Zimdahl, R.L., 1999. My View Weed Sci. 47:1). Key concepts of prevention, control, and management are presented in Chapter 10, followed by presenta- tion of mechanical, nonmechanical, and cultural control techniques, as well as the new topic (in this edition) of herbicide-resistant crops. Chapter 11 continues discussion of control but by biological means. Chapter 12 intro- duces important concepts related to chemical control of weeds, and Chapter 13, one of the longest and most difficult, classifies herbicides based on how they do what they do—their mode of action and their chemical family. Chap- ters 14 and 15 are central to understanding of the interactions of herbicides and plants and herbicides and soil. Herbicide application is covered briefly in Chapter 14. Herbicide formulation is presented in Chapter 16. Chapter 17 returns to the ecological theme, but this time with information on the interaction between herbicides and the environment, including effects on water, humans, and global change. A central, and intentionally unanswered, question is how one balances and judges the potential harmful and beneficial aspects of herbicide use. The chapter concludes with a discussion of herbicide safety. Chapter 18 presents the legislative decisions that were required to address some of the questions raised in Chapter 17. Chapter 19 brings things together by discussing weed management systems, many of which are largely conceptual and not yet prescriptive. They evolve and improve with time. New sections on the influence of molecular biology on weed management and weed management decision aids have been added. Finally, Chapter 20 presents a view of the future of weed science. It is meant xviii Preface to provoke thought and discussion. It is not an infallible prediction of what will be. There is a strong, growing trend in weed science away from exclusive study of annual control techniques toward understanding weeds and the systems in which they occur. Control is important, but understanding endures. Herbicides and weed control are important parts of the text, but it is hoped that understanding the principles of management and the biology and ecology of the weeds to be managed will be seen as the dominant themes. That is the primary objective of the book: to introduce the concepts fundamental to weed science and provide adequate citations so interested readers can pursue specific interests and learn more. The study of weeds, weed management, and herbicides is a challenging, demanding task that requires diverse abilities. Weed science involves far more than answering the difficult question of what chemical will selectively kill weeds in a given crop. Weed science includes work on selection of methods to control weeds in a broad range of crops, on noncrop lands, in forests, and in water. Weed scientists justifiably claim repute as plant physiologists, ecologists, botanists, agronomists, organic and physical chemists, molecular biologists, and biochemists. However, lest the reader be intimidated by that list of disciplines, I hasten to add that this text will emphasize general princi- ples—the fundamentals—of weed science and not attempt to include all appli- cable knowledge. It is tempting, and would not be much more difficult, to incorporate extensive, sophisticated knowledge developed by weed scientists. While this knowledge is impressive and valuable, it is beyond the scope of an introductory text. It is hoped that the book conveys some of the challenges of the world of weeds and their management and the importance of weed problems to agri- culture, society, and to meeting the demand to feed a growing world popula- tion. The aim has been to include most aspects of weed science without exhaustively exploring each. The book is designed for undergraduate weed science courses. It is hoped that the text is not too simple for sophisticated readers and that omissions of depth of coverage do not sacrifice accuracy and necessary detail. Readers should note that in nearly all cases I have used units of measure from the original reference rather than changing all to one measurement system. Several colleagues provided helpful suggestions on this and earlier editions of the Fundamentals of Weed Science. I thank all of them, even though some comments were difficult to hear. The first edition had some inaccuracies, and those have been corrected. I thank the following colleagues for suggestions and critical review of portions of the manuscript included in this and earlier editions: Dr. Kenneth A. Barbarick, Dr. K. George Beck, Dr. Cynthia S. Brown, Dr. Sandra K. McDonald, Dr. Philip Westra, Dr. Scott J. Nissen, and Mr. Steven Preface xix Markovits of Colorado State University; Dr. William W. Donald USDA/ARS, University of Missouri; Dr. David L. Mortensen, Pennsylvania State University; Dr. Robert F. Norris, University of California—Davis; Dr. Gregory L. Orr, Fort Collins, CO; Dr. Keith Parker, Syngenta Corp., Greensboro, NC; Dr. Alan R. Putnam, Gallatin Gateway; Montana, Dr. Albert E. Smith Jr., University of Georgia; Dr. Malcolm D. Devine, formerly University of Saskatchewan, presently Research Director, National Research Council, Plant Biotech Inst., Saskatoon, Canada; and Dr. Steven Brunt, BASF Corp, Research Triangle Park, NC. The book has been improved because of their efforts. Any errors of interpretation of fact are solely my responsibility. Robert L. Zimdahl Fort Collins, Colorado 2006 This book is dedicated to the memory of Ann Osborn Zimdahl and to the loving presence of Pamela Jeanne Zimdahl How little we know of what there is to know. I wish that I were going to live a long time... I’d like to be an old man and to really know I wonder if you keep on learning or if there is only a certain amount each man can understand. I thought I knew about so many things that I know nothing of. I wish there was more time. For Whom the Bell Tolls by Ernest Hemingway This page intentionally left blank  CHAPTER 1 Introduction See them tumbling down, Pledging their love to the ground Lonely but free I’ll be found Drifting along with the tumbling tumbleweeds. Cares of the past are behind Nowhere to go but I’ll find Just where the trail will wind Drifting along with the tumbling tumbleweeds. I know when night has gone That a new world’s born at dawn. I’ll keep rolling along Deep in my heart is a song Here on the range I belong Drifting along with the tumbling tumbleweeds. “Tumbling Tumbleweeds” Composed in 1932 by Bob Nolan and recorded by the Sons of the Pioneers Everybody knows what weeds are. Songs and poems have even been written about them, but weeds have never received the recognition and respect they deserve. The fact that many people earn a living and serve society by control- ling and managing weeds is often greeted with amusement, if not outright hysterical laughter. Even scientific colleagues who work in other esoteric disciplines find it hard to believe that another group of scientists could be concerned exclusively with what is perceived as a mundane and ordinary part of the environment. Weeds have been with us since the advent of settled agriculture some 10,000 years ago. It has been suggested that the most common characteristic Fundamentals of Weed Science Copyright © 2007 by Academic Press, Inc. All rights of reproduction in any form reserved. 1 2 Fundamentals of Weed Science of the ancestors of our presently dominant crop plants is their ”weediness”— their ability to proliferate and thrive in disturbed habitats, most notably around human dwellings (Cox, 2006). Bailey (1906, p. 199), to whom agricultural science owes so much, spoke of the Sisyphean battle against Russian thistle in the western United States: What I have thus far stated is only a well-known truth in organic evolution— that the distribution of an animal or plant upon the earth, and to a great extent the attributes of the organism itself, are the result of a struggle with other organisms. A plant which becomes a weed is only a victor in a battle with farm crops; and if the farmer is in command of the vanquished army, it speaks ill for his generalship when he is routed by a pigweed or a Russian thistle. I am never surprised when a student who enrolls in a course about weeds questions why the course is recommended, or even required, and what it is about. Students who enroll in chemistry or English have a reasonably good idea of what they will learn in the class and how it fits into their curriculum. But this is not the case for weed science students. Of course, students who live on farms and ranches already know a lot about weeds and the problems they cause, but they do not always comprehend the complexities of weed manage- ment or the generalship required. Therefore, in this course, it is important that the subject be established and connected to students’ prior knowledge of agri- cultural, biological, and general sciences. From the beginning, a textbook, the teacher, and the student should strive to establish relationships among weed science, agriculture, and society. This book introduces the fundamental con- cepts of weed science and explores how they have changed over the years. The story of agriculture is a story of struggle. It is the story of struggles— those that have ensued “in consequence of the sudden overturning of estab- lished conditions, and the substitution therefor of a very imperfect and one-sided system of land occupancy” (Bailey, 1906, p. 200). This is what we know as modern agriculture. Agricultural history, although a fascinating subject, is too large a topic to address adequately in this book, so only small bits are included. Those who are interested in studying agricultural history are referred to Goodwin and Johnstone (1940) and Rasmussen (1975). The history of weed control was reviewed by Timmons (1970, republished in 2005) and Appleby (2005). Formidable obstacles have been placed between humans and a continuing food supply. These include physical constraints such as a lack of good highways and transportation, economic constraints such as a lack of credit and opera- tional funds, environmental constraints such as too much or too little water or too short a growing season, and biological constraints such as problems with fertility, varieties, soil pH, or salinity. One of the most formidable environmen- tal constraints has been what are known as pests. Surveys by the Food and Agriculture Organization of the United Nations (1963, 1975) showed that in Introduction 3 the 1960s and 1970s more than one-third of the potential annual world food harvest was destroyed by pests. The $75 billion lost was equivalent to the value of the entire world’s grain crop (about $65 billion) and the world’s potato1 crop (about $10 billion). This means that insects, plant diseases, nematodes, and weeds deprived humans of food worth more than the entire world crop of wheat, rye, barley, oats, corn, millet, rice, and potatoes. These losses were only up to harvest and do not include damage during storage—another huge quan- tity. Current, less-complete estimates show that losses due to pests of all kinds have increased since the first FAO estimates were made. History is filled with examples of human conflicts with pests, from biblical to modern times. Examples include locusts (Melanoplus spp.), which still plague the world, to late blight [Phytophthora infestans (Mont.) D. By.], which caused the Irish potato famine of the 1840s. The continuing worldwide pres- ence of Colorado potato beetles (Leptinotarsa decemlineata Say) and the 1970s epidemic of Southern corn leaf blight (Helminthosporium maydis Nisik and Miyake) illustrate that the battle goes on. In fact, the battle has become even more intense as agriculture has changed, with the introduction of chemical pesticides and as an increasing population creates demand for ever greater demand for high-quality food. One must respect the prescience of writer Jonathan Swift (1667–1745; see Williams, 1937), who said the following: Hobbes clearly proves that every creature Lives in a state of war with nature,... So, Nat’ralists observe, a Flea Hath smaller Fleas that on him prey; And these have smaller Fleas to bite ’em: And so proceed ad infinitum. De Morgan (1850), who probably had read, but did not cite, Swift’s poem, expressed the ubiquity of pests several years later: Great fleas have little fleas upon their backs to bite ’em, And little fleas have lesser fleas, and so ad infinitum, And the great fleas themselves, in turn, have greater fleas to go on; While these again have greater still, and greater still, and so on. 1 Common and scientific names of all crops and weeds are listed in Appendixes 1 and 2. 4 Fundamentals of Weed Science The subject of this book is weeds, visible but unspectacular pests, whose presence may be formidable but whose effects are not. Weeds have always been with us and are mentioned in some of our oldest literature: Cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life; thorns and thistles shall it bring forth to thee; and thou shalt eat the herb of the field. Genesis 3:17–18 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? Matthew 7:16 And thorns shall come up in her palaces, nettles and brambles in the fortresses thereof.... Isaiah 34:13 Jesus also spoke of weeds in his parables (Matthew 13:18–23). The biblical “thistles, thorns, and brambles” are common weeds and have been identified as such by biblical scholars (Moldenke and Moldenke. 1952). Weeds were and are still considered serious threats in the continuing battle to produce enough food for the world’s inhabitants. The “tares” in the following parable (Matthew 13:24–30) are the common weed poison ryegrass, a continuing problem in cereal culture: The kingdom of heaven is likened unto a man which sowed good seed in his field: But while he slept, his enemy came and sowed tares among the wheat, and went his way. But when the blade was sprung up, and brought forth fruit, then appeared the tares also. The Greek word tares is translated as “darnel”—a weed that grows among wheat. It is a grass that resembles wheat or rye but has smaller, poisonous seeds. The weed called tares in Europe today is a different species. No agricultural enterprise or part of our environment is immune to the detrimental effects of weeds. They have interfered with human endeavors for all of history. In much of the world (including my garden), weeds are control- led by hand or with a hoe. The figure of a person holding a hoe may be as close as we can come to a universal symbol for “farmer,” even though most farmers in developed countries no longer weed with hoes. For many, both the hoe and the weeding done with it symbolize the practice of agriculture. Con- trolling weeds is probably the farmer’s most arduous task, as expressed by Edwin Markham in his poem “The Man with the Hoe”: Introduction 5 Bowed by the weight of centuries he leans Upon his hoe and gazes on the ground, The emptiness of ages in his face, And on his back the burden of the world. Who made him dead to rapture and despair, A thing that grieves not and that never hopes, Stolid and stunned, a brother to the ox? Who loosened and let down this brutal jaw? Whose was the hand that slanted back this brow? Whose breath blew out the light within this brain?... O masters, lords and rulers in all lands, How will the future reckon with this man? How answer his brute question in that hour When whirlwinds of rebellion shake all shores? Four major advances in agriculture have significantly increased food production: 1. The introduction of mineral fertilizer. Early work on plant nutrition and soil fertility proceeded directly from the pioneering studies of Justus von Liebig (1942), who questioned prevailing theories about plant nutrition. 2. Agricultural mechanization, which began in the United States with Eli Whitney’s invention, the cotton gin, in 1793, McCormick’s reaper in 1834, and Deere’s moldboard plow in 1837. 3. Genetic research in plant and animal production. The Austrian monk Gregor Mendel, who pursued his studies in quiet and seclusion, had no dreams of pragmatic application or economic gain. His discoveries, most notably in the development of plant hybrids, have had a huge, generally positive, effect on our ability to produce food. The nearly simultaneous and independent rediscovery of Mendel’s work by De Vries in Holland, Correns in Gemany, and Tschermak in Austria in 1900, while examining the literature to confirm their own discoveries, produced enormous benefits for agriculture. 4. The use of pesticides and plant growth regulators. These moved beyond mechanization to the chemicalization of agriculture and led to the develop- ment and growth of weed science. Weed science did not develop exclusively because of herbicide development, nor is its continued development de- pendent on herbicides, although they are an important part of knowledge of weeds and their management. Weed science is vegetation management—the employment of many tech- niques to manage plant populations in an area. This includes dandelions in turf, poisonous plants on rangeland, and johnsongrass in soybean crops. Weed 6 Fundamentals of Weed Science The introduction of mineral fertilizer increased food production. science might be considered a branch of applied ecology that attempts to modify the environment against natural evolutionary trends. Natural evolu- tionary or selection pressure tends toward the lower side of the curve (see Figure 1.1; Shaw et al., 1960) toward what ecologists call climax vegetation, the specific composition of which will vary with latitude, altitude, and environ- ment. A climax plant community does not, and probably cannot, provide the kind or abundance of food 6.5+ billion humans want or need. Therefore, we attempt to modify nature to grow high-value crops for food and fiber. In the beginning, there were no weeds. If one impartially examines the composition of natural plant communities or the morphology of weed flowers, one can find beauty and great aesthetic appeal. The flowers of wild onion, poison hemlock, dandelion, chicory, sunflower, and several of the morning glories are very attractive and worthy of artistic praise for symmetry and color. But what right do we have to call plants with beautiful flowers “weeds”? Who has the right to say a certain type of plant should be destroyed? By what authority do we so easily assign the derogatory term weed to a plant and accuse it of interfering with agriculture, increasing costs of crop production, reducing yields, and maybe even detracting from our quality of life? Nature does not regard weeds as a separate category. One widely accepted definition is a plant that grows where it is not wanted (Buchholtz, 1967). Students should be aware of the anthropocentric dimension of this definition. Desire is a human trait, and therefore a particular plant is labeled a weed only in terms of human attitude. Ecologists speak of “weedy plants,” but often their Introduction 7 Mechanization has increased agricultural productivity. FIGURE 1.1. The food productivity potential of vegetation (Shaw et al., 1960). 8 Fundamentals of Weed Science The flowers of many weeds are beautiful and have great aesthetic appeal. This is the flower of wild carrot or Queen Anne’s Lace. use of the term is influenced by preconceptions of the role of vegetation on a particular site. People decide that a plant that grows in a certain location is not desirable and is therefore a weed. Weeds are regarded as the lowest of the kingdom of flowering plants not because they are innately harmful but because they are harmful to us. Homeowners have been the ones to declare that dandelions and crabgrass are unacceptable in their lawns. Does grass really care what other plants it must share the soil with? Hayfever sufferers blame ragweed or sagebrush in the western United States for their misery. And only those who get an itchy rash from poison ivy insist that it should be eradicated. Farmers claim, with economic justification, that they want their crops to grow in a weed-free environment because it will maximize their yield and profits. So it is people who decide what plants are weeds and when, where, and how they will be controlled. This book discusses many aspects of weeds, their biology, and their con- trols, but it differs from other weed science texts in significant ways. Most current weed science textbooks devote at least 50% of their content to herbi- cides and their use; in some, it is as much as 75%. One notable exception is Aldrich and Kremer’s book (1997), which does not include any major section Introduction 9 The dandelion is considered a weed by many. on herbicides. Because of the undeniable success of chemical weed manage- ment, however, this author believes that it deserves a place in any complete weed science textbook. Omitting that topic will produce students who are only partially prepared for modern weed management. This book discusses herbi- cides and their use but only as part, albeit an important part, of the fundamen- tals of weed science. This book also claims that killing weeds with herbicides is the modern way, as opposed to first understanding their biology and ecology. Weed control has been a major concern almost since the beginning of farming. The Weed Science Society of America has recently identified 17 important early publications on weeds (from 1895 to 1965), 12 of which dealt with their destruction, control, or eradication. As you read this book, you will see that although these important topics are included, the primary emphasis is on understanding everything about weeds. One can establish a relationship between pesticide use and agricultural yield. Perhaps a better way to put it is that one can find a relationship between good pest management (regardless of how it is accomplished) and agricultural yield. One should not always equate good weed control with herbicide use. Good weed control depends on cultural knowledge—what a good farmer or plant grower knows. Cultural knowledge is different from the scientific 10 Fundamentals of Weed Science knowledge that leads to herbicide development and successful use. Both kinds of knowledge—scientific to tell us what can be done and cultural to tell us what should be done—are essential to good weed management. One can also postulate a relationship among the way weeds and other pests are controlled, the practice of pest management, and a nation’s food supply. Figure 1.2 shows the world’s tropical and subtropical areas, their major crops, and the percent of the world’s total crop grown in each area. The region’s ability to control weeds is shown in Figure 1.3, with data for the world and four major areas. Each segment in Figure 1.3 is divided into good, moderate or acceptable, low, and very poor weed management. The world’s tropical and subtropical regions (Figure 1.2) are home to a majority of the world’s popula- tion and produce most of some of the world’s most important crops. But although pest management science has made remarkable progress since Figure 1.2 was prepared in 1971, these areas identified still suffer from underdevelop- ment of weed science and other agricultural technology (Figure 1.3). In a UN/FAO survey (Labrada, 1996) of 70 countries in Asia, Africa, and Latin America, which had 43.7% of the world’s arable land and 65.8% of the world’s people, an almost total lack of weed control technology and knowledge were found. The founder of Latin prose, Cato the Elder, reminds us in his work on agriculture that “it is thus with farming: If you do one thing late, you will be late in all your work.” We are late in implementing advanced weed manage- FIGURE 1.2. Crop production in the world’s tropics (Holm, 1971). Introduction 11 FIGURE 1.3. The level of weed control practices in the world and four regions (Labrada, 1996). ment techniques in much of the world, and agriculture cannot progress to its full potential without them. The agricultural productivity of the developed world is not an accident. US agriculture and that of other advanced nations grew out of a propitious com- bination of scientific advancement, industrial growth, and abundant resources of soil, climate, and water. One should not regard it as just good luck that we Americans can pay our food bills and still have money left over while many folks in the world are starving. In much of the world, weeds are controlled by hand or with crude hoes. The sizes of a farmer’s holding and yield per unit area are limited by several things and paramount among them is the rapidity with which a family can weed its crops. More human labor may be expended to weed crops than on any other single human enterprise, and often much of that labor is expended by women. Weed control in the Western world and in some other areas is performed by sophisticated machines and by substituting chemical energy for mechanical and human energy. There is a relationship between the way farmers control weeds and the ability of a nation to feed its people. Weed science is part of that relationship. Good weed management is one of the essential ingre- dients to increasing food production. The early flights of the Apollo spacecrafts and subsequent space flights gave those back on Earth a view of the whole planet, floating in the great, black sea of space. Many had imagined but had never seen such a view before. Space 12 Fundamentals of Weed Science exploration opened exciting new vistas and opportunities for someday living on other planets, but, for now, we are confined to Earth. About 1965, world food production began to lose the race with an expanding population, just as T.R. Malthus (1798) predicted it would. Each year, the apocalypse that Malthus predicted is prevented, but it remains a daily specter for many in the world. The world’s population now exceeds 6.5 billion people, and it will continue to grow, albeit at a slower rate. More than 85% of the world’s people live in poor, developing countries, and about 95% of the population growth will occur in those countries. As world population expands, food production is barely keeping pace and often slipping behind. About 10% of the world’s 33 billion acres of land are arable, and while the area devoted to productive agriculture can be expanded, the cost will be great. One must also recognize that the world may lack the social and political will to handle the complex problems that may arise from expansion onto previously untilled land. Such expansion is certainly part of the solution to the world food dilemma, but an equally important one is to use appropriate, available technology and to develop new technology. If all the world’s people are going to enjoy higher standards of living and be able to watch their children mature without fear of debilitating disease, malnutri- tion, or starvation, we must use intelligently all present agricultural technology and continue to develop better, safer technology. Shared technology and knowledge will permit our neighbors in this world to farm in ways that realize full agricultural and human potentials. Weed science is not a panacea for the world’s agricultural problems. The problems are too complex for any simple solution, and students should be suspicious of those who propose simple solutions to complex problems. In fact, the goals should be not to solve but to diminish, not to cure but to allevi- ate, and to at least anticipate the “brute question” and have some answers when “whirlwinds of rebellion strike all shores.” The work of the weed scien- tist is fundamental to solving problems of production agriculture in our world. Weeds have achieved respect among farmers who deal with them every year in every crop. Weeds and weed scientists have achieved respect and credibility in academia and the business community. The world’s weed scientists are and will continue to be in the forefront of efforts to feed the world’s people. LITERATURE CITED Aldrich, R.H. and R.J. Kremer. 1997. Principles in Weed Management, 2nd ed. Iowa State Univ. Press, Ames, IA. 455 pp. Appleby, A.P. 2005. A history of weed control in the United States and Canada—A sequel. Weed Sci. 53:762–768. Bailey, L.H. 1906. The Survival of the Unlike: A collection of evolution essays suggested by the study of domestic plants. Macmillan Co. & Co., Ltd., London, UK. Introduction 13 Buchholtz, K.P. 1967. Report of the terminology committee of the Weed Science Society of America. Weeds 15:388–389. Cato the Elder. De Agri Cultura. 2nd Century BC. Cox, S. 2006. Civilization’s weedy roots. Pp. 8–10, in The Land Report 84 (Spring). De Morgan, A.C. 1850. A budget of paradoxes. p. 453, item 662.1. International Thesaurus of Quotations, 1970. Compiled by R.T. Tripp. Food and Agriculture Organization of the United Nations. 1963. Production Yearbook. Food and Agriculture Organization of the United Nations. 1975. Production Yearbook. Goodwin, D.C. and P.H. Johnstone. 1940. A brief chronology of American agricultural history. U.S. Dept. of Agriculture Yearbook of Agriculture. Pp. 1184–1196. Holm, L. 1971. The role of weeds in human affairs. Weed Sci. 19:485–490. Labrada, R. 1996. Weed management status in developing countries. Second Int. Weed Cont. Congress. 2:579–589. Liebig and After Liebig. 1942. Washington, D.C. American Association for Adv. of Sci. 111 pp. Malthus, T. R. 1798. An essay on the principle of population as it affects the future improvement of society, with remarks on the speculations of Mr. Godwin, M. Condorcet, and other writers. Macmillan and Co., Ltd., 1966 ed. 396 pp. Markham, Edwin. 1899. The man with the hoe. Pp. 303–304, in The pocket book of verse. 1940. Pocket Books, Inc., New York. Moldenke, H.N. and A.L. Moldenke. 1952. Plants and the Bible. Dover Publications, Inc. New York. Pp. 70–72, 133–134, and 153. Rasmussen, W.D. 1975. Agriculture in the United States: A documentary history. Four volumes. Random House. New York. Shaw, W. C., J. L. Hilton, D. E. Moreland, and L. L. Jansen. 1960. Herbicides in plants. Pp. 119– 133, in The nature and fate of chemicals applied to soils, plants and animals. USDA, ARS., 20–9. Timmons, F.L. 1970. A history of weed control in the United States and Canada. Weed Sci. 18:294–307. Republished—Weed Sci. 2005;53:748–761. Williams, H. 1937. The poems of Jonathan Swift. Vol. II. On poetry: A rhapsody. Pp. 639–659. Specific reference on p. 651. This page intentionally left blank  CHAPTER 2 Weeds—The Beginning FUNDAMENTAL CONCEPTS The most basic concept of weed science is embodied in the term weed. Weeds are defined in many ways, but most definitions emphasize behavior that affects humans. All weeds share some characteristics. Weeds express their undesirability in at least nine distinct ways. Although it is difficult to estimate total weed cost, in the United States, losses due to weeds exceed $8 billion per year. OBJECTIVES To understand the definitions of weeds. To identify the common characteristics of weeds. To understand how weeds cause damage To appreciate the enormous cost of weeds and how costs are estimated.... and nothing teems But hateful docks, rough thistles, kecksies, burs, Losing both beauty and utility. And as our vineyards, fallows, meads, and hedges Defective in their natures, grow to wildness; Even so our houses, and ourselves, and children, Have lost, or do not learn, for want of time, The sciences that should become our country. King Henry V, Act 5, Scene 2. Play by William Shakespeare Fundamentals of Weed Science Copyright © 2007 by Academic Press, Inc. All rights of reproduction in any form reserved. 15 16 Fundamentals of Weed Science I will go root away the noisome weeds, which without profit suck the soil’s fertility from wholesome flowers. Richard II, Act 3, Scene 3. Play by William Shakespeare There are laws in the village against weeds The law says a weed is wrong and shall be killed The weeds say life is a white and lovely thing And the weeds come on and on in irrepressible regiments. “Weeds” Poem by Carl Sandburg I. DEFINITION OF THE WORD WEED To be fully conversant with a subject, one must understand its basic concepts, and the most basic concept of weed science is embodied in the word weed itself. Each weed scientist has a clear understanding of the term, but there is no universal definition that is accepted by all scientists. In 1967 the Weed Science Society of America defined a weed as “a plant growing where it is not desired” (Buchholtz, 1967). In 1989 the Society’s definition was changed to define a weed as “any plant that is objectionable or interferes with the activities or welfare of man” (Humburg, 1989, p. 267; Vencill, 2002, p. 462). The European Weed Research Society defined a weed as “any plant or vegetation, excluding fungi, interfering with the objectives or requirements of people” (EWRS, 1986). Although the definitions are clear, they are not accepted by all scientists. These definitions leave their interpretations with people, so they must be the ones to determine when a particular plant is growing where it is not wanted or where it interferes with their activities or welfare. The Oxford English Dictionary (Little et al., 1973) defines a weed as a “her- baceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegeta- tion.” The human role is again clear because it is we who determine use or beauty and which plants are to be regarded as superior. It is important that weed scientists and vegetation managers remember the importance of defini- tions as determinants of their views of plants and attitudes toward them. How one defines something largely determines his or her attitude toward the thing defined, and, for the weed scientist and vegetation manager, deter- mines which plants are weeds and therefore must be controlled. Weeds, like other plants, lack consciousness and cannot enter the court of public opinion to claim rights. Humans can assign rights to plants and serve as their counsel to determine or advocate their rights or lack thereof in our environment. Our Weeds—The Beginning 17 attitude toward weedy plants need not always be shaped by another’s definition because people seldom agree on definitions. Once in a golden hour, I cast to earth a seed. Upon there came a flower, The people said a weed. Read my little fable: He that runs may read Most can raise the flowers now, For all have got the seed. And some are pretty enough, And some are poor indeed: And now again the people Call it but a weed. “The Flower” Poem by Alfred Lord Tennyson Not all people agree about what a weed is or what plants are weeds. Harlan and de Wet (1965) assembled several definitions to show the diversity of definitions of the same or similar plants. The array of definitions emphasizes the care weed scientists and vegetation managers must take in equating how something is defined with a right or privilege to control. Definitions from plant scientists W.S. Blatchley 1912 “A plant out of place or growing where it is not wanted.” A.E. Georgia 1916 “A plant that is growing where it is desired that something else shall grow.” W.W. Robbins et al. 1942 “These obnoxious plants are known as weeds.” W.C. Muenscher 1946 “Those plants with harmful or objectionable habits or characteristics which grow where they are not wanted, usually in places where it is desired that something else should grow.” J.L. Harper 1960 “Higher plants which are a nuisance.” E.J. Salisbury 1961 “A plant growing where we do not want it.” G.C. Klingman 1961 “A plant growing where it is not desired; or a plant out of place.” Definitions by enthusiastic amateurs R.W. Emerson 1912 “A plant whose virtues have not yet been discovered.” F.C. King 1951 “Weeds have always been condemned without a fair trial.” Ecological definitions A.H. Bunting 1960 “Weeds are pioneers of secondary succession, of which the weedy arable field is a special case.” 18 Fundamentals of Weed Science W.S. Blatchley 1912 “A plant which contests with man for the possession of the soil.” T. Pritchard 1960 “Opportunistic species that follow human disturbance of the habitat.” E.J. Salisbury 1961 “The cosmopolitan character of many weeds is perhaps a tribute both to the ubiquity of man’s modification of environmental conditions and his efficiency as an agent of dispersal.” Godinho (1984) compared the definitions of the French words d’aventice and le mauvaise herbe with the English weed and the German unkraut. No single definition was found for weed and unkraut because both words have two distinct meanings: 1. In the ecological context, weed, unkraut, and d’aventice mean a plant that grows spontaneously in an environment that has been modified by man. 2. In the weed science context, weed, unkraut, and malherbe (Italian) or le mauvaise herbe mean an unwanted plant. In some languages weeds are just bad (mal) plants. In Spanish, it is mala hierba or malezas, and in Italian, malherbe. One must agree with Godinho (1984), Fryer and Makepeace (1977), Anderson, (1977), and Crafts and Robbins (1967) that neither the word weed nor the plants to which the word is assigned are easy to define. Aldo Leopold (1943, as cited in Falder and Callicott, 1991) made the point well in an article written in 1943 that was critical of the 1926 bulletin Weeds of Iowa. Many of the native plants of Iowa are included in the bulletin, and Leopold noted that these plants, in addition to their inherent beauty, have value as wildlife food, for nitrogen fixation, or as makers of stable plant com- munities. He admits that many of the plants people consider weeds are common in pastures, but soil depletion, overgrazing, and needless disturbance of advanced successional stages often make control necessary. Leopold (1943) argues that the definition of weed is part of the problem because not all plants that some call weeds “should be blacklisted for general persecution.” Leopold’s view is supported by McMichael (2000), who noted, with supporting evidence, that “in many rural cultures, noncrop plants (often termed weeds) represent food, fodder, and medicine. About 3,000 of the 350,000+ recognized plant species have been or are cultivated, and one cannot assume that the rest are weeds. Specific, unknown, and noncultivated plants must also be considered. The ulterior etymology of the word weed is unknown, but an exposition of what is known was provided by King (1966). He traced the word to a Germanic romance language and Asian roots, but he concluded that weed is an “example of language as an accident of usage.” He was unable to find a common word in any ancient language for the collective term weed. Weeds—The Beginning 19 It is logical to assume that even if one cannot define weed, it should still be possible to identify the origin of individual species and determine certain characteristics of weeds. They come from both native and naturalized flora. Some plants succeeded as weeds because they were able to evolve forms adapted to disturbed environments more readily than other species. Baker’s (1965, 1991) definition emphasizes success in disturbed environments, a point he reiterated in the later paper: A plant is a “weed” if, in any specified geographical area, its populations grow entirely or predominantly in situations markedly disturbed by man (without, of course, being deliberately cultivated plants). Thus, for me, weeds include plants which are called agrestals by some writers of floras (they enter agricultural land) as well as those which are ruderals (and occur in waste places as well as along roadsides). It does not seem to me necessary to draw a line between these categories and accept only the agrestals as weeds (although this is advocated by some agricul- turally oriented biologists) because in many cases the same species occupy both kinds of habitat. Ruderals and agrestals are faced with many similar ecological factors, and the taxa which show these distributions are, in my usage, “weedy.” If one considers weeds in the Darwinian sense of a struggle for existence, they represent one of the most successful groups of plants that have evolved simultaneously with human disruption of areas of indigenous vegetation and habitats and creation of disturbed habitats (King, 1966). Aldrich (1984) and Aldrich and Kremer (1997, p. 8) offered a definition that does not deny the validity of others but introduces a desirable ecological base. A weed is “a plant that originated in a natural environment and, in response to imposed or natural environments, evolved, and continues to do so, as an inter- fering associate with our crops and activities.” This definition provides “both an origin and continuing change perspective” (Aldrich, 1984). Aldrich wants us to recognize weeds as part of a “dynamic, not static, ecosystem.” His defini- tion departs from those that regard weeds as enemies to be controlled. Its eco- logical base defines weeds as plants with particular, perhaps unique, adaptations that enable them to survive and prosper in disturbed environments. Navas (1991) also included biological and ecological aspects of plants and effects on man in his definition. A weed was defined as “a plant that forms populations that are able to enter habitats cultivated, markedly disturbed or occupied by man, and potentially depress or displace the resident plant populations which are deliberately cultivated or are of ecological and/or aesthetic interest.” Although all do not agree on precisely what a weed is, most know they are not desirable. Those who want to control weeds must consider their definition. When the term weed is borrowed from agriculture and applied to plants in natural communities, a verification of negative effect on the natural commu- nity should be a minimal expectation. Simple yield affects are not acceptable, but the effects of the presumed weed in a natural community can be estimated in terms of a management goal such as establishment of presettlement 20 Fundamentals of Weed Science conditions, preserving rare species, maximizing species diversity, or maintain- ing patch dynamics (Luken and Thieret, 1996). Many recognize the human role in creating the negative, often deserved, image. Weeds are detrimental and often must be controlled but only with adequate justification for the site and conditions. II. CHARACTERISTICS OF WEEDS Crop agriculture is based on a very few plants that thrive in a disturbed habitat (a cropped field) and produce an abundance of seed. Weeds also thrive in disturbed habitats and produce an abundance of seed that is not useful to humans (Manning, 2004, p. 55). Why is it that some plants that thrive in dis- turbed habitats are weeds? What is it that makes some plants capable of growing where they are not desired? Why are they difficult to control? What are their modes of interference and survival? The most consistent trait of weedy species is not related to their morphology or taxonomic relationships. It is, as Baker (1965) noted, their ability to grow well in habitats disturbed by human activity. They are plants that are growing where someone does not want them, and often that is in areas that have been disturbed or altered inten- tionally. Weeds grow especially well in gardens, cropped fields, golf courses, and similar places. Their ability to grow in habitats that have been disturbed by man makes them a kind of ecological Red Cross: They rush right into dis- turbed places to restore the land. Two nonindigenous species, kudzu and purple loosestrife, illustrate the ability of weeds to spread to new areas and habitats. (See Chapter 7 for a dis- cussion of the role of these plants as invasive species.) Both were introduced to the United States, and both now grow all over the country (see Figure 2.1; U.S. Congress, 1993). Not all weeds possess every single characteristic that is considered undesir- able, but in addition to growing in disturbed habitats, all have at least some of the following characteristics (see Baker, 1965): 1. Weeds have rapid seedling growth and the ability to reproduce when young. Redroot pigweed can flower and produce seed when less than 8 inches tall. Crops cannot do either. 2. Quick maturation or only a short time in the vegetative stage. Canada thistle can produce mature seed two weeks after flowering. Russian thistle seeds can germinate very quickly between 28° and 110°F in late spring (Young, 1991). It would spread more, but the seed must germinate in loose soil because the coiled root unwinds as it pushes into soil and is unable to do so in hard soil. Weeds—The Beginning 21 FIGURE 2.1. US distribution of kudzu and purple loosestrife (U.S. Congress, 1993; Thompson et al., 1987; also see Anonymous, 1990). 3. Dual modes of reproduction. Most weeds are angiosperms and reproduce by seed. Many also reproduce vegetatively (e.g., Canada thistle, field bind- weed, leafy spurge, quackgrass). 4. Environmental plasticity. Many weeds are capable of tolerating and growing under a wide range of climatic and edaphic conditions. 5. Weeds are often self-compatible, but self-pollination is not obligatory. 6. If a weed is cross-pollinated, pollination is accomplished by nonspecial- ized flower visitors or by wind. 7. Weeds resist detrimental environmental factors. Most crop seeds rot if they do not germinate shortly after planting. Weed seeds resist decay for long periods in soil and remain dormant. 8. Weed seeds exhibit several kinds of dormancy or dispersal in time to escape the rigors of the environment and germinate when conditions are most favorable for survival. Many weeds have no special environmental requirements for germination. 22 Fundamentals of Weed Science 9. Weeds often produce seed that is the same size and shape as crop seed, making physical separation difficult and facilitating spread by man. 10. Some annual weeds produce more than one seed crop per year, and seed is produced as long as growing conditions permit. 11. Each generation is capable of producing large numbers of seed per plant, and some seed is produced over a wide range of environmental conditions. 12. Many weeds have specially adapted long- and short-range seed dispersal mechanisms. 13. Roots of some weeds are able to penetrate and emerge from deep in the soil. While most roots are in the top foot of soil, Canada thistle roots routinely penetrate 3 to 6 feet and field bindweed roots have been recorded over 10 feet deep. Roots and rhizomes are capable of growing many feet per year. 14. Roots and other vegetative organs of perennials are vigorous with large food reserves, enabling them to withstand environmental stress and inten- sive cultivation. 15. Perennials have brittleness in lower stem nodes or in rhizomes and roots, and, if severed, vegetative organs will quickly regenerate a whole plant. 16. Many weeds have adaptations that repel grazing, such as spines, taste, or odor. 17. Weeds have great competitive ability for nutrients, light, and water and can compete by special means (e.g., rosette formation, climbing, allelopathy). 18. Weeds are ubiquitous. They exist everywhere that we practice agriculture. 19. Weeds resist control, including resistance to herbicides. In spite of the anthropomorphic aspects of the definitions of weed and the multiple traits that weeds share, weed scientists have a clear idea of which plants are weeds. It seems that weeds are everywhere in almost every place, and many books have been written about weeds: Common weed seedlings of the Central High Plains (Nissen and Kazarian, 2000) Major Weeds of the Philippines (Moody et al., 1984) Major Weeds of Thailand (Noda et al., 1985) Striga Identification and Control Handbook (Ramaiah et al., 1983) The Arable Weeds of Europe—with their Seedlings and Seeds (Hanf, 1983) The Identification of Weed Seedlings of Farm and Garden (Chancellor, 1966) Weeds of Colorado, A Comprehensive Guide to Identification (Zimdahl, 1998) Weeds of Hawaii’s Pastures and Natural Areas (Motooka et al., 2003) Weeds—The Beginning 23 Weeds of Karnataka (Krishna Sastry et al., 1980) Weeds of Nebraska and the Great Plains (Stubbendieck et al., 1994) Weeds of North India (Arora et al., 1976) Weeds of Rice in Asia (Caton et al., 2004) Weeds of the West (Whitson et al., 1991) The Weed Science Society of America has published a weed identification CD that includes an interactive format for identification of 1,000 weeds of North America (https://timssnet.allenpress.com/ECOMWSSA/timssnet/products/tnt_ products.cfm), click on identification, photo gallery. III. HARMFUL ASPECTS OF WEEDS Definitions of weeds usually include trouble with crops, harm to people, or harm to animals. Most people do not consider plants to be bad. They are assigned the descriptive, derogatory term weed because of something they do to us or to our environment; they interfere with the activities or welfare of man. If they were benign we wouldn’t be so concerned about them because there would be no detrimental effects. The nature of weeds’ harmful effects will be explored briefly in this section. That harmful effects exist is not ques- tioned. It is important to understand specific effects so appropriate action can be taken. A. PLANT COMPETITION From an agricultural perspective, we are concerned about weeds because they compete with crop plants for nutrients, water, and light. If they did not, those who grow things would be more willing to tolerate their presence. Weed-crop competition will be discussed in Chapter 6. If weeds did not compete, they would not need to be managed because crop yield would not be affected by their presence. But it is, and often complete crop failure (100% loss of market- able yield) can occur if weeds are not controlled. B. ADDED PROTECTION COSTS Weeds increase protection costs because they harbor other pests. A partial listing of diseases, insects, and nematodes that use weeds as alternate hosts is in Tables 2.1, 2.2, and 2.3. Weeds harbor a wide range of organisms thereby increasing opportunities for those organisms to persist in the environment and reinfest crops in succeeding years. TABLE 2.1. Plant Diseases Harbored by Specific Weeds. Plant disease Weed host Crop infested Reference Blackleg Black nightshade Potato Dallyn and Sweet, 1970 Common Lambs quarters Mare’s tail Redroot pigweed Smartweed Wilt diseases Netseed lambs Potato, alfalfa Oshima et al., 1963 quarters Common purslane Redroot pigweed Stem canker Netseed Lambs Potato, beans Oshima et al., 1963 quarters Soft rot Annual sowthistle Chinese cabbage Kikumoto and Dayflower and other Sakamoto, Common Lambs vegetables 1969 quarters Powdery mildew Wild oats Wheat, oats, barley Eshed and Wahl, 1975 Stripe mosaic virus Common Lambs Barley ARS, 1966 quarters Leaf curl virus Common Lambs Sugarbeet ARS, 1966 quarters Cucumber mosaic vi