AGR205 weed management ch 8.pdf

AGR-205 Weed Management By Chip Bubl, Extension agent, Columbia County, Oregon State University. Adapted by J.D. Green, Extension weed specialist, University of Kentucky. Creating a...

AGR-205 Weed Management By Chip Bubl, Extension agent, Columbia County, Oregon State University. Adapted by J.D. Green, Extension weed specialist, University of Kentucky. Creating a fire hazard along railways and next to power In this chapter: substations Presenting allergy or poison hazards for humans or animals Understanding Weeds 01 (through skin contact, ingestion, or inhalation) Types of Weeds 03 Harboring diseases, rodents, and insect pests Imparting off-flavors to water Weeds and Your Landscape 04 Managing Weeds 05 Why Do We Have Weeds? Weed Management for Specific Situations 09 People have been cultivating plants for almost 10,000 years. For More Information 10 When the first agriculturists tilled ground, planted seeds, and irrigated, they created an artificial environment that favored crops. They also selected and bred certain plants to improve the plants’ food, fiber, or medicinal characteristics. The very E very garden has weeds, and every gardener wonders what to do about them. Gardening involves lots of small decisions that can have a cumulative effect on those weed problems. This practices that favored growth of these earliest crops inadver- tently encouraged other plants that also liked plowed soil. Many of these plants became very aggressive competitors on farms chapter will explore the origin of weeds, their adaptation and and in gardens. impact, and the techniques you can use to manage weeds in As agricultural practices and products spread, plants we now your landscape. call weeds extended their range. Native plants that liked tilled ground joined the weed inventory and moved well beyond their Understanding Weeds original habitats. It is amazing how many of “our” weeds are equally well known in Asia, Europe, Africa, and South America. To control weeds, you first need to understand them and be Table 8.1 shows the original home of some common weeds. able to identify them. Many weeds traveled the world in feed and seed, on equip- ment, in ships’ ballast, or in the bellies of domesticated animals. Why Are Weeds a Problem? In addition, many plants that once had value as food, fiber, The plants we call weeds are often aggressive, persistent medicine, or ornamentals remained to become a nuisance plants—they are not shrinking violets. Weeds do have some long after their usefulness had diminished. Once weeds get a redeeming qualities, but these qualities are often overwhelmed by the plants’ “weedy” attributes. Table 8.1. Origin of several common weeds. The most useful definition of a weed is that it is a plant that Weed species Origin is a hazard, nuisance, or causes injury to people, animals, or a Canada thistle Eurasia desirable crop. A plant may be defined as a weed in some cases Pigweed Tropical Americas but not in others. For example, clover can be a valuable addi- Field bindweed Europe tion to a pasture and is tolerated in many lawns. But if you are Common purslane Western Asia a park manager faced with reducing liability due to bee stings, Common chickweed Europe you may feel compelled to remove clover in some parts of the Dandelion Europe park. Another example is maple seeds. When they sprout in an Quackgrass Mediterranean herbaceous perennial flower bed, they are weeds, but they may Barnyardgrass Eurasia well have come from trees that are part of the planted landscape. Crabgrass Europe Weeds can cause a range of problems in your garden or Poison ivy Native community, including the following: Johnsongrass Mediterranean Competing with desirable crops Common lambsquarters Eurasia Reducing the aesthetic qualities of a landscape Musk thistle Eurasia Obstructing sight lines on roadways Yellow foxtail Europe Interfering with water drainage from roads and low-lying Trumpet creeper Native communities Morningglory Tropical Americas CHAPTER 08 Weed Management foothold in a new area, they spread by wind, water, animals, in with a good reference (see “For More Information” section at trade goods, and in equipment. the end of this chapter) will help you put a name to some of the Plants that might be tame in their native landscape can challenging plants you encounter. You may find it helpful to give become nasty when moved to new locations. The new envi- some of your worst weeds specific nonsense names until you ronment might suit them better, or they might have left their get a more accurate identification. Then, it will be easy to mesh natural enemies behind. Kudzu was originally introduced to your new knowledge with your old identification. help stabilize steep slopes and prevent erosion. It now grows Weed books usually first classify plants into two groups and uncontrolled in much of the southern United States and has a miscellaneous category: become established in several Kentucky counties, where it is Monocots (Figure 8.1) include grasses, lilies, and the like. unsightly and chokes out native vegetation. Purple loosestrife Dicots (Figure 8.2) are broadleaf species. was originally planted as a pond ornamental, but now it aggres- “Other” weeds include mosses, horsetails, and ferns. sively competes with native vegetation in wetland areas. Plants are further subdivided within each major category by Nations establish quarantines to exclude new weed species. family—for example, the buckwheat, rose, and borage families. State departments of agriculture designate some weeds as nox- Within these family groupings are the genera, and within the ious and implement programs to restrict their spread. There genera are the species. may be specific requirements for control of certain noxious Scientific (botanical) names are in Latin and include the weeds in some locations. genus name followed by the species name. For example, com- Once weedy species are established in a region, it is virtually mon dandelion is known as Taraxacum officinale. The scientific impossible to eradicate or remove them completely. Instead, name is used worldwide and eliminates the confusion caused gardeners must live with them and work to lessen their negative when the same common name refers to several plants. impact. Various techniques offer a range of options to achieve You don’t have to be a botanical expert to work through a that goal. weed identification book, although it does help to learn some of the key characteristics of the major families. Weed Identification Once you know a plant, you can gather important details If you can accurately identify a plant that is causing prob- about its life cycle and how it spreads within the landscape lems in your landscape, you have taken the first step toward or garden. With practice, you can learn to distinguish weed good management. Fortunately, most problems are caused seedlings from your planted vegetables and flowers. by a surprisingly short list of weeds. Often, a little time spent Figure 8.1. Examples of monocots: bermudagrass (Cynodon dactylon), crabgrass (Digitaria spp.), and nimblewill (Muhlenbergia schreberi J.F. Gmel.). 8-2  Weed Management CHAPTER 08 Table 8.2. Weeds classified by life cycle. Annuals Common name Botanical name Grass weeds Annual bluegrass Poa annua Barnyardgrass Echinochloa crus-galli Crabgrass Digitaria sanguinalis Yellow foxtail Steria glauca Broadleaf Annual sowthistle Sonchus oleraceus weeds Birdsrape mustard Brassica rapa Carolina geranium Geranium carolinianum Catchweed bedstraw Galium aparine Common chickweed Stellaria media Common lambsquarters Chenopodium album Common mallow Malva neglecta Common purslane Portulaca oleracea Figure 8.2. Wild violet (Viola Figure 8.3. Common chickweed, Common ragweed Ambrosia artemisiifolia sp.), an example of a dicot. an example of an annual weed. Cressleaf groundsel Senecio glabellus Hairy bittercress Cardamine hirsuta Weed Life Cycles Ladysthumb Polygonum persicaria Mayweed chamomile Anthemis cotula Most gardens have a mix of annual and perennial weeds, plus Nodding spurge Euphorbia nutans a few biennials. When you understand the life cycle of trouble- Pineapple weed Matricaria matricarioides some weeds, you can begin to make intelligent decisions about Prostrate knotweed Polygonum aviculare control strategies. Later sections of this chapter examine some Shepherdspurse Capsella bursa-pastoris of those strategies. Table 8.2 lists some of our worst weeds by Smooth pigweed Amaranthus hybridus common and botanical names and their life cycles. Biennials Common name Botanical name Broadleaf weeds Bull thistle Common mullein Cirsium vulgare Verbascum thapsus Types of Weeds Musk thistle Carduus nutans Weeds include annuals, biennials, and perennials. Prickly lettuce Lactuca serrioia Teasel Dipsacus sylvestris Annuals Wild carrot (Queen Anne’s Daucus carota lace) Annuals go from seed to seed in less than a year, often in Perennials Common name Botanical name periods as short as 45 days. Once they have thrown off their Grass weeds Bermudagrass Cynodon dactylon usually vast quantities of seed, the plants die. As you might Johnsongrass Sorghum halepense suspect, annual weeds are numerous because their growth Quackgrass Elytrigia repens habit parallels our agricultural cropping pattern. Velvetgrass Holcus lanatus Annual weeds can be found in both winter and summer. Broadleaf Blackberry Rubus spp. Winter annuals germinate in late fall through early spring weeds Buckhorn plantain Plantago lanceolata and go to seed in spring and early summer. Some common Canada thistle Cirsium arvense examples include hairy bittercress, henbit, several mustards, Creeping buttercup Ranunculus repens annual bluegrass, and common chickweed (Figure 8.3). Clearly, Curly dock Rumex crispus the cooler temperatures and lower light intensity of winter Dandelion Taraxacum officinale are not an obstacle to these weeds. They prosper when other Field bindweed Convolvulus arvensis competitive weeds and plants are absent. A few winter annuals, Hedge bindweed Calystegia sepium especially common groundsel, seem to germinate in flushes Oxalis (woodsorrel) Oxalis corniculatus throughout the year. Poison ivy Toxicodendron radicans Summer annuals get started in the spring and summer and Red sorrel Rumex acetosella go to seed in the summer and fall. Many of our most annoying Others Common name Botanical name weeds are in this group, including crabgrass, pigweed, purslane, Horsetail Equisetum arvense and lambsquarters. Most summer annuals germinate quickly, Star-of-Bethlehem Ornithogalum umbellatum ahead of many flower and vegetable seeds. Others do not Wild garlic Allium vineale germinate until May or June and then grow aggressively and Yellow nutsedge Cyperus esculentus complete their life cycle without delay. Note: This list is not exhaustive, but it does include many of the most common As discussed below, annuals have sophisticated mechanisms weeds that gardeners should recognize. to ensure good year-to-year seed survival. 8-3 CHAPTER 08 Weed Management Biennials Weeds and Your Landscape Biennial species are less common. They take more than Weeds can disturb the appearance of a landscape. Most gar- one year but less than two to complete their life cycle. Most den soils contain a lot more weed seeds than the vegetable or start from seed in the fall or spring and grow through the sum- flower seeds you plant. Studies on commercial vegetable farms mer, fall, winter, and next spring. They overwinter as rosettes. show that viable weed seed populations in the top six inches of In the second summer, biennials flower and die. Examples soil average 900 per square foot, with some fields having more include wild carrot (Queen Anne’s lace), bull thistle, and musk than 7,000 viable seeds per square foot! thistle (Figure 8.4). What is worse, weed seeds germinate very quickly when the ground is tilled. Two weeks after planting, it may be very Perennials difficult to find your flowers and vegetables amid all the weeds. These weeds often are the most difficult to manage. Woody Some poorly tended gardens may end up with 10 to 20 times species generally go dormant in the winter and begin growth in more weight in weeds than in desired plants. spring from aboveground stems. Aboveground parts of herba- ceous perennials may die back, but their underground storage Weed Competition organs survive the winter. Many are deep rooted and continue Weeds compete directly with garden plants for light, nutri- to grow during summer droughts. The life span of perennials ents, and water. A successful weed grows aggressively to capture varies. They spread from seed and often from creeping roots, whatever resource is in shortest supply. Crop plants may end up tubers, bulbs, and rhizomes. Tilling of perennial weeds often stunted and unable to produce a normal product. Paired test spreads them, and mulches may have little impact. Weeds in this plots of vegetables that were either weeded or left untouched group include quackgrass, poison ivy, Canada thistle, horsetail, provide some eye-opening results (Table 8.3). and field bindweed (Figure 8.5). Research in England showed that a delay in weeding could reduce final yield by 3 percent per day, depending on the crop, weed species, and weed density. Some plants (onions, for example) need a certain leaf area to produce a marketable crop, while others (e.g., potatoes and artichokes) can send up new leaves and recover to produce a near-normal yield if weeds are removed early enough. Some characteristics that help weeds compete include the following: Aggressive vegetative growth from seeds Abundant and rapid reproduction Good means of dispersing seeds Long-lived seeds and other plant parts Wide adaptability to soil types and climates Ability to time germination to coincide with favorable conditions Ability to thrive in disturbed or bare soil Table 8.3­. Vegetable yields in weeded and unweeded plots.1 Yield (lb) Crop Weedy Nonweedy Carrots 27.9 503.3 Beets 45.9 240.3 Cabbage 129.1 233.6 Onions 3.6 67.7 Tomatoes 23.2 164.2 Potatoes 52.7 148.3 Figure 8.4. Musk thistle, an Figure 8.5. Field bindweed 1 Plot sizes not specified, but weedy and nonweedy plots were equal in size. example of a biennial weed. (Convolvulus arvensis), an With the exception of weed management, both plots were treated the example of a perennial weed. same. 8-4  Weed Management CHAPTER 08 Rapid Root and Top Growth In the broadest sense, weed management has the following three objectives: Weeds grow quickly to capture sunlight, water, space, and Preventing introduction of new weeds nutrients. They often can alter their branching pattern, leaf size, Discouraging weeds so they can’t compete with desired plants and leaf orientation to win the battle for light. In soil short of Preventing weeds from going to seed, thus reducing over moisture, weeds’ root growth can stunt crop roots. time the weed seed burden in the soil Sophisticated Reproductive Strategies Prevention Not only do weeds produce tremendous numbers of seeds, Most garden soils already have many weeds, and other but they also have ways to prevent all of their seeds from ger- weed seeds can be blown in, but the alert gardener can take minating during years with less favorable weather. Seeds can be some steps to prevent introduction of new weeds. Examine buried in undisturbed soil for an amazing length of time and still any soil you plan to bring to your lot. Pay particular attention germinate. Lotus seed in Manchurian lakebeds has germinated to the presence of Johnsongrass, Canada thistle, and Star-of- 1,000 years after the seed was produced. Common lambsquar- Bethlehem, which will cause years of agony if found in your ters seed from Egyptian tombs also has been viable after many landscape (Figure 8.6). years. Table 8.4 shows weed seed production and survival rates. Watch container plants from nurseries, garden exchanges, These survival rates are under optimal conditions, and most and plant sales. They can be a source of several tough plants, par- weed seeds are nowhere near these rates. Nevertheless, as the ticularly Oxalis, buttercups and the weedy veronicas. Remove saying goes, one year of weeds leads to seven years of hoeing. any sign of these plants before placing new trees, shrubs, or flowers in your landscape. Chemical Warfare between Plants Finally, cultivate a relationship with your neighbors that Some weeds produce compounds in their leaves and/or allows for a coordinated approach to creeping perennial roots that can kill or harm other plants. This phenomenon is weeds such as field bindweed, English ivy, blackberries, and called allelopathy. The compounds aren’t effective on all the Japanese knotweed. species with which a particular weed might be competing, but they at least can improve the weed’s odds. Some weeds that have allelopathic qualities include quackgrass, Canada thistle, pigweed, kochia, purslane, and smartweed. Managing Weeds Weeds are part of the dynamic and shifting garden land- scape. As discussed earlier, many weeds are especially adapted to a cultivated environment. A gardener should plan for weeds. It is possible to develop a fairly comprehensive weed management strategy that takes into account landscape objec- tives, weeds already present, available tools, and personal gardening philosophy. Table 8.4. Weed seed production and seed survival in soil. Weed species Seeds per plant Seed survival (yrs) Spotted knapweed 1,100 7–10 Lambsquarters 72,450 40 Purslane 52,300 40 Dandelion 15,000 6 Pennsylvania smartweed 19,300 30 Canada thistle (per stem) 680 21 Pigweed 117,400 10 Barnyardgrass 7,160 3 Crabgrass 25,000 3 Note: Seed survival means that some viable seed remains. Generally, however, most seeds germinate or lose viability within three to 10 years or less, depend- ing on soil conditions. A few, however, will hang on to aggravate future gardeners. Figure 8.6. Star-of-Bethlehem (Ornithogalum umbellatum). 8-5 CHAPTER 08 Weed Management Cultural and Mechanical Control Advantages—Less weed seeds are brought to the surface. Leaving untilled areas (either in sod or heavily mulched) also You can manipulate the landscape in ways that reduce reduces the area that needs to be managed for weeds. weed success if you work from the premise that weeds thrive Disadvantages—Most of the tilling and weeding must be in disturbed ground. Cultural and mechanical control meth- done by hand. Some gardeners plant too closely and crops are ods can help you reduce the impact and spread of weeds stunted, much as with weed competition. without herbicides. Several are discussed in this section. Using Transplants Transplants can be one of the best weed management strat- Rotation egies. Vegetables or flowers can be started in a cold frame or Rotating vegetable crops can reduce weed infestations. greenhouse and set out into the freshly worked garden. Noncompetitive crops such as carrots and onions should Advantages—The transplant has a head start against germinat- follow more aggressive vegetables such as winter squash or ing weed seeds and, if it is managed well in the transplanting corn. Fallowing (keeping part of the garden clean-tilled or in process, it should effectively compete against many weeds. a summer cover crop) can help reduce weed problems for the Disadvantages—It takes time to produce transplants or money next season. to buy them, and not all plants transplant well. Solarization is a way to reduce weed- and disease-infested garden areas. In July, thoroughly till and moisten the soil. Stretch clear plastic over the soil as tightly as possible. The heat captured Mulches under the plastic steams the soil, killing many weed roots and Mulching should be part of any landscape planting because seeds. Leave the plastic on through August. The area can be of the weed suppression it provides. When bare soil is cov- planted in the fall or the following spring. ered, many weed seeds either won’t germinate or can’t grow through mulch. Weeding Mulch materials may be organic (such as shredded bark Cultivating soil to control weeds has been practiced for or leaves, pine needles, compost, or newspapers) or synthetic thousands of years. Agricultural yields improved tremendously (such as plastic or landscape fabrics). They can be applied in when cultivating equipment became widely available. Tilling both winter and summer for year-round weed control. Mulch and hand pulling normally are gardeners’ first weapons in the materials may be home generated or purchased, but purchased weed wars. materials can be somewhat expensive. Advantages—A germinating weed is very vulnerable to root Advantages—If used effectively, mulches significantly reduce disturbance from hoeing, hand pulling, or rototilling. It also time spent pulling weeds. Fewer weed seeds are brought to the helps break up soil crusts that may limit vegetable or flower surface than with hoeing or rototilling. Mulching correctly can seedling growth. cool the soil and help manage soil moisture. Plastic mulches can Disadvantages—Mechanical weed removal must be done work well on well-drained and aerated soils and around some early and often to be effective. Cultivating perennial weeds deep-rooted trees and shrubs. can spread reproductive underground portions of the plants, Disadvantages—If organic mulches are applied in a layer of six causing more weeds to grow, and tilling brings new weed seeds or more inches, they may produce acids that can hurt plants. to the surface. Also, excessive cultivation destroys soil tilth and Plastic mulches can create a zone of lifeless soil that often causes compaction. leads to root disease due to their inability to “breathe” around When tilling, take care not to damage your plants’ roots. For shallow-rooted species, such as rhododendrons. Weed seeds example, it generally is unwise to hoe close to shallow-rooted can blow in and germinate in the organic mulch covering gener- rhododendrons and blueberries. ally used over synthetic mulches to improve their appearance, Remember that some plant compounds are toxic to the skin. thus reducing weed control. Plastic and landscape fabrics can You should wear gloves when hand weeding. Also, a sharpened give shelter to mice (voles) or rats. hoe makes for more effective weeding. Choosing the Right Mulch Bed Planting Which mulch will work best depends on your landscape. Planted beds are areas designed with untilled areas between Woody landscapes—Twice-yearly applications of organic them. Soil working is reduced, and mulches are used extensively. mulches such as bark go a long way toward reducing weed Flower or vegetable crops often are planted close together problems. As the material decays, it improves soil tilth and to compete against weeds. Shading is effective in prevent- aeration around landscape plants. Fall applications of mulch ing some weed species from germinating. Transplants are can reduce winter and spring weed problems. used extensively. In general, plastic mulch is not the best choice for woody landscape beds. 8-6  Weed Management CHAPTER 08 Landscape fabrics are better than plastic mulches in a Competitive Plantings of Groundcovers, Herbaceous Perennials woody landscape bed, since water and air can pass through In permanent landscapes, it often is best to eliminate bare them. Laminated fabrics are more expensive, but they are areas under trees and shrubs by planting them with ground- porous, wick water through the fiber, and prevent roots from covers or herbaceous perennials. Many species do well as penetrating. When choosing a fabric, make sure it cannot be understory plants and can provide both foliage and flowers broken or stretched with your finger. Easily perforated fabrics (e.g., hostas, ferns, and daylilies). are not effective. For lawns, more tolerant gardeners now can choose mixes Annual gardens—Plastic mulch does have a place in annual that include certain broadleaves (e.g., yarrow) with good lawn vegetable and flower gardens. It usually is used along with drip character. These mixes tend to repel some of the weedier irrigation and bed planting. Make slits in the plastic, then place broadleaf species. transplants in the slits next to the water source. Add fertilizer Use caution when selecting groundcovers for these areas. to the soil before transplanting or later through the irrigation Some plants, such as creeping euonymus (winter creeper), system. Dispose of the plastic after the growing season. English ivy, and periwinkle (vinca minor), can become Black plastic is preferred for weed control because it reduces weedy escapees. light to the soil, thus preventing weed growth. Clear plastic Advantages—The landscape can be more visually interesting, sometimes is used where weeds are few, since it warms the soil and over time, the need for weeding is reduced. faster than black plastic but does not control weeds. Be sure to Disadvantages—Establishing plants can be costly. Herbaceous stretch the plastic tightly over the soil surface to get the benefits perennial weeds such as Canada thistle, bindweed, and quack- of soil warming as well as weed control. grass can cause problems. Some shallow-rooted plants, such Landscape fabrics can be used in the same way as plastic. as rhododendrons, don’t like aggressive understory plant- Since they allow moisture to pass through, they don’t require ings. Lawns planted in both grass and broadleaf species do a drip system. However, they are thicker than plastic and tend not look exactly like traditional lawns. Also, they may not to cool the soil instead of warming it. This cooling can be a tolerate the herbicides usually recommended to get rid of problem for crops such as tomatoes, peppers, and melons, undesirable plants. which require very warm soil. For most vegetable and flower gardens, organic mulches Cover Crops are the best choice, because they tend to improve the soil as Cover crops are generally grown in winter on annual veg- they break down. In the summer, many gardeners use organic etable and flower beds. Fast-growing crops such as winter grains mulches to conserve moisture and control weeds. It is best to are the most competitive with weeds. The cover crop may be a apply these mulches around heat-loving plants such as peppers winter-hardy grain, a legume, or a combination of both. Cover after the soil is warm. crops are by no means 100 percent successful in weed suppres- Winter organic mulches can be pulled back or tilled in before sion, so there may be some weeds to contend with. planting in the spring. Advantages—Cover crops smother winter weed growth. They Organic mulches can cause nitrogen deficiency as they capture excess fertilizer, improve soil tilth, and when turned decompose, and leaves from a few trees can inhibit growth under, increase organic matter. They are also generally inexpen- of some plants and seeds if they are part of organic mulches. sive. In warm areas, some cover crops can be cut and left on Walnut trees are one example. the surface as a weed-suppressing mulch during the growing season, and some crops, such as tomatoes, can be transplanted Effect of Water Management and Irrigation through the residue. The residue enhances the environment for Weeds need water to germinate and grow. When a garden beneficial soil insects. is irrigated with sprinklers, the entire area is usually watered Disadvantages—Gardeners in most areas have to till in the and must be weeded. When water is directed only to desired cover crop before they can plant a spring/summer garden, plants, such as with drip irrigation, much of the garden stays dry, which can delay planting in a wet spring. Rye grain used as a so that in those drier areas, weed problems are much reduced. cover crop might reduce germination of some small-seeded Drip systems are excellent tools for reducing weed growth. They vegetables such as lettuce. Also, slugs and other insects may are often combined with bed planting and/or plastic mulches prosper if the residue isn’t turned under. in vegetable and annual flower plots to control weed growth. Advantages—They are fairly easy to set up, and they have come down in cost. Chemical Control Disadvantages—Despite improvements and reduced cost, drip Herbicides are another tool for managing weeds. The extent systems still can be costly to install and complicated to maintain. to which you choose to use them depends on your personal In some cases, they might not provide an adequate moisture philosophy, garden objectives, and particular weed problems. zone for desirable plants. Herbicides may have a place in your garden or landscape areas, 8-7 CHAPTER 08 Weed Management but you should always know what the alternatives are and what must enter through green tissue. If this product is applied to makes the most sense given your situation. bare soil, it has no effect on germinating seeds. Also, glyphosate All herbicides have detailed label instructions on mixing, cannot be picked up by roots in most soils. application timing, weeds controlled, plants around which they Soil-active products are absorbed through roots or through can be used, and other significant issues concerning their safe the growing tips of germinating seeds. and effective use. The label is the legal document administered A number of products are both soil-active and foliar-active, by the EPA that defines the use of a product and your responsi- although one absorption route is usually more important. bilities as a consumer. It is crucial that you read the instructions Again, the label will describe how to apply the herbicide. on the label before you purchase a product and follow them as you mix and apply it. Preemergent and postemergent Herbicides control weeds by interfering with critical plant Finally, it is important to know when to use an herbicide functions, thus resulting in the death of plants. Not all herbi- relative to the growth of the weeds and the crop. A preemergent cides act in the same manner. herbicide is applied before weeds sprout. The desirable plants You need to understand some important terms and concepts (landscape trees and shrubs, vegetables, flowers, bulbs, etc.) before purchasing and applying herbicides. may or may not be present. Careful label reading is important! A postemergent product is used after weeds are up and Selective and Nonselective actively growing. Selectivity of postemergent herbicides may A selective herbicide controls certain plants and not others. depend on crop age, application rate, or other factors. It’s impor- For example, many lawn herbicides control broadleaf plants, tant to understand application timing as it relates to both weed such as dandelions, without damaging grasses. A few products and crop growth. Again, read and follow the label. control some (though not all) weedy grasses without harming turf. Other selective herbicides affect germinating seeds and Herbicide interactions sprouting herbaceous plants but not established woody trees Herbicide results are influenced by a number of factors. A and shrubs. A handful of products control grasses without plant with a waxy leaf (such as poison ivy) or a hairy leaf (such damaging broadleaf species. as stinging nettle) may not absorb an herbicide as well as a plant If an herbicide is selective, the label will provide extensive with a smooth leaf. Likewise, a plant with narrow, upright leaves information about plants around which it can safely be used may be hard to cover adequately with a spray mixture, or a and weeds and plants it is likely to affect. In addition, there will weed’s growing point may be protected under the soil surface. be instructions on when to use the herbicide to obtain desired Environmental conditions can affect an herbicide, rendering results and avoid problems. it useless against the target plant or causing unintended dam- Nonselective herbicides can potentially damage any type of age to nontarget plants. In general, as temperature increases, plant. Some last a long time, having a residual effect; others do herbicides work more effectively. not. The label details how the herbicide acts. Some herbicides can become volatile (gaseous) at certain It is important to remember that some products may be temperatures. For example, some formulations of 2,4-D and selective at certain rates but lose that selectivity as the amount some of its related chemistries can volatize off leaves shortly of herbicide applied increases. after application if temperatures climb into the mid-80s. Since small amounts of this herbicide can visibly distort sensitive Systemic and Nonsystemic plants, high temperatures and high humidity combined with a Most herbicides are systemic, which means they circulate little wind can cause serious problems in your own or neighbor- from the point of initial absorption to other parts of the plant to ing landscape areas and surrounding gardens. have their effect. They may be sprayed on the leaves and move Wind drift by itself can be a problem when using any herbi- to the roots or vice versa. cide. As you pump a sprayer, the pressure increases, which in Nonsystemic or contact herbicides affect only the part of turn creates a smaller spray droplet. The smaller the droplet, the plant they touch. When a contact product is sprayed on the more likely it is to become airborne and move away from leaves, it generally kills those leaves but does not travel to the where you want it to land. Again, this drift can cause unintended root system. They may control young annual or biennial weeds, consequences and problems for neighborhood landscapes. but while they may cause established perennial weeds to lose Moisture in modest quantities is needed to move soil-applied leaves, the weeds will resprout. Very few contact products are herbicides such as pendimethalin into the soil. However, if available for home landscapes or gardens. a downpour occurs shortly after a soil-applied herbicide is applied, the chemical may wash downslope instead of enter- Foliar-active and soil-active ing the soil. This runoff can damage lawns or other plants in Herbicides also are classified by the way they move into its path. When you use foliar chemicals, there must be enough plants. Many common products are foliar-active, meaning they time between application and rain to allow the plant to absorb enter through leaves. For example, glyphosate (e.g., Roundup) the herbicide. Eight hours of dry weather is normally enough. 8-8  Weed Management CHAPTER 08 While eight hours is ideal, significant plant damage can occur Oryzalin and pendimethalin—These preemergent herbicides in considerably less time. There are many stories of misapplying act on germinating seeds. They are most effective on annuals. herbicides and then trying to wash them off, only to find that It is very important to read the labels for oryzalin and pen- the plant was already damaged. dimethalin carefully. These products differ in their need for Sometimes, herbicides work poorly in dry weather. Drought- mixing into the soil, the weeds they control (some, but not all, stressed plants conserve water by reducing transpiration grasses and broadleaves), and the plants around which they through leaves. In this condition, they are less able to absorb can be used. herbicides. Several systemic products have specific statements Pendimethalin is sold as Pendulum, Pre-M, and several other on the label about their reduced effectiveness when plants are names. Oryzalin is marketed as Surflan and Weed Preventer. drought stressed. Plants sensitive to these herbicides cannot be planted for several months after use. Specific herbicides Fluazifop—Fluazifop controls many grass species but does not Always read the label to determine whether or not the harm most broadleaf plants. It is applied after grasses emerge application site is appropriate for the product considered for and are rapidly growing. It takes seven to 14 days or more to use. Many of the herbicides listed below are not labeled for use see results. It is sold as Grass-B-Gon and other brand names. in vegetable gardens. Sethoxydim—Sethoxydim controls grasses and does not harm Note: All herbicides listed may not be available in your area most broadleaf plants. It is applied after grass has emerged and or suitable for your situation. Consult with your local Extension is growing rapidly. The most common trade name is Poast. agent for current herbicides recommended for use in Kentucky. 2,4-D—This systemic, foliar, postemergent herbicide affects broadleaves, especially herbaceous annuals, biennials, and Weed Management for Specific Situations perennials. It is a common component in lawn products, since Your particular landscape or situation determines how you it doesn’t damage established grass. It is sold under numerous manage weeds. trade names and is often combined with other closely related chemicals—mecoprop (MCPP), dichlorprop (2,4-DP) and/or dicamba—to broaden the spectrum of weed control. Lawns Glyphosate—Glyphosate is a systemic, nonselective, foliage- Choose grass species suited for your area. A vigorous lawn applied herbicide. It potentially affects any plant with which it reduces weed invasion. Good fertility, watering, and mowing comes in contact. Glyphosate is absorbed through leaves, green will keep your turf in top condition. Plan an annual program bark (usually a very young tree or shrub), or freshly cut stems. of overseeding weak areas, especially shade areas. Work to The chemistry of glyphosate is such that it becomes tied up on reduce insect damage to your lawn; weakened areas offer little soil particles. Little chance exists that glyphosate can be picked competition to encroaching weeds. up by roots unless they are somehow exposed to it. Lawn blends that contain grass-cover combinations may Glyphosate tends to follow the flow of sugars in plants. If provide a stable plant community that resists invasion by more it is applied to an actively growing woody plant, the chemical weedy species. These combination lawns do not have the feel of tends to move to the new leaves (where the sugars are headed) pure grass, however, and some people find them unacceptable. and stunt the terminal growth. In that circumstance, it may not Other homeowners swear by them. move to the root system in sufficient quantities, and the plant If the grass you plant can tolerate a mowing height of 1¾ survives. Thus, it is best used on perennial plants as they begin inches or more, the shade cast by the grass will inhibit germina- flowering or in the fall as they start storing sugars for winter. tion of many broadleaf weeds. Use a fertilizer low in phosphorus Glyphosate can be used on some grasses almost any time to avoid stimulating clover (assuming you don’t want clover in they are green and on annuals when they are actively growing. your lawn). Some broadleaf weeds can be managed by hand It works faster (in seven to 10 days) in warm weather. It also can pulling, but smaller species may be hard to control in this way. be effective in colder weather, but results might not be evident Weedy perennial grasses such as quackgrass, bermudag- for a month or more. It is sold under several brand names. rass, and nimblewill can be very invasive. Once these plants Triclopyr—This product is systemic and predominantly foli- establish, you have few options short of complete renovation. age absorbed. In some cases, it can be absorbed through bark. Some gardeners spot-spray the patches with glyphosate and Triclopyr is active against broadleaf plants, especially woody then overseed them. species. It has no effect on established grass. It often is used on Summer annual grasses such as crabgrass are generally part blackberries and poison ivy. It is sold alone or sometimes mixed of most well-maintained lawns, and can be more prominent in with 2,4-D for control of brush and other harder-to-control closely mowed turfgrasses. broadleaf weeds. Read the label carefully to see whether the Usually, lawns are weakest in shady areas and where drain- formulation is an ester or an amine. Avoid applying ester for- age is a problem. Annually overseed lawns in those areas to mulations in hot weather to reduce potential volatility and drift. maintain a viable turf. Moss often grows where grass is weak. 8-9 CHAPTER 08 Weed Management Many homeowners use broadleaf herbicides, either alone Complex landscapes that cover most of the ground generally or in combination with fertilizer (“weed and feed”). To reduce have fewer weed problems as the plants become established— the amount of herbicide used, it generally is much better to which can be a great reason to buy more plants! It does help to spot-spray weedy areas rather than apply herbicides over the have a plan and know which species will work best. In some entire lawn each time you fertilize. cases, competitive plantings may limit your herbicide choices. Herbicide-treated lawn clippings should be used cautiously The herbicides most commonly used in woody landscapes as mulch or in compost. Unintended damage to nontarget are isoxaben, pendimethalin, oryzalin, trifluralin, and spot plants can occur. For example, clopyralid-containing products application of glyphosate. Be sure you understand how these can persist for more than a year. products work to avoid damaging desirable plants. If young children or pets use the lawn, be cautious with herbicides! Annual Flower and Vegetable Gardens Annual vegetable and flower gardens can be weed night- Renovation mares. Working the garden in the spring offers an opening for Lawn renovation often is done to reestablish turfgrasses weed seeds. Their aggressive growth can quickly dominate where weedy grass species have taken over. The weedy spe- a garden. cies must be killed (especially perennials) before a new lawn Weeds are best managed in flower and vegetable gardens by is planted. a combination of hoeing, hand pulling, use of vigorous plants A single rototilling generally spreads rather than controls that shade the ground as they mature (including extensive use problem grasses, but repeated tillage over three to four weeks of transplants), drip irrigation, mulches, and relentless atten- can give acceptable control in dry weather. tion that keeps weeds from going to seed. Winter weeds can Some homeowners use glyphosate to kill an existing weedy be managed by mulches or cover crops. lawn, and then they dethatch and overseed. Repeat applica- In general, herbicides are not a good option for home veg- tions of glyphosate are sometimes necessary before seeding. etable gardening due to the complex of different plants that They don’t use rototilling unless the lawn needs to be reshaped. are grown. With good temperatures and water, a new lawn can be up and Some gardeners use glyphosate before the first spring culti- growing in three to four weeks. vation to control persistent perennial weeds, especially quack- grass. Trifluralin (Preen or Treflan) is labeled for use around Woody Landscape Areas some (but not all!) flowers and vegetables. Oryzalin has some home-garden labels for flowers. Read the labels carefully and Weeds in landscape beds can be managed with a mix of follow instructions if you use herbicide products. techniques that include mulching, water placement, competi- tive planting, hand pulling, and herbicides applied as both spot and broadcast treatments. For More Information Mulches should be your first line of defense against weeds. They reduce the germination of weed seeds and protect the University of Kentucky Cooperative Extension Service publications: soil. Organic mulches such as bark probably are the most effec- Weeds of Kentucky Turf (AGR-12) tive and cause the least problems. Hoeing in mulch is easy and Weed Control for Kentucky Home Lawns (AGR-208) disrupts most annual weeds. Landscape fabrics may be used Other resources: around annual and perennial flowers but should not be used around woody plants, where they often encourage surface root McCarty.LB, et.al. Color Atlas of Turfgrass Weeds. Ann Arbor growth. Perennial weeds generally are not deterred by organic Press. 1991. mulches and may defeat landscape fabrics as well. Haragan, Patricia Dalton. Weeds of Kentucky and Adjacent Drip irrigation puts water around desirable plants but doesn’t States: A Field Guide. University Press of Kentucky, 1991. water everything, and the dry areas will have much less potential for weed growth. When bare areas are planted with robust plants, weeds struggle to compete. Revised 01-2024