Chapter 4 - Propagation Environments.pdf

Propagation Environments Douglass F. Jacobs, Thomas D. Landis, and Tara Luna 4 An understanding of all factors influencing plant growth in a nursery environ- ment is needed for the su...

Propagation Environments Douglass F. Jacobs, Thomas D. Landis, and Tara Luna 4 An understanding of all factors influencing plant growth in a nursery environ- ment is needed for the successful growth and production of high-quality con- tainer plants. Propagation structures modify the atmospheric conditions of temperature, light, and relative humidity. Native plant nurseries are different from typical horticultural nurseries because plants must be conditioned for outplanting on stressful sites where little or no aftercare is provided. This set of circumstances makes conditioning and hardening (see Chapter 12, Harden- ing) especially important, and these horticultural treatments require changing or modifying propagation structures. Two essential processes in plants are photosynthesis and transpiration. Pho- tosynthesis is the process in which light energy from the sun is converted into chemical energy in the presence of chlorophyll, the green pigment in leaves. During photosynthesis, sugars A propagation environment is any area that are produced from carbon dioxide has been modified to grow plants. It may or from the air and water from the soil while oxygen is released back into the may not involve a structure like a greenhouse. air (figure 4.1). Photosynthesis is a leaky process because, to allow the intake of carbon dioxide, water vapor is lost through pores, or stomata, on the leaf surfaces. This process is called transpira- tion. To maximize the photosynthesis necessary for plant growth, growers must manage any limiting atmospheric factors in the propagation environment. Greenhouse operated by the Confederated Salish and Kootenai Tribes in Montana by Tara Luna. 55  LIMITING FACTORS Managing all the various factors that can be limiting to plant growth is the key to successful nursery manage- ment. To do this, the best possible propagation environ- ment must be designed for a specific nursery site (Landis 1994). It is helpful to separate these limiting fac- tors of the environment into those in the atmosphere and those in the growing medium (figure 4.2). Atmospheric Atmospheric limiting factors include light, tempera- ture, humidity, carbon dioxide, and organisms that are determined by the climate at the nursery if plants are grown outside. As discussed in this chapter, propagation structures can be built to modify the local climate so that plants will grow more rapidly. For example, a green- house will modify light, temperature, and wind com- pared to the outside environment, which affects not only temperature but also humidity and carbon dioxide levels inside the greenhouse. The greenhouse also affects the organisms that interact with the crop. For example, although a greenhouse structure can exclude insect pests, it also creates a more humid environment Figure 4.1—Two important processes occur in the leaves of green plants. In photosynthe- sis, sunlight triggers a chemical reaction in which water from the soil and carbon dioxide for new pests such as algae and moss. In this chapter we from the air are converted to sugars and oxygen,which are released back to the atmosphere. discuss modifying light, temperature, carbon dioxide, During the process, water vapor is lost from the leaves in a process known as transpiration. and humidity with propagation structures. Illustration by Jim Marin. Growing Medium Growing medium limiting factors include water and mineral nutrients. The type of propagation environment can certainly affect water use; the details are discussed in Chapter 10, Water Quality and Irrigation. Mineral nutri- ents are supplied through fertilization (see Chapter 11, Fertilization); both water and mineral nutrients are held for plant uptake in the growing medium (see Chapter 5, Growing Media). Biotic Organisms can be limiting in either the atmosphere or the growing medium. Animal pests, including insects, can be excluded from a nursery through prop- Figure 4.2—It is useful to think of the nursery environment in terms of factors that might er design, and beneficial microbes, such as mycorrhizal be limiting to plant growth. Limiting factors in the soil include water and mineral nutrients fungi, can be promoted. Beneficial organisms are cov- whereas, temperature, light, carbon dioxide, and humidity can be limiting factors in the at- mosphere. Other organisms can either be beneficial or detrimental in both places. Illustration ered in Chapter 14, Beneficial Microorganisms, and pests by Jim Marin. are discussed in Chapter 15, Holistic Pest Management. 56 PROPAGATION ENVIRONMENTS  Matching Propagation Environments to the Site lishing young seedlings in their containers. Green- Whether building a new nursery or modifying an ex- houses are expensive to operate, however, because of isting one, it is critical to analyze the limiting factors high energy requirements. After young seedlings are on the site. For example, the Hopi Reservation in north- established, they could be moved to a shadehouse or eastern Arizona is at high elevation with sunny, cloud- open compound to continue their growth. During hard- less days for most of the year. Winters can be very cold ening, the crops must be acclimated to the ambient with snow, and high winds are very common year- environment and this is usually done in the same round. Here, the most limiting site factors are the in- shadehouse or open compound. In cold climates, the tense sunlight, freezing temperatures, and high winds; crops might need to be moved to an overwintering therefore, a strong greenhouse to withstand snow and structure to protect their root systems from freezing wind loads, with an aluminized shadecloth to mini- temperatures and the shoots from winter desiccation. mize heat buildup, is desired. Many potential problems can be averted by careful A completely different propagation structure would nursery design and planning. be required on the Yurok Reservation in the coastal rainforest of the northern California coast, where fog is common and it rarely gets below freezing. Here, the A good nursery manager knows how to limiting factors are low sunlight with heavy winter rains, so a structure with a very clear covering to max- “think like a plant,”and create a propagation imize light transmission while protecting the crop environment that modifies all physical and from heavy rains would be ideal. As expected, the costs of nursery development biological factors that may be limiting to increase with greater control of the propagation envi- ronment. A nursery that is well matched to its environ- plant growth. ment, however, will be much less expensive to operate than a poorly designed one. T YPES OF PROPAGATION ENVIRONMENTS When most people think of container nurseries, they The Challenge of Growing Many Species and Stock Types think of greenhouses; however, many other propaga- Native plant nurseries differ from other types of tion environments are available. For our purposes, we horticulture in which high quantities of a few crops are define “propagation environment” as any area that is grown in large greenhouses. Most tribal nurseries grow modified to encourage the growth of nursery stock. small numbers of a wide variety of plants in one loca- This environment could be as simple as a shady area tion. Often, these crops must be started on various under a tree or it could be a greenhouse with full envi- dates, so, at any one time, a nursery might have every- ronmental controls. It is important to realize that you thing from germinating seeds to large plants. Although do not need a greenhouse to grow native plants. Many some species are grown from seeds, others in the same simpler and inexpensive propagation structures can be nursery might have to be grown from rooted cuttings. designed to create the type of growing environments So, a good native plant nursery will have to be designed that crops require. Understanding different types of with many relatively small propagation environments propagation structures and how they work is critical in which plants of similar requirements can be grown. whether designing a new nursery facility or modifying When starting a new nursery, designing a variety of an existing one. propagation environments is a luxury; unfortunately, Container nursery facilities can be distinguished by most existing nurseries have to modify existing propa- their relative amount of environmental control: mini- gation structures. mally controlled, semicontrolled, or fully controlled. At a given nursery, different propagation environ- These facilities differ not only in their complexity but ments are needed for different growth stages at differ- also in their biological and economical aspects (table 4.1). ent times of the year. For example, a greenhouse is an ideal environment for germinating seeds and estab- DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 57  Minimally Controlled Environments A minimally controlled environment is the simplest and least expensive of all types of propagation envi- ronments. The most common type is an open growing compound. It consists of an area where plants are ex- posed to full sunlight and usually nothing more than an irrigation system and a surrounding fence. Open Growing Compounds Nurseries use open compounds as hardening areas to expose crops grown inside structures to ambient conditions. Some nurseries, such as the Temecula Band of the Luiseno Indians in southern California, use an open compound that incorporates trees for natural shade (figure 4.3A). Plants can be grown directly on the ground using landscape fabric to control weeds over a A layer of gravel to provide drainage. The Banksavers Nursery of the Stillaguamish Tribe in coastal Washing- ton State grows a variety of riparian and wetland plants in an open compound (figure 4.3B). Some nurs- eries prefer to grow their stock on pallets or benches to improve air pruning of the roots. If the nursery soil is heavy and poorly drained, then drainage tiles should be installed. Irrigation is provided by way of sprinklers for smaller containers or driplines for larger ones; plants obtain nutrients from controlled-release fertilizers that are incorporated into the growing media. The com- pound should be fenced to minimize animal damage, B and, in windy areas, a shelterbelt of trees can improve the coverage of the irrigation system. Although open compounds are an inexpensive way to grow plants, they have the highest risk of freezing injury. Frost protection with irrigation is possible, however, the excess water can cause serious disease problems. For this reason, open growing compounds are more popular in milder cli- mates; for example, in Louisiana, where the Clifton- Choctaw Nursery grows longleaf pine (figure 4.3C). Wetland Ponds Artificial ponds are another type of minimally con- C trolled environment. They are used for growing ripari- an and wetland plants and are especially good for propagating sedges and rushes. Wetland ponds can be Figure 4.3—(A) The simplest nurseries are open compounds that use natural shade but have irrigation and are fenced. (B) Open compounds, like this one used by Roy Tyler of the aboveground tanks, such as wading pools or cattle Clifton-Choctaw Tribe in Louisiana, are most appropriate in areas with milder climates, troughs, or they can be constructed with pond liners where the risk of cold injury is minimal. (C) Even in colder climates, open compounds are either in an excavated area or at ground level using a often used for hardening crops grown in greenhouses or other structures. Photo A by Tara Luna, B by Charles Mathern,and C by Thomas D.Landis. raised perimeter (figure 4.4A). These simple propaga- 58 PROPAGATION ENVIRONMENTS Table 4.1—Operational considerations for selecting a propagation environment Factors Minimally Controlled Semi Controlled Fully Controlled BIOLOGIC AL Ambient climate Mild Moderate Any Growing season Summer Spring to fall Year-round Cropping time 6 to 24 months 3 to12 months 3 to 9 months Risk of crop loss High Low Low ECONOMIC Construction costs Low Medium High Maintenance costs Low Medium High Energy use Low Low to medium High tion environments use growing media amended with controlled-release fertilizer and require only periodic flood irrigation. For example, the Shoshone-Bannock Tribes in southeastern Idaho grow a variety of wetland and riparian plants in their nursery (figure 4.4B). Semicontrolled Environments This next category of propagation environments is called “semicontrolled” because only a few of the limit- ing factors in the ambient environment are modified. Semicontrolled environments consist of a wide variety of growing structures ranging from simple cold frames A to shadehouses. Cold Frames Cold frames are low-to-the-ground structures consist- ing of a wood or metal frame with a transparent covering (figure 4.5). As their name suggests, they have no heating source except the sun. Cold frames are the most inex- pensive propagation structure and are easy to build and maintain. Because temperatures inside can rise quickly, cold frames can be used to extend the growing season in spring. Seeds can be germinated and cuttings can be rooted weeks before they could be in an open compound. Cold frames are also used in late summer or autumn for hardening plants moved out from greenhouses and can B be used for overwintering crops. Cold frames are labor intensive, however, because they need to be opened and closed daily to control temperature and humidity levels Figure 4.4—(A) Wetland ponds can be constructed in the outdoor nursery for growing wetland species or (B) by using plastic tubs inside a greenhouse, such as this one operated (figure 4.5). by the Shoshone-Bannock Tribes of Idaho. Photo A by Thomas D.Landis,B by J.Chris Hoag. The ideal location for a cold frame is a southern exposure with a slight slope to ensure good drainage and maximum solar absorption. A sheltered spot against the wall of a building or the greenhouse pro- DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 59  A B Figure 4.5—Cold frames are an inex- pensive alternative to a greenhouse. (A) Cold frames should be placed in a shel- tered location for additional protection. (B) Coverings may be removed or (C) held open to manage humidity and heat lev- els. Photos by Tara Luna. C 60 PROPAGATION ENVIRONMENTS vides additional protection. Some nurseries sink the Cold frames require careful management of tempera- frame 6 to 12 in (15 to 30 cm) into the ground to use the ture and humidity levels. A thermometer that can be earth for insulation. Other nurseries make their cold conveniently read without opening the cover is manda- frames lightweight enough to be portable so they can tory. In a cold frame, cool-season plants grow best at 55 move them from one section of the nursery to another. to 65 °F (13 to 18 °C), while warm-season plants grow well It is relatively easy to build a cold frame. Frames are at 65 to 75 °F (18 to 24 °C). If air temperature goes above usually made of wood such as redwood or cedar that 85 °F (29 °C), the top must be opened to allow ventilation. will resist decay; the new recycled plastic lumber also Monitor the temperature to ensure that the cold frame works well. Never use creosote-treated wood or wood does not cool down too much, but close it early enough treated with pentachlorophenol because these sub- before the solar heat is lost. stances are toxic to plants. The cold frame should be built so that it is weathertight and the top lid should be Hot Frames or Hot Beds constructed so that it can be propped open at different Structurally similar to cold frames, hot frames are heights to allow for various levels of ventilation, water- heated with electric heating cables and are primarily ing, and the easy removal of plants. The cover must be used as an inexpensive way to root cuttings. They are able, however, to be attached securely to the frame to also ideal for overwintering nonhardy seedlings or resist wind gusts. Old storm windows make excellent newly rooted cuttings. Cold frames can easily be con- covers for cold frames. Heavy plastic film is an inex- verted into hot beds. Start by removing the soil to a pensive covering but usually lasts only a single season. depth of 8 or 9 in (20 cm). Lay thermostatically con- Hard plastic or polycarbonate panels are more durable trolled heating cables horizontally in loops on top of 2 and will last for several years. Cold frame kits may also in (5 cm) of sand (figure 4.6). Be sure that the cable be purchased and are easily assembled; some kits even loops are evenly spaced and do not cross each other. contain automatic ventilation equipment. Cover the cable with 2 in (5 cm) of sand and cover the Figure 4.6—A hot bed is structurally similar to a cold frame but is heated by electric heating cables. Illustration courtesy of John W.Bartok,Jr. DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 61  Shadehouses Shadehouses are the most permanent of semicon- trolled propagation environments and serve several uses. Shadehouses are used for hardening plants that have just been removed from the greenhouse and also serve as overwintering structures. In the Southwestern United States, many tribal nurseries have shadehouses similar to that of the White Mountain Apache Tribe in Arizona (figure 4.8A). Shadehouses, however, are also being used to propagate plants, especially in locations where sunlight is intense; for example, at the Colorado River Indian Tribes Nursery in Arizona (figure 4.8B). One popular shadehouse design consists of upright poles with a framed roof; snow fencing is installed over Figure 4.7—Hoop houses, like this one at the Colorado River Indian Tribes Nursery in Ari- zona, can be used for a variety of propagation environments by changing or removing the the roof and sides to provide about 50 percent shade coverings. Photo by Tara Luna. (figure 4.8C). Other shadehouses consist of a metal pipe frame covered with shadecloth. When used for sand with a piece of wire mesh (hardware cloth). Trays growing, they are equipped with sprinkler irrigation of cuttings or seedlings can be placed directly on top of and fertilizer injectors (figure 4.8D). When the shade is the wire mesh. installed on the sides of the structure, shadehouses are very effective at protecting crops from wind and so Hoop Houses and Polyethylene Tunnels reduce transpiration. In areas with heavy snow, remov- Hoop houses and polyethylene (“poly”) tunnels are able shadecloths are used for the roofing and sides so very versatile, inexpensive propagation environments. that they can be removed to avoid damageduring win- The semicircular frames of polyvinyl chloride (PVC) or ter months. This can be done after plants are put into metal pipe are covered with a single layer of heavy poly storage. and are typically quite long (figure 4.7). The end walls are made of solid material such as water-resistant ply- Fully Controlled Environments wood. The cover on hoop houses is changed during the Fully controlled environments are propagation growing season to provide a different growing environ- structures in which all or most of the limiting environ- ment, eliminating the need to move the crop from one mental factors are controlled. Examples include structure to another. Generally, in early spring, a clear growth chambers and greenhouses. Fully controlled plastic cover is used during seed germination and environments are often used because they have the seedling establishment. As the days become longer advantage of year-round production in almost any cli- and warmer, the plastic cover can be pulled back on mate. In addition, most crops can be grown much sunny days to provide ventilation. After the danger of faster than in other types of nurseries. These benefits frost is past, the plastic cover is removed and replaced must be weighed against the higher costs of construc- with shadecloth. Sometimes, a series of shadecloths, tion and operation. Murphy’s law of “Anything that can each with a lesser amount of shade, are used to gradu- go wrong will go wrong” certainly applies to nurseries, ally expose crops to full sun. During hardening, the so the more complicated a structure is, the more prob- shadecloth is completely removed to expose the plants lems that can develop. This concept is particularly true to the ambient environment. When covered with white in the remote locations of many tribal lands, where plastic sheeting, hoop houses can be used for overwin- electrical power outages are more common and it is tering. See Chapter 13, Harvesting, Storing, and Shipping, difficult, time consuming, and expensive to obtain spe- for further discussion on this topic. cialized repair services (see table 4.1). The following is a brief description of common greenhouse designs. Some good references that provide much more detail 62 PROPAGATION ENVIRONMENTS  A B C D Figure 4.8—(A) Shadehouses are semicontrolled environments that are used for hardening and overwintering plants and (B) are also used for propagation. (C) They are constructed of wood frames with snow fencing or (D) metal frames with shadecloth and are equipped with irrigation systems that can also apply liquid fertilizer. Photos by Thomas D.Landis. include Aldrich and Bartok (1989), Landis and others Tribes in Montana, use greenhouses. In addition to (1994), and Bartok (2000). growing native trees, tribes have grown many other kinds of native plants in different types of green- Growth Chambers houses, but all the greenhouses are transparent struc- Growth chambers are high-cost options that are tures that allow natural sunlight to be converted into used almost exclusively for research. The high cost of heat (figure 4.9A). On the other hand, greenhouses are the equipment and the expense to operate them make poorly insulated and require specialized heating and them unsuitable for normal nursery production. cooling systems. Most people think that keeping a greenhouse warm during cold weather is the most crit- Greenhouses ical consideration, but it is not that difficult to accom- Tribes with large forestry and reforestation pro- plish with modern heating systems. Actually, keeping a grams, such as the Mescalero Apache Tribe in New greenhouse cool during sunny days in spring and sum- Mexico, the Red Lake Band of the Chippewa Indians in mer is more of a challenge, and so ventilation systems Minnesota, and the Confederated Salish and Kootenai must be carefully engineered. In climates with periods DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 63  A B C Figure 4.9—(A) Greenhouses, like this one on the Hopi Reservation in Arizona, are the most sophisticated propagation environments. (B) Retractable roof greenhouses allow crops to be exposed to the outside environment during hardening.(C) Workers with special- ized skills are needed from the initial surveying to (D) the final construction. Photo A by Thomas D.Landis,B and C by Tara Luna,D by Joseph F.Myers. D 64 PROPAGATION ENVIRONMENTS of clear winter weather, greenhouses can heat up con- If wheeled equipment will be used to move plants, siderably during the day so, for this reason, it is usually concrete walkways between the benches are necessary. not a good idea to use them for overwintering plants. Note that black asphalt heats up rapidly and becomes Retractable-roof greenhouses, the newest type of soft, so concrete is a better but more expensive option. propagation structure, have become popular in tem- Full concrete floors will eliminate many pest problems, perate regions. Their major advantage is that the roof especially algae, moss, and liverworts that thrive in the can be opened to expose plants to the outside environ- humid nursery environment. Make sure that floors are ment when cooling is required (figure 4.9B). They are engineered to drain freely to prevent standing water, particularly useful during the hardening phase because which is a safety hazard. Full floors can be engineered the crop does not have to be moved to another struc- with drains so that all water and fertilizer runoff can be ture such as a shadehouse. Some nursery managers contained on site; runoff containment is a legal require- without retractable-roof greenhouses remove the plas- ment in some parts of the country. tic from their greenhouses to help harden their crops. Framing Mater ials. Ideal framing supports the cov- ering with minimal shading and heat loss while allow- ENGINEERING CONSIDERATIONS ing ease of access and handling. Framing materials It is important to understand that greenhouse con- include galvanized steel, aluminum (lightweight but struction is a very specialized business. Even a licensed expensive), and treated wood. contractor who is skilled at general construction will Greenhouse Kits. The heating and cooling systems be challenged by the specialized demands of building a of fully controlled greenhouses must be carefully engi- greenhouse (figures 4.9C and D). Here are a few gener- neered to match both the size of the structure and the al terms that anyone building a greenhouse or shade- ambient environment. Be aware that inexpensive house should be familiar with. greenhouse kits often have vents or fans that are too Design Loads. Dead loads are the weight of the struc- small for the size of the greenhouse. Kit greenhouses ture, whereas live loads are caused by building use. Live were designed for some “average” environment and loads include people working on the structure and the will probably have to be modified to handle the limit- weight of equipment, such as irrigation systems, ing environmental factors on your site. Before purchas- heaters, lighting systems, and even hanging plants. ing a greenhouse kit, it is a good idea to hire an Weather-related loads (snow and wind) must also be experienced consultant, speak with a knowledgeable taken into consideration. In developed areas, be aware company representative, and discuss designs with that greenhouses may be regulated by municipal build- other growers or professionals. ing codes and zoning; this is another good reason to work with a professional firm before buying a green- GREENHOUSE COVERINGS house. A wide variety of greenhouse coverings are available Foundations, Floors, and Drainage. The foundation and the selection of a particular type is usually based connects the greenhouse to the ground and counteracts on cost, type of structure, and the environmental con- the design load forces. Inexpensive floors can consist of ditions at the nursery site. gravel covered with ground cloth, but the ground Poly tarps are relatively cheap but require replace- beneath the floor must drain water freely. Nursery crops ment every 2 to 4 years depending on the grade of plas- require frequent irrigation and in most systems much of tic. Double layers of poly sheeting that are inflated with this water will end up on the floor. Drain tiles might be a fan are stronger and provide better insulation longer needed to make sure that the nursery floor will not turn than a single layer (figure 4.10A). The two layers are into a bog. It may be necessary to design the greenhouse attached to the framing with wooden furring strips or so that all wastewater drains into a pond or constructed specially designed fasteners. This process is relatively wetland to prevent contamination of water sources.This simple so many growers change their own coverings. water can sometimes be used for other purposes on the Because they are so well insulated and airtight, poly site, such as growing plants in wetland ponds as greenhouses require good ventilation to prevent con- described previously in this chapter. densation. DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 65  A B Figure 4.10—(A) Greenhouses are covered with transparent coverings such as plastic sheeting or (B) hard plastic panels to maximize the amount of sunlight reaching the crop. Photo A by Tara Luna,B by Thomas D.Landis. Polycarbonate (“polycarb”) panels, the most popular midity, keep the area warm. If the chambers are out- permanent greenhouse covering, have about 90 per- side, the covering further protects cuttings from drying cent of the light transmission properties of glass. Poly- winds and rain. Usually, bottom-heating cables are carb is strong and durable but is one of the more placed below the flats of cuttings or rooting medium to expensive coverings (figure 4.10B). warm the medium, which speeds root development. These are the most common greenhouse coverings, Experience has shown that cuttings of many native and a more detailed description with costs, engineer- plants root better when the growing medium is kept ing and operational considerations can be found in warmer than the shoots. See Chapter 9, Vegetative Prop- Landis and others (1994) Some good references that agation, for more information on rooting cuttings. provide much more detail on environmental controls The two most common rooting chambers in native include Aldrich and Bartok (1989), Landis and others plant nurseries are enclosed chambers without irriga- (1994), and Bartok (2000). tion and chambers with intermittent-mist systems. SPECIALIZED PROPAGATION ENVIRONMENTS Enclosed Rooting Chambers This section discusses smaller propagation environ- Because it is easy to construct and very economical, ments that have very specific functions. Often, they are a simple enclosed rooting chamber is essentially the constructed inside greenhouses or shadehouses. same as the hot frame discussed earlier. Because they rely on manual operation, enclosed systems require Rooting Chambers diligent daily inspection to regulate humidity and air The most common type of vegetative propagation is temperatures, and the rooting medium must be the rooting of cuttings. Often, this form of propagation watered as needed. They are typically covered with requires a specialized environment known as a rooting shadecloth to moderate temperatures, but, if heat or chamber that creates specific conditions to stimulate humidity becomes excessive, enclosed chambers need root initiation and development. Because cuttings do to be opened for ventilation. One design is known as a not have a root system (figure 4.11A), rooting chambers “poly propagator” because it is covered with plastic must provide frequent misting to maintain high humid- sheeting and is a simple and inexpensive way to root ity to minimize transpiration. Root formation is stimu- cuttings (figure 4.11B). lated by warm temperatures and moderate light levels; these conditions maintain a high level of photosynthe- Intermittent-Mist Rooting Systems sis.Therefore, many rooting chambers are enclosed with These propagation environments are the most com- poly coverings that, in addition to maintaining high hu- mon way to root cuttings and are either enclosed (figure 66 PROPAGATION ENVIRONMENTS 4.12A) or open (figure 4.12B). Rooting cuttings is much easier in these environments because intermittent-mist rooting chambers have a high degree of environmental control (figure 4.12E). Clock timers (figure 4.12D) con- trol the timing and duration of mistings from special- ized nozzles (figure 4.12E). These frequent mistings maintain very high humidity that reduces water loss from the cuttings, while evaporation of the small droplets moderates air and leaf temperature (figure 4.12F). Bottom heat is supplied to the rooting zone of the cuttings by means of insulated electrical cables at the bottom of the rooting medium (figure 4.12G) or A with specially designed rubber heating mats placed under the rooting trays. Mist systems require high water pressure that is supplied through PVC pipes and delivered through special nozzles (figure 4.12E). Very practical timing of the mistings is controlled by a series of two timeclocks that open and close a magnetic solenoid valve in the water line to the nozzles. One clock turns the system on during the day and off at night and the other con- trols the timing and duration of the mists. Because the aperture of the mist nozzles is so small, a cartridge fil- ter should be installed in the water line after a gate valve. A thermostat controls the temperature of the heating cables or mat, which are protected by a wire mesh (figure 4.12C). Because of the proximity of water and electricity, all B employees should receive safety training. All wiring used for mist propagation must be grounded and must adhere to local building codes. Electrical outlets and Figure 4.11—(A) Rooting cutting requires a specialized propagation environment be- cause cuttings lack a root system.(B) The “poly propagator” is the most simple and inexpen- components must be enclosed in waterproof outlets. sive rooting chamber; this one being used by Shannon Wetzel is at the Salish Kootenai The high humidity encourages the growth of algae and College Nursery in Montana. Illustration by Steve Morrison,photo by Tara Luna. mosses, so the mist propagation system should be cleaned regularly. Mist systems require water low in dissolved salts; “hard” water can result in whitish the most critical, in which growers control their crops deposits that can plug mist nozzles. See Chapter 10, is by the type of container. Container volume, plant Water Quality and Irrigation, for more information on spacing, and characteristics such as copper coating can this topic. greatly affect the size and quality of the crop (see Chapter 6, Containers). Different crops, as well as differ- MANAGING THE PROPAGATION ENVIRONMENT ent stock types of the same species, may require differ- In nurseries, a variety of horticultural techniques can ent growing media. For example, a very porous media be used to modify the propagation environment. The containing perlite or pumice is used for rooting cut- type of propagation environment dictates the extent to tings, whereas a media with more water-holding ca- which environmental conditions may be controlled. pacity is required for germinating seedlings (see Ways of controlling the propagation environment are Chapter 5, Growing Media). A steady supply of high- discussed in other chapters. The main way, and one of quality water is one of the most critical needs of grow- DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 67 ing plants (see Chapter 10, Water Quality and Irrigation). It is possible to greatly accelerate the growth of native plants with fertilizer, especially for very slow-growing species (see Chapter 11, Fertilization). Certain organisms can be extremely important for the health and growth of some nursery crops. For example, the rooting of some difficult-to-root native plants has been shown to improve after treatment with a beneficial bacterium (Tripp and Stomp 1998) (see Chapter 14, Beneficial Micro- organisms). Because of the high light intensity in green- houses, controlling the light and temperature can be challenging. A Modifying Light in the Greenhouse Light affects plants in several ways. As mentioned earlier, light is necessary for photosynthesis, which provides energy for plant growth. For light-loving species, more light equals more growth (figure 4.13A), but greenhouse light levels are often too intense to grow some native plants (table 4.2). As a result, growers apply shadecloths to lessen light intensity and the re- sultant heat (figure 4.13B). Shadecloths are rated by the amount of shade they produce, ranging from 30 to 80 percent. Black has been the traditional color but now shadecloth also comes in white, green, and reflective metal. Because black absorbs sunlight and converts it into heat that can be conducted into the propagation structure, black shadecloth should never be installed directly on the covering (figure 4.13C) but instead should be suspended above it to facilitate air move- ment. White shadecloth absorbs much less heat than black, and other colors absorb intermediate amounts of heat. New aluminized fabrics do a great job of reflecting incoming sunlight (figure 4.13D). Applying a series of B shadecloths, each with a lesser amount of shade, over a period of time is a good way to gradually harden nurs- Figure 4.12—(A) Intermittent-mist systems can be either enclosed or (B) in outdoor ery stock and prepare it for outside conditions. growing compounds. (C) Their environments can be easily controlled. (D) Programmable clock timers control the timing and duration of (E) specialized mist nozzles, which (F) keeps humidity levels high,reduces transpiration,and provides cooling.(G) Heating cables or mats Supplemental Lighting under the growing medium keep rooting medium temperatures high. Another way that sunlight affects plants is the relative Photos A,B,D-G by Thomas D.Landis,C byTara Luna. length of day and night, which is known as “photo- period.” Some native plants, especially those from high latitudes or high elevations, are very sensitive to daylength, a process controlled by the plant pigment phytochrome. When days are long, shoot growth occurs, but, when daylength drops below a critical level, shoot growth stops (figure 4.14A). Native plants 68 PROPAGATION ENVIRONMENTS  C F D G E DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 69 Table 4.2—Shade requirements of a variety of native plant species Scientific Name Common Name Shade Requirement Sun Shade Either Artemisia tridentata Big sagebrush X Carex aquatilis Water sedge X Prunus virginiana Chokecherry X Dryopteris filis-mas Male fern X Chimaphila umbellata Pipsissewa X Gymnocarpium dryopteris Oakfern X Ceanothus sanguineum Redstem ceanothus X Rubus parviflorus Thimbleberry X Pteridium aquilinum Bracken fern X A B C Figure 4.13—(A) Sunlight provides the energy necessary for plant growth but is con- verted to heat inside propagation structures. (B) By reducing light intensity, shadecloth cools the environment. (C) Black shadecloth absorbs heat that is radiated back into the propagation environment,but (D) new aluminized shadecloth diffuses light without gener- ating heat. Photos by Thomas D.Landis,illustrations by Jim Marin. D 70 PROPAGATION ENVIRONMENTS  A B Figure 4.14—(A) The relative length of day compared to night affects plant growth like a switch; long days stimulate growth, but short days cause dormancy.(B) Native plants from northern climates or high elevations are particularly sensitive to daylength and will quickly set a dormant bud under short day conditions.(C) Nurseries use photoperiod lights C to artificially keep days long and their crops in an active state of shoot growth. Photos by Thomas D.Landis,illustration by Jim Marin. from northerly latitudes or high elevations are particu- houses. Vents and fans are used to keep air moving in- larly sensitive to daylength and will “set bud” (stop side the greenhouse (figure 4.15B) and exhaust heat shoot growth) quickly when days begin to shorten (fig- from the structure (figure 4.15C). In dry environments, ure 4.14B). This process is genetically controlled and wet walls use the power of evaporation to cool incom- protects plants against early fall frosts. ing air (figure 4.15D). Growers can also use short bursts Container nurseries use photoperiod lights to of their irrigation system for cooling (figure 4.15E). extend daylength to force continued shoot growth (fig- Heating greenhouses is much easier than cooling ure 4.14C). The lights are turned on as soon as seeds them. During cold weather, heaters keep the propaga- germinate and are shut off when height growth is ade- tion environment at the ideal temperature for growth quate and the hardening phase begins. Several differ- (figure 4.16A). Rising fuel costs are becoming more of a ent lighting systems are used in nurseries; for more concern and nurseries are adjusting their growing details see Landis and others (1992). schedules and using other management strategies to reduce fuel costs. Many growers start their crops in Modifying Temperature heated greenhouses and then move them outside as One of the most challenging aspects of nursery soon as the danger of frost has passed (figure 4.16B) management is controlling temperature in propaga- tion structures. Temperature directly affects chemical Temperature Monitoring and Control Systems reactions involved in plant metabolism and also af- Fortunately, temperature is very easy to measure fects rates of transpiration. As just discussed, sunlight and should be monitored at all times during the is converted into heat, but this can be managed with growth of the crop. Automatic sensing instruments are shadecloth. Control units (figure 4.15A) automatically available that can be connected with cooling and heat- operate cooling and heating systems within green- ing equipment to trigger a cooling or heating cycle for DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 71  A B C D E Figure 4.15—(A) Fully controlled greenhouses contain heating and cooling systems and sophisticated controllers.(B) Greenhouses heat up quickly during sunny weather and must be de- signed for cooling with interior circulation fans and (C) exhaust fans and vents.(D) In dry climates,fans pull air through wet walls,where it is cooled by evaporation.(E) A quick application of irrigation water can also be used to cool crops. Photo A by Jeremy R.Pinto,B-E by Thomas D.Landis. A Figure 4.16—(A) The heat from green- house heaters is circulated through the grow- ing area. (B) Rising fuel costs are causing many nurseries to start their crops in a heated greenhouse and then move them to an open compound like this one at the Confederated Tribes of the Colville Reservation Nursery in Washington State. Photo A by Thomas D.Landis,B by B R.Kasten Dumroese. 72 PROPAGATION ENVIRONMENTS  B A Figure 4.17—(A) Monitoring and controlling temperature is critical to successfully growing a crop of native plants, and monitoring equipment is inexpensive. Many nurseries use ther- mometers that record daily maximum and minimum temperatures and (B and C) small, pro- grammable,self-contained temperature sensors. Photo A by Thomas D.Landis,B and C by David Steinfeld. C the greenhouse. Mechanical thermostats consist of a single-chip recording devices can be submersed in temperature sensor and switch that can be used to water and are resistant to dirt and impact, they can be activate motorized vents, fans, and unit heaters within used to monitor most temperatures encountered in the a greenhouse. Thermostats provide the best and most nursery. The data recorded on the sensors must be economical form of temperature control. Sophisticated downloaded to a computer and can then be easily control systems that can maintain a designated tem- placed into a computer spreadsheet. The small size of perature through a series of heating and cooling stages the sensor can also be a drawback; they are easy to mis- is a very necessary and wise investment, considering place. Attach a strip of colorful flagging to indicate current fuel costs. where they are located and write any necessary infor- Thermometers that record the maximum and mini- mation on the flagging with a permanent marker. mum temperatures during the day are simple and eco- nomical instruments (figure 4.17A) that can help EQUIPMENT MAINTENANCE growers monitor subtle microclimates within the prop- Even if you purchase the best “automatic” environmen- agation environment. For example, the south side of tal control equipment, it must be monitored and main- the greenhouse is usually warmer than the north side tained. The hot and humid nursery environment is or areas closest to the vents or cooling system. Thus, particularly hard on equipment; regular maintenance you could plan your greenhouse space by placing ensures longevity, reduces costly repairs, and may help species that require slightly cooler temperatures for avoid disasters. When selecting equipment, it is help- germination and growth on the north side of the ful to consult with other nurseries in your area that are greenhouse and use the south side of the greenhouse growing similar species. for species that prefer warmer temperatures. New Routine maintenance of all greenhouse and nursery devices, such as self-contained, programmable tem- operation equipment should be a top priority. Some- perature sensors, are revolutionizing the ways in one who is mechanically inclined should be given the which temperature can be monitored in nurseries (fig- responsibility for equipment maintenance. Write ures 4.17B and C). Many of these sensors are small everything down. The best place to do this is in a daily enough to be placed within a container or storage box log book. See Chapter 3, Crop Planning and Developing and can record temperatures (between -40 and +185 °F Propagation Protocols, and Chapter 16, Nursery Manage- [-40 and 85°C]) for more than 10 years. Because these ment, for more details. These log books can be filed DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 73 away by year and will prove invaluable when solving being grown. In fully controlled greenhouses, tempera- problems, budgeting, and developing maintenance ture and humidity are usually provided by automatic plans. A system of “promise cards” specifies when heating, cooling, and ventilation equipment. Auto- servicing needs to be done and can be incorporated matic control systems are a necessary investment in into the nursery computer system. Keep a supply of greenhouses. The diversity of species grown in native spare parts on hand, especially parts that may not be plant nurseries requires the provision of ambient con- readily available or may take a long time to receive. It is ditions during the different phases of growth, includ- a good idea to have a spare cooling fan motor on stand- ing germination and establishment, rapid growth, and by. A handy supply of hardware items such as washers, hardening. Thus, most native plant nurseries have a screws, and bolts is also good idea. Familiarize all variety of propagation structures to meet these needs. employees on the operation of all equipment so that With careful planning, these structures can be used for problems can be detected early. The instruction manu- a variety of purposes throughout the year. als for all equipment need to be kept on hand. SUMMARY A propagation environment must be carefully designed and constructed to modify the limiting factors on the nursery site. Each site is unique and there is no ideal type of nursery structure. Crop size, species, length of crop rotation, and the number of crops grown per year are important design considerations. The need for dif- ferent propagation environments for different species and at different growth stages should also be consid- ered. If only a few species with similar growing require- ments are to be grown, then a large single growing structure is feasible. If the plan is to grow many differ- ent species with very different growth require- ments, then a variety of propagation structures will be needed. Having several smaller propagation structures provides more flexibility; these structures can be added over time. Propagation structures are modified based on the location and prevailing environmental conditions at the nursery and for the species that are 74 PROPAGATION ENVIRONMENTS LITERATURE CITED APPENDIX 4.A. PLANTS MENTIONED IN THIS CHAPTER Aldrich,R.A.;Bartok,J.W.,Jr.1989.Greenhouse engineering.NRAES-33.Ithaca,NY:Cornell big sagebrush,Artemisia tridentata University,Northeast Regional Agricultural Engineering Service.203 p. bracken fern,Pteridium aquilinum Bartok,J.W.,Jr.2000.Greenhouse for homeowners and gardeners.NRAES-137.Ithaca,NY: chokecherry,Prunus virginiana Cornell University,Northeast Regional Agricultural Engineering Service.200 p. longleaf pine,Pinus palustris Landis,T.D.1994.Using “limiting factors”to design and manage propagation environments. male fern,Dryopteris filix-mas The International Plant Propagators’Society,Combined Proceedings 43:213-218. oakfern,Gymnocarpium dryopteris Landis,T.D.;Tinus,R.W.;McDonald,S.E.;Barnett,J.P.1992.The container tree nursery man- pipsissewa,Chimaphila umbellata ual: volume 3, atmospheric environment. Agriculture Handbook 674.Washington, redstem ceanothus,Ceanothus sanguineus DC:U.S.Department of Agriculture,Forest Service.145 p. rushes,Juncus species Landis,T.D.;Tinus,R.W.;McDonald,S.E.;Barnett,J.P.1994.The container tree nursery man- sedges,Carex species ual:volume 1,nursery planning,development,and management.Agriculture Hand- thimbleberry,Rubus parviflorus book 674.Washington,DC:U.S.Department of Agriculture,Forest Service.188 p. water sedge,Carex aquatilis Tripp, K.E.; Stomp, A.M. 1998. Horticultural applications of Agrobacterium rhizogenes (“hairy-root”) enhanced rooting of difficult-to-root woody plants.The International Plant Propagators’Society,Combined Proceedings 47:527-535. ADDITIONAL READING Clements, S.E.; Dominy, S.W.J. 1990. Costs of growing containerized seedlings using dif- ferent schedules at Kingsclear,New Brunswick.Northern Journal of Applied Forestry 7(2):73-76. DOUGLASS F. JACOBS, THOMAS D. LANDIS, AND TARA LUNA 75