Commercial Production of Vegetable Transplants (B 1144) University of Georgia Extension Producing greenhouse-grown containerized transplants is an increasingly popular way to establish vegetable crops. Compared to field-grown transplants, greenhouse transplants have several advantages. They can be produced earlier and more uniformly than field-grown plants. Their growth can be controlled more easily through fertility and water management and they can be held longer and harvested when needed. 2017-01-30 13:58:30.987 2006-06-02 14:27:26.0 Commercial Production of Vegetable Transplants | Publications | UGA Extension Skip to content

Commercial Production of Vegetable Transplants (B 1144)

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W. Terry Kelley, Extension Horticulturist
George E. Boyhan, Extension Horticulturist
Darbie M. Granberry (retired), Extension Horticulturist
Stormy Sparks, Extension Entomologist
David Langston, Extension Plant Pathologist
Stanley Culpepper, Extension Weed Scientist
Paul E. Sumner, Extension Engineer
Greg Fonsah, Extension Economist

Is Growing Greenhouse Transplants for Me?

W. Terry Kelley

Greenhouse grown containerized transplants is an increasingly popular way to establish vegetable crops. Compared to field-grown transplants, greenhouse transplants have several advantages. They can be produced earlier and more uniformly than field-grown plants. Their growth can be controlled more easily through fertility and water management and they can be held longer and harvested when needed. Because containerized plants are less crowded and healthier; stockier plants can be produced. Finally, container-grown greenhouse plants have a media-enclosed root ball, which retains moisture and root integrity at transplanting, reducing transplant shock. Transplants often result in better stands and earlier harvests; factors, which can increase profits to offset additional production costs associated with transplanting. However, if crops are lost due to frosts or other factors, replanting further increases production costs.

If you plan to use transplants, you may purchase them from a reputable transplant grower or grow them yourself. In deciding if you should grow vegetable transplants, consider (1) your overall operation, (2) the economic feasibility of growing the number of transplants you need, (3) your management skills and the availability of investment capital, (4) the specific vegetable transplants you intend to grow, and (5) the time and other resources you can spend on transplant production.

First determine if this operation fits your total business scheme. Production of greenhouse transplants is very management and resource intensive. Do you have the time, knowledge, management skills and financial resources necessary to do the job well? Start-up costs for growing greenhouse transplants are generally high. If you cannot devote sufficient resources to transplant production, it will be an extremely risky venture.

Second, weigh the risks and benefits of growing transplants. Potential problems can range from diseases and pests to structure and equipment failure. Crop management is much more intensive and less forgiving. An entire crop can be lost in a matter of days. Problems in the greenhouse can be transferred to the field and poorly grown transplants can become a liability after they are transplanted in the field.

Growers of greenhouse transplants generally fall into one of three categories: (1) producers of transplants solely for commercial sale, (2) growers of transplants for personal use, and (3) growers of transplants for their own operations as well as for commercial sale. The time that must be devoted to the production of greenhouse transplants depends on the category in which a grower falls. Since the third category is merely a hybrid of the other two, categories one and two are discussed.

If you grow transplants for sale, consider if the operation will be profitable. There must be a sufficient market for transplants just as for any other commodity. There is also the possibility that your customers may hold you responsible if their crop fails. Growers producing transplants for themselves have the same investment and loss potential risks. They may also have the added cost of buying scarce, expensive replacement plants if the crop fails. There are advantages, however, to growing your own plants. You know the crop history and variety as well as if the plants are free of diseases, insects and weeds. Contamination of your farm with pests from imported plants is also avoided. Successful vegetable producers who grow their own transplants can have healthy plants when they need them.

Once the risks, benefits and feasibility of growing transplants are determined, then decide if you want to proceed. For further discussion on the economics of growing greenhouse transplants, refer to the section titled "The Economics of Growing Transplants in a Greenhouse."

Containers and Media

W. Terry Kelley and George E. Boyhan

Containers

The production of containerized vegetable transplants has changed dramatically in the past several years. Most container-grown vegetable transplants were produced in peat-based containers, but now the vast majority are grown in hardened plastic or polystyrene (Styrofoam) containers. Generally, peat containers, clay pots, peat pellets, fiber blocks and plastic pots are not used for mass production. Therefore, discussion here is limited to polystyrene and plastic types.

Since both plastic and polystyrene containers are a considerable financial investment, most are reused many times. Because of reuse, containers must be properly sanitized after each use or disease problems are likely to occur.

Most containers are sterilized using a 10 percent chlorine bleach solution. Problems sometimes arise when chlorine rinses are used. If polystyrene containers are not aerated properly, chlorine can soak into crevices and cause toxicity in future plantings. Use proper rates and throughly rinse with water after washing to reduce the possibility of toxicity.

Unlike polystyrene containers, plastic containers do not have the small pores and crevices that may harbor disease organisms and soak up residual chlorine. Plastic containers have edges, however, that can be difficult to clean and may provide a suitable environment for pathogens. Plastic containers are usually made up of two parts: a reusable plastic tray or flat with inserts of various sizes and/or configurations. The inserts are typically used only once.

Numerous types (shapes, sizes and configurations) of containers are available. Select the type that best fits your system. Plastic and polystyrene containers most often come in straight row arrangements. Polystyrene containers normally have inverted pyramid-shaped cells that taper toward the bottom. They may have cell sizes as small as 0.8 inch square or as large as 6 inches square. The number of cells in a container depends on the cell size. They may have from 12 to 338 cells per container. A polystyrene tray with a 1½-inch cell size has 128 cells, while a tray with a 2½-inch cell size has 72 cells.

Cells in plastic trays are arranged similarly and hold the same volume of soil as the same size polystyrene cells. Since plastic cell walls are thinner, plastic trays typically have more cells per tray than similar polystyrene containers. For example, cell media volume of a 512-cell plastic container is roughly equal to the media volume of a similar sized 338-cell polystyrene container.

More recently, single piece plastic flats are being used in transplant production (Figure 1). These flats are typically larger than standard flats (11" x 22") with a size of 13" x 26". They have greater rigidity than the inserts used with 11" x 22" flats. These single piece flats are reusable like polystyrene containers, but are considered more durable and easier to clean. As with polystyrene and plastic inserts, a variety of sizes are available.

Figure 1. One-piece hard plastic flat. Figure 1. One-piece hard plastic flat.

Different plant species require differing amounts of space, nutrients and water. Certain cell sizes are more suitable for some plant species than others. Larger cells hold a greater volume of media, which enables them to retain more water and nutrients. Therefore, transplants growing in larger cells require less frequent watering and fertilizing. This helps reduce the likelihood of moisture or nutrient stress. Also, larger-celled containers normally produce stockier and earlier plants. Because of larger root volume, these plugs experience less transplant shock in the field. Proper watering is more critical with larger sized cells however, especially in the early stages when over-watering can be a real problem resulting in a greater chance of root disease development.

Generally, smaller cells are used for plants such as cabbage, broccoli, cauliflower, collard, kale and lettuce. Trays with 1 to 1½-inch cells are well suited for producing transplants of these crops. These trays generally have from 200 to 338 cells per tray. Use larger cell sizes, 1½ to 2½ inches, for production of tomato, pepper, watermelon, muskmelon, cucumber and squash transplants. These trays generally have from 72 to 200 cells per tray.

Some containers have round cells that may be in offset rows. There are no particular advantages or disadvantages to this arrangement. In situations where plants need additional room to grow, these containers may be used. However, containers with a larger cell size will serve the same purpose.

Other than sanitation difficulties, there are few disadvantages to plastic or polystyrene trays. Plants can sometimes be difficult to remove from them, particularly if roots grow through the bottom of the tray or into crevices. Transplants can be more easily removed if they are moistened prior to transporting to the field. This will also help reduce stress if there is a delay in transplanting. The trays are easy to handle, however, in a number of greenhouse setups.

Media

Generally, a commercially prepared soilless media is used for containerized transplant production. This media must be free of insects, pathogens, nematodes and weed seeds. Most commercial mixes and components for commercial mixes meet these criteria.

Media have different properties that vary according to the contents. Be careful to choose a media best suited for the intended use. For instance, several media are available specifically for starting transplants from seed. Generally, they contain finer shredded peat particles than media used for bedding plants or potting soil. Check the media description or ask the sales representative which media best suits your purpose. Select a media that drains well and provides good aeration but still has moderate water-holding capacity.

Media are available with starter nutrients (charged) or without starter nutrients (non-charged). Although either type can be used, many growers prefer a non-charged media so they can add their own starter solutions and know for sure the fertility levels are correct. Also, growers using non-charged media know the exact amounts of nutrients plants have received (provided they have precisely monitored fertilizer application/injection).

Georgia law requires media sold in this state to be labeled according to contents. According to the way they are formulated, nutrient content of charged mixes varies. Some may contain only major nutrients while some may include a mixture of trace elements as well. Growers can get customized mixes by special order (contract) with a commercial formulator.

Some charged mixes contain slow-release fertilizer that provides nutrients to plants after seed germination. Media with slow-release nitrogen is generally not recommended since it may eliminate the option of withholding nitrogen to control plant growth. It is essential to know the concentrations of all the nutrients in a media for effective fertilizer management.

There are many sources and kinds of media. Desirable prepackaged media are generally light-weight, well drained, well aerated, and hold moisture and nutrients well. The basic constituents of media are peat moss, perlite, vermiculite and a wetting agent. Other ingredients such as washed granite sand and processed bark ash can also be found in some mixes. Perlite is a processed volcanic mineral that provides good drainage to the media while holding air in the rooting zone. Most of the water from perlite stays on the surface of the particles, making it readily available to plants. Vermiculite is a mica-type mineral that can hold large amounts of air, water and nutrients.

Media with finely ground perlite, vermiculite and peat moss are best for starting vegetable transplants. Peat-lite mixtures, containing only peat moss and perlite, are not acceptable because the media is too coarse. Media without these components may not provide the needed nutrient and water-holding capacity to produce acceptable transplants. Additional ingredients are at the grower's discretion.

Although prepackaged media may be available in different sizes (Figure 2), it is usually shipped in bags or bales of either 3 or 4 cubic feet. The number of trays that can be filled by a bag/bale of media varies according to tray size and cell volume. Prices of media also vary according to contents, size of bags/bales and quantity of media purchased. We recommend transplant growers purchase commercially available media from a reputable company with an established quality control program.

Figure 2. Transplant growing media ready for trays 
      is usually in bags. Figure 2. Transplant growing media ready for trays is usually in bags.

It is possible for growers to mix their own soilless media. This requires additional time, labor and management, and any actual cost savings will likely be minimal. Except in special situations, commercially prepared media is preferred over home mixing.

Do not use field soils for growing containerized plants. Field soils generally drain poorly and are often contaminated with diseases and weed seed. Even though this soil can be treated to help control pest problems, soil treatment is labor-intensive, costly and does nothing to improve soil drainage.

Selecting an appropriate media is especially important when growing transplants using the float system. Media containing clumps and large particles are not recommended for float production. In the float system, plants are irrigated by water drawn from the surface of the water reservoir through holes in the bottom of the tray into the media-filled cells. For this to occur, the media must form a continuous column from the bottom to the top of the cell. Because large particles prevent media from completely filling cells, they create air pockets. Cells containing air pockets do not take up water properly and stay too dry for good plant growth -- "dry cells." The media must have enough water-holding capacity to allow capillary action to draw up the water to the roots, but it must be porous enough to allow sufficient gas exchange.

Transplant Production Systems

George E. Boyhan and Darbie M. Granberry

The two transplant production systems used by Georgia transplant growers are (1) containerized production in protective structures (greenhouses, cold frames and hotbeds) and (2) in-ground transplant production in the open field (commonly referred to as field transplant production).

Container-Grown Plants in Greenhouses

This system is the most intensive and produces the earliest plants. During the past decade, major Georgia transplant growers have shifted from field to greenhouse production. This trend is expected to continue. Consequently, the value of field-grown transplants will continue decreasing and the value of greenhouse-grown containerized transplants will continue increasing.

Containerized greenhouse plants can be grown using the conventional "rail" (sometimes called "rack") system or the "float" system.

Rail System

In the rail system, rows of rails, usually aluminum T-rails, are precisely spaced to support the trays (flats) on each end (Figure 3). In this system, often referred to as the "Speedling System," trays have drainage holes in their bottoms and plants are watered and fertilized from above, usually by an overhead boom. Georgia transplant growers began using the rail system in the 1970s and used it almost exclusively until introduction of the float system in the late 1980s. Although interest in the float system is increasing, the rail system is still the most popular greenhouse transplant production system in Georgia.

Figure 3. Aluminum T-rails are usually used to 
      support transplant trays. Figure 3. Aluminum T-rails are usually used to support transplant trays.

Growers also use various other types of benches, that support trays at various heights. There are even moving benches available that use space more efficiently since only one isle is needed that can be created anywhere in the greenhouse as the tables are rolled out of the way. Some operations put flats directly on the ground. Usually this is on a concrete floor that will often have in-floor heating.

Float System

With the float system, growers fill polystyrene (Styrofoam) trays with media and then seed them. Trays are then irrigated and placed in a warm environment. After seeds germinate, trays are placed on a reservoir of water where they "float," either continuously or intermittently, until they are ready to go to the field. Soluble fertilizer is dissolved in the water and, as plants float, they take up water and nutrients (fertigation) from the reservoir. The float system requires less labor and management for irrigation and fertilization. Another advantage is that foliage does not get wet during irrigation. This can help reduce the incidence and severity of foliar diseases. If water in the reservoir becomes contaminated with disease-causing organisms, however, disease problems can be difficult to control (see "Disease Management" section).

One of two water management systems may be used for float production of transplants. They are the intermittent float system (sometimes called ebb and flow) and the continuous float system. The intermittent float system costs more to build and operate. Water is pumped into the reservoir to float and subsequently irrigate the plants for a brief period. Then the water is pumped or drained back into a holding tank. In the continuous float system, the water remains in the reservoir the entire time.

The float system appears to have potential for vegetable transplant production, but it has not been fully vetted for this purpose. Several kinds of vegetable transplants, including eggplant, tomato, pepper, cabbage, broccoli, cauliflower, pumpkin and squash have been produced using the float system. In some cases, such as seedless watermelon production, this type of system is not suitable since these seed need to be germinated in moist but not wet media. There is evidence that this system produces transplants more quickly than the conventional method, but the plants do not do as well upon transplanting. At this time, we do not recommend this system other than for experimental purposes.

Although the continuous float system is successfully used for tobacco transplant production, the intermittent float system may be better for vegetable transplant production. Growers experimenting with the float system are encouraged to try the intermittent float system first.

Whether one uses the float system or the rail system, the major advantage of producing transplants in greenhouses is that temperatures can be controlled and moderated for earlier, less stressed transplants.

Field Production of Bare-Root Plants

Because there is no overhead cost for protective structures, field production is the least expensive way to produce transplants. However, there are two significant restraints to field transplant production: (1) unpredictable frosts limit production of transplants for the early spring (2) soil type is critical, heavy clay soils are unsuitable for this type of production. Field-grown transplants have traditionally been used for early spring plantings in the northeastern United States and for late spring and summer plantings in the South (Figure 4). Virtually all Vidalia onions are produced as field grown transplants, however, most are produced for on-farm use with very few available for resale.

Figure 4. Pulling and packing field-grown transplants 
      requires much hand labor. Figure 4. Pulling and packing field-grown transplants requires much hand labor.

Site Selection and Soil Preparation

Field production sites should receive full sunlight but be protected from strong winds by windbreaks. The soil should be well drained and free of perennial weeds, nematodes and diseases that may adversely affect transplant growth. When selecting a site, consider the prior crop and prior herbicide applications. Do not use land containing residues of potentially damaging "carry over" herbicides. If there is any doubt, carefully check labels of previously applied herbicides (see "Weed Management" section for information on weed control and herbicide use).

If possible, seed transplants on new ground. Otherwise, select land that grew crops suitable in a vegetable rotation. Use the following list to help select the best possible rotation crop.

First Choice - Pasture or Sod
Second Choice - Grain Crop
Third Choice - Cotton
Fourth Choice - Soybeans
Fifth Choice - Peanuts
Sixth Choice - Transplants or the same vegetable crop

Since seed are planted directly into field soil, prepare the soil well to ensure that it is free of clods and debris. Deep-turn (moldboard plow or switch plow) the soil to bury litter. Use a rotary tiller or similar implement to form a smooth, raised bed free of clods. Raised beds warm up quicker in the spring and also help prevent flooding. The best soils tend to be sandy or sandy loams in nature. This type of soil when moistened is ideal for harvesting (pulling) the plants. Heavier soils such as clays make this process more difficult or impossible.

Liming and Fertilization

Base lime and fertilizer rates on soil analysis. Deep plowing on Coastal Plain soils (south Georgia) often brings acid subsoil to the surface. So take soil samples after plowing so lime recommendations are adequate. A soil pH in the range of 6.0 to 6.5 is desirable. For best results, incorporate recommended lime two to three months before planting. Apply and incorporate recommended fertilizer 7 to 10 days before seeding. Monitor transplants closely as they grow and, if needed, apply top dressings of 15 to 30 lb nitrogen per acre. Currently specific fertilizer recommendations for field grown transplant production are only available for Vidalia onions. For other listed crops (Table 1) plan on using to ½ the recommended rates for the entire crop.

Seeding and Irrigation

Plant seed close together in narrow rows (Table 1). Seed to the same depth you would for crop production (about two to three times the diameter of the seed). After seeding, water the bed uniformly to a depth of 2 inches. Keep the top ½ inch of soil moist until plants emerge. This may require several irrigations per day during hot, dry weather. Applications at 10 a.m., 2 p.m. and 4 p.m. are often used. Frequent irrigations during hot weather not only supply water for growth but also help lower soil temperatures and cool young plants.

Table 1. Spacing for Field Seeding Transplants
Crop Inches between Rows In-Row Spacing
Inches between Plants
Broccoli 6-9 1
Cabbage 6-9 1
Cauliflower 8-9 1
Collard 6-9 1
Onion 4-6 0.125-0.5
Pepper 9-12 5
Tomato 9-12 5

After seedlings emerge, irrigate beds more thoroughly. If possible, wet the soil to a depth of 6 inches. Be careful not to injure or wash away small plants with excessive water force and do not "drown-out" transplants by allowing water to pond on the soil. Let the soil surface dry between irrigations. To help harden transplants, reduce irrigation and allow the soil to dry slightly during the five to seven days before pulling.

Frost Protection

Transplants growing in the field are especially susceptible to frost damage. Although they increase production costs, spun-bonded polyester row covers provide limited protection (a few degrees) from cold weather. These wide, lightweight covers can be laid directly over transplants prior to injurious cold temperatures. Continuous overhead irrigation also provides limited cold protection, but do not use center pivot systems in this fashion. They are not strong enough to withstand the ice that will build up and they do not supply continuous water over the entire crop.

Cold Frames and Hotbeds

Cold frames are constructed close to the ground (usually 1 to 3 feet high). They can be constructed of a number of materials including wood, PVC pipe, concrete, etc., and can have a variety of coverings including polyethylene film, rigid plastic, or glass. Hotbeds are similar but have an internal source of heat, usually buried heating cables. Seed may be planted directly into the ground or into containers placed inside the cold frame or hot bed.

Very few of Georgia's vegetable transplants are produced in cold frames and hotbeds. They are sometimes used when relatively small numbers of transplants are needed. If you need additional information on using cold frames and hotbeds for transplant production, contact your local county Extension office.

Water and Fertilizer Management for Production of Containerized Transplants

Darbie M. Granberry and George E. Boyhan

Water Management

You may wish to pre-moisten peat-based media, which can be difficult to wet when completely dry and in fact may require the use of a surfactant to wet initially. After the flats are filled with media and seeded, irrigate to provide moisture for seed germination. Keep media moist but not soggy during germination. Over-watering encourages seed rot and often causes poor seedling vigor or seedling death. Keeping soils too dry during germination can prevent or delay germination and cause erratic stands.

Seed need adequate levels of oxygen and water during germination; hence, proper water management is crucial. Media selection is an important part of managing root zone moisture level. Monitor seedlings regularly to maintain optimum soil moisture. Sub-irrigation with float or intermittent float systems can result in too much water, and overhead irrigation can result in dry spots if not properly designed and maintained.

After germination, irrigate whenever the surface of the media becomes dry. To prevent over-watering, let the top inch of media dry between irrigations. When irrigating from above, continue applying water until it drips from the bottom of the flats. This will ensure that each flat is adequately watered and will also help prevent the accumulation of fertilizer salts in the media. Schedule irrigations early in the day so foliage will dry before nightfall. If leaves are still wet at sundown, they will probably stay wet all night. Moisture on leaves tends to increase disease incidence and severity.

Seedless watermelon are extremely difficult to germinate and various methods have been proposed to improve germination, including scarification of seed and careful control of media moisture. Never saturate media with water; this appears to reduce germination and final stand. Typically, media will be pre-moistened so it is damp but not wet. Triploid seed are sown in this media and the temperature is maintained above 70 degrees F, particularly at night. Often these flats or trays are stacked and wrapped in pallet plastic to maintain high temperatures. No additional water is required for 48 to 72 hours, at which time they are unwrapped and placed in the greenhouse benches. Light watering to maintain moisture is sufficient until seedlings emerge. Over-watering can reduce germination by more than 50 per

Status and Revision History
Published on Jan 5, 2004
Re-published on Feb 26, 2009
In review Jan 5, 2010
Re-published on Feb 9, 2010
Reviewed on Jan 4, 2014
Reviewed on Jan 30, 2017