Plug and Transplant Production for Organic Systems

By Lane Greer, updated by Katherine L. Adam, NCAT Agriculture Specialists

men tending transplants in a greenhouse

Photo: Peggy Greb, USDA ARS


Since the few large commercial suppliers of plugs do not produce organic plugs, growers must produce their own or buy them locally. This publication presents information on raising vegetable and ornamen­tal plugs and transplants, but it is not intended as an introduction to the subject. Rather, it is a comple­mentary piece of information, focusing on organic rather than conventional production methods and on conformity to the Final Rule of the National Organic Program. Although much of the research cited covers ornamentals, the information applies to vegetables as well.


Equipment: Seeders
Nutrition: Organic Fertilizers for Container Systems
Lighting and Growth Regulation
Scheduling: Holding Plugs
Pest Management


The traditional way to raise a lot of seedlings is to sprout seeds in trays, then transplant these fragile plants into larger packs or pots. This method is very labor-intensive and results in consider­able mortality from transplant shock or root loss.

Since the 1980s, most seed germination has been done in plug trays; by 1998, 81% of annual seedlings were grown from plugs.(1)   A plug is a containerized transplant with a self-enclosed root system.(2) The advantages to growing seedlings from plugs are many: less time and labor to transplant, reduced root loss, more uniform growth, faster crop establishment, and increased production. There are disadvantages, too. Much more attention has to be paid to scheduling and to cultural practices. While labor is decreased, mechanization and the need for specialized, well-trained workers increases.

There are a number of pros and cons to con­sider when deciding whether to grow plugs from seed or to purchase plugs and grow them to transplant size. The advantages of produc­ing one’s own plugs include rapid production, efficient use of greenhouse space, choice of species and cultivars, and self-reliance. The disadvantages can include extra labor to han­dle an exacting crop and increased heating costs in winter (since plugs are quite sensi­tive to temperature fluctuations). According to Kessler and Behe (3):

The decision should be based partially on market considerations, labor availability and expertise, the number of plants to be produced, the cost per plug, and the specialized equip­ment and facilities required. This investment is often not economically practical unless pro­duction is large or plugs are marketed to other growers. For most small to medium sized grow­ers, especially [beginners], it is often more economical to purchase plugs from special­ized growers and concentrate on producing fin­ished containers. The issue of grow versus pur­chase should be reviewed periodically as the needs and facilities of the grower change.

Commercial Suppliers of Organic Plugs

Mountain Valley Growers, Inc.
Can supply 128 types of ornamental and culinary herb plugs. Also certified organic seeds for basil and other annual herbs, sunflow­ers, and gourmet greens. Stocks 24 varieties of lavender. Web sales only; nursery is not open to the public.

Greystone Gardens
After research evaluating organic plug production techniques and economic feasibility (SARE grant FNE03-468), Brenda and Mike Hedges started offering organic vegetable and flower plugs grown in their 3000-sq.-ft. greenhouse in 2005. Large selection of certified organic plugs for vegetables and ornamentals. Some vegetatively propagated varieties offered only as conventional stock grown under organic methods.

Patchwork Farm and Greenhouse
In business since 1976. Member of Slow Food of Central Pennsylvania.

The basic considerations in plug production include:

  • Container size
  • Media
  • Seeding and germination
  • Nutrition
  • Temperature
  • Watering and moisture management (especially during germination)
  • Light and supplemental lighting
  • Growth regulation
  • Scheduling
  • Pest management, especially of fun­gus gnats, shore flies, and root rots.

The book Plug and Transplant Production, by Drs. Roger Styer and David Koranski (2), contains the most extensive information on all the basics listed above. Because it focuses on specifics, it is good for troubleshooting. The book comes with three pullout tables—
one each for vegetables, bedding plants, and cut-flower plugs. These tables provide extremely detailed information on specific crops. This publication should be on the shelf of every serious plug and transplant grower. See Reference 2 for information on how to order this book.

In 1990, and again in 1996, the editors of GrowerTalks magazine (4) compiled some per­tinent articles on plug production into books entitled GrowerTalks on Plugs (5) and Grower­Talks: PlugsII.(6) The 1990 edition is no lon­ger in print, but it may be available through interlibrary loan. The 1996 edition is avail­able from Ball Publishing (see Reference 4). This edition focuses on the basics mentioned above, as well as automation (transplanters, shipping, boom irrigators), plug holding, stress, container size, growing your own vs. buying, seeds and germination, pest control, and culture by crop for 11 species of bed­ding plants.

Greenhouse Grower (7) and Greenhouse Man­agement and Production (8) are periodicals that address plug production and related issues. Greenhouse Grower produces an annual bonus issue that targets plugs and propagation. GrowerTalks holds an annual plug conference that lasts for three days, usu­ally in October. Although the conference is expensive (usually around $550 for all three days), it would be worth the money to talk with growers from around the country to see what they recommend and discuss solutions to problems. For more information, contact Ball Publishing at the address listed in the References section.

Information on conventional bedding plant production is available from Bedding Plants International (9); they sell books, videos, and other educational materials. BPI also holds an annual conference.

The New Organic Program

Under the Final Rule of the National Organic Program, organic plug production must take place in a certified organic operation. Organic plug production begins with certified organic seed or certified organic plants from which propagation material is obtained through layering or other means of vegetative propagation. If certified organic seed (or vegetative propagation mate­rial) of the desired cultivar is not commercially available, untreated seed or slips may be used for plugs, as long as they are not derived from genetically engineered plants. Though high cost is not deemed “unavailability,” in practice most certifiers will accept as evidence of unavailability proof that growers have tried unsuccessfully to get organic material from at least three different suppli­ers. The federal organic “seeds and planting stock practice standard” (§205.204) can be found at A grower should always determine the name of the seed supplier’s organic certifier, perhaps by asking to see the certificate. The official list of accredited certifiers is at www.ams.usda/NOP.

Federal organic regulations also address composting and the use of raw manures. These may have implications for plug production of early-harvested food crops. Federal standards also dic­tate what may and may not be used as growing media, fertilizer, or pest control inputs. For more details, it is advisable to read the relevant sections of the Final Rule. For information on some com­mercial products that meet the criteria set forth in the Final Rule, visit the OMRI website at and view their listings.


Plugs are produced in multi-cell plastic trays, also called flats. The trays are available in a wide range of cell sizes and depths—any­where from 50 to 800 cells in a single tray. The type of flat chosen will depend on the species being grown and the length of time spent growing it. Pansies, for instance, are most often grown in 288, 392, or 406-plug trays.(3) The time from sowing to ready-for-transplant stage is usually about 6 weeks in 288 trays and 5 weeks in 392 or 406 trays. Larger cells provide more moisture and nutrients to the plant, but make much less efficient use of greenhouse space than the smaller cells.

Rigid foam cell trays (such as Speedling® flats) are an alternative to plastic trays. Though more expensive per unit, they have the advan­tage of greater durability and “stand-alone” ability—particularly important when using automated equipment. They are quite bulky, however, and will not nest for storage. Light­weight plastic trays, in contrast, are easy to store in the off-season and cost less, but they are less durable and usually require an undertray to maintain sufficient rigidity for convenient handling. Manufacturers offer other innovative designs, as well. Winstrip® trays, for example, have small slits down two sides of each plug to improve aeration, and holes between cells to allow air movement up through the tray.(10) Plugs grown in Win­strips usually have less root spiraling.

Occasionally, growers choose a soil-block sys­tem. Developed in Europe some years ago, soil blockers press a soil mix together into cubes that are handled in much the same manner as peat pellets. Commercial-scale blocking equipment is expensive, and many growers find the plants’ tendency to grow their roots into adjoining blocks annoying. Small-scale blocking equipment has proved popular with some market gardeners. For more information on soil-block methods, see the book The New Organic Grower, by Eliot Coleman.(11)

Table 1 gives some idea of the sizes, cell shapes, and major suppliers of plug trays. See Suppliers of Plug Trays at the end of this publication for more information.

Table 1. Plug tray suppliers, sizes, and cell shapes

Company Sizes Available Cell Shapes Available
Winstrip 50, 72, 72, 128, 162, 216, 288 Square
Dillen 72, 84, 98, 105, 128, 144, 162, 288, 512 Square, Round
Landmark 50, 72, 84, 98, 128, 144, 162, 200, 288, 384, 392, 406, 512, 648, 800 Square, Round, Octagonal
Blackmore 128, 144, 200, 216, 288, 338, 384, 406, 512, 648, 800 Star, Square Deep, Octagonal, Square, Octahedron, Waffle
Summit Plastic 72, 128 Round
Growing Systems 51, 73, 96, 104, 135, 170, 198, 200, 273, 288, 400, 512 Square
Speedling 72, 128, 162, 200, 242, 288, 338, 388, 392, 595 Square

Research on Containers

Research conducted at the University of Geor­gia in 1995 showed that copper-treated plug flats affected the root growth and flowering of three species of bedding plants. Armitage and Gross reported that root spiraling and seedling height were reduced in impatiens, geraniums, and petunias. The copper treat­ment delayed the flowering of the impatiens and petunias by 12 days, and the gerani­ums by 21, but did not affect their mature heights.(12)  The National Organic Standard [§205.601(i)(2)] permits copper sulfate and most fixed coppers (copper hydroxide, cop­per oxide, copper oxychloride) in crop pro­duction, as long as copper does not accumu­late in soils or become a pollution hazard.

Container size can affect transplant char­acteristics. Researchers at the University of Florida tested four sizes of Speedling flats and found that lettuce grown in the two larger sizes took longer to reach harvestable size, but yield was not affected. In the spring-grown lettuce, there was no difference in head weights, but fall-grown heads started in the larger cell sizes were heavier. The researchers also tested media compression and found that compressing the media in the cells was not justified, since it was more costly and did not increase yields.(13)

In research done at the University of Ken­tucky, marigold plugs were grown in 512, 406, and 288-plug flats over a 16-day period. Size did not affect seedling growth until the 13th day, but after 16 days, seedlings grown in the 512 flats were about two-thirds the size of those grown in 288 flats. Container size also affected root branching: plants from the 288 trays had roots with fewer branches.(14)

Of greatest importance to the end user and the plug grower, however, is whether yield after transplanting is affected by container size. Researchers have yet to prove that con­tainer size affects ultimate crop yield.(15) However, studies conducted in England in the mid-1990s showed that organically grown vegetable transplants benefited from larger cell sizes.(16) The larger cells held more media and more nutrients.

Plug flat color has little effect on medium sur­face temperature, and temperature is impor­tant when germinating seeds. Researchers in Tennessee tested black, gray, and white flats, and found no significant differences among the three colors.(17)

Grower Jay Martin in Maryland mixes Pro-Mix Lite® and compost in proportions ranging from 12:1 to 15:1 (mix:compost), depending on the crop.(18) For example, brassicas, which are quick to finish, are grown in a mix with less compost; slower-growing solanaceous crops are raised in a mix with more compost.


In organic production, plant health depends on the quality of the planting mix. Most pro­ducers opt for a soilless mix to reduce the risk of soil-borne diseases. The mix should contain sufficient amounts of the major nutri­ents to carry the young crop for the time it will be in the greenhouse.

Purchasing a commercially prepared organic mix is the easiest way to get started. How­ever, most commercial potting mixes contain synthetic ingredients and do not meet organic standards. One alternative is to arrange a special order from a commercial supplier who agrees to exclude starter fertilizers and wet­ting agents. However, since freight is often prohibitive, many growers choose to mix their own. For more information on potting mixes, ask for the ATTRA publication Potting Mixes for Certified Organic Production.

Equipment: Seeders

Mechanical seeders are necessary when planting large numbers of plugs. Seeders are quite expensive, costing from $1,000 to $50,000. There are four types:

  • vacuum template ($1,000–$5,000)
  • vacuum tips or needles ($8,000–$13,000)
  • vacuum cylinder or drum ($18,000–$50,000)
  • electric eye ($10,000–$13,000)

Here is how the vacuum template works. The seed is scattered over a template that has small indentations on its surface. A vacuum holds the seeds in the indentations, while extra seed is knocked off. When the vacuum is turned off, the seeds drop into place in the plug tray, so the entire tray is planted at once. There are also manual wand seeders that use the vacuum needle system to sow one row of seed at a time. They cost less than $1,000.(1) For a list of suppliers of both kinds of seed­ers, see Suppliers of Seeders.

It is possible to make a simple seeder out of plastic. Dr. Charles Marr developed a plant­ing template in the early 1990s at Kansas State University.(19) Here are his specifica­tions:

The template consists of two sheets of 3-mm acrylic plastic cut to rectangular dimensions of the seed flat. The upper sheet has a 6-cm-tall “wall” glued to the outside with a small opening in the wall at one end, so excess seeds can be poured out. The bottom sheet is held in place by four glued tabs on each side, so that the bottom sheet could slide laterally. The bottom sheet is left slightly longer with a slot cut as a handle.


Grower Profile—The Henrys’ Plant Farm

The Henrys’ Plant Farm in Kansas, near the Missouri border, has been operated by Marcia and Roy Henry, and now their oldest son, Brian, for more than 20 years. The Henrys have 18 greenhouses with almost 13,000 square feet of growing space and sell more than 400 variet­ies of perennials and 100 of annuals, including 60 different tomatoes and 40 peppers. Marcia manages the operation of the greenhouses. Four theme gardens entertain visitors.

Marcia uses a vacuum seeder with five plates to accommodate differ­ent seed sizes. Holes that are the same size or slightly smaller than the seed are drilled in the plastic. The method of operation is simple: The top and bottom sheets are kept out of alignment. Seeds are poured onto the top sheet and rolled around until all the cavities are filled. Excess seed can be poured off. The bottom sheet is then moved into line with the top, and the seeds fall through both sheets and onto the seed mix in the plug trays.

Marcia’s production schedule is the result of many years of good record keeping.(20) She begins December 1 with geraniums, bego­nias, and perennials. At the beginning of January, she plants pansies, snapdragons, and vinca. By the end of the month she is potting up geranium cuttings and transplanting seed geraniums. On February 1, she plants petunias. With most of the best sellers and the tomatoes, she plants again on February 15. She starts most of the other annual flowers and vegetables in March.

She has found pre-printed tags available from her seed company to be helpful. Although the tags are expensive ($1,000 in 1997), custom­ers appreciate the extra information they provide. She saves money by ordering her media by the truckload, which is 40% cheaper than buy­ing it bagged. She also saves money on heating costs by contracting for propane gas each year.

Nutrition: Organic Fertilizers for Container Systems

There are four basic ways to fertilize con­tainerized plants: incorporate, topdress, liq­uid feed, and foliar feed. In bedding plant culture, fertilizer incorporation in the mix combined with liquid feeding should provide sufficient nutrition.

Organic fertilizers that can be incorporated to provide nitrogen include alfalfa meal, blood meal, cottonseed meal, feather meal, hoof and horn meal, soybean meal, and animal manures, among others. Materials that pro­vide phosphorus include oak leaves, bone meal, shrimp wastes, residues from raw sugar, and various forms of rock phosphate. Greensand, granite meal, soybean meal, ash from orange and potato skins, unleached wood ashes, and Sul-Po-Mag® all provide potassium. For more information on these and other materials, see the ATTRA pub­lication Alternative Soil Amendments.

Another ATTRA publication, Potting Mixes for Certified Organic Production, provides many recipes for soil and fertilizer mixes. If formulating your own recipe, mix several experimental batches with different fertiliz­ers and rates of incorporation and try them on a test group of plants.

Fertile Soil (21), by Robert Parnes, is an in-depth publication on organic fertiliz­ers. Parnes’s book provides detailed tables on the nutri­ent content of various manures and plant and animal byproducts.

Liquid feeding is done by proportioning nutrients through drip lines, proportioning through watering hoses, or drenching (from a measuring cup or bucket). Soluble fertil­izers can be applied at each watering in a diluted solution, or on a 7- to 10-day basis with a concentrated solution.

Nitrogen is the main nutrient supplied through liquid feeding. Soluble organic nitro­gen sources include fish powder, fish emul­sion, guano, and worm castings. Phospho­rus is available for liquid feeding by using high-phosphorus guano or micronized soft rock phosphate.

Foliar feeding can be used to supplement soil and liquid fertilization, especially where certain nutrients are deficient and must be incorporated into the plant quickly. Filtered solutions of manure, seaweed, fish powder, and fish emulsion can be used (manures must have been composted by an approved NOP method). Seaweed is an excellent foliar mate­rial because it contains growth hormones (auxins, gibberellins, and cytokinins) as well as trace elements. Research suggests that foliar feeding programs enhance plant resistance to pest and disease attack.

Recent Research on Fertilization

A study conducted in 1998 at the University of Georgia found that nitrogen, rather than phosphorus and potassium, mainly deter­mined the growth of plugs. The report rec­ommended that growers focus their fertility programs on nitrogen and reduce phosphorus and potassium applications.(22)

Often, there are seasonal variations in growth, and the fertilization system must take this into account. A recent study in Florida showed that spring-grown tomato seedlings respond linearly to increased nitro­gen fertilizer.(23) In other words, the more N applied, the more the plants grow. (The researchers used 15 to 75 mg/liter of N.) In fall-grown plants, however, the opposite is true. Researchers believe that increased light and higher temperatures in the fall were responsible for some of the differences in growth patterns. Other research has shown that anywhere from 75 to 400 mg/liter of N produces the largest tomato seedlings and often increases early yields.(23) Too much nitrogen, however, attracts aphids.

In an English study, cabbage seedlings were fertilized with conventional fertilizer, dried blood (applied in a liquid form), and hoof and horn meal (incorporated into the potting media).(16)  Dried blood applied at a rate of 3 grams/liter, three times a week, increased plant growth about as much as conventional fertilizer. Table 2 presents the rest of the findings.

Table 2. Fresh weight of cabbage plants at transplanting (16)

Fertilizer Regime

Fresh Weight (g) of 10 plants

Control (no fertilizer)


Conventional fertilizer


Blood 1 (1.5 g/liter 3x/wk.)


Blood 2 (3 g/liter 3x/wk.)


Blood 3 (15 g/liter 3x/wk.)


Hoof and horn (3 g/liter media)


In 1993, Premier Peat Moss in Canada con­ducted research on organic wastes from the food industry, comparing their ability to fer­tilize greenhouse tomato transplants.(24)  The researchers found that meal from blood, feathers, meat, crab shells, fish, cottonseed, and whey by-products increased shoot weight 57 to 83% over non-fertilized plants. The results of this study are shown in Table 3.

Table 3. Effect of organic waste products on shoot weight of tomato transplants (24)

Fertilizer N-P-K

Shoot wt. (grams)

Crab-shell meal 8.2-1.5-0.5


Blood meal 12.5-1.1-1.0


Dried whey sludge 5.3-2.5-0.9


Feather meal 13.6-0.3-0.2


Fish meal 10.1-4.5-0.5


Meat meal 7.7-3.1-0.7


Cottonseed meal 6.5-1.1-1.6


Fish-scale meal 10.0-3.7-0.1


Distiller’s dried grains 4.3-0.9-1.1


Soybean meal 7.5-0.7-2.4


Wheat bran 2.9-1.4-1.3


Alfalfa meal 2.5-0.3-1.9


Canola meal 6.0-1.1-1.3


None (control)  0-0-0



Thorough, even watering is extremely impor­tant to success in plug and transplant pro­duction. Water stress sets plants back and increases the chance that they will fare poorly once planted outdoors. Automatic or semi-automatic watering saves a great deal of work, but it will not eliminate the need for “touch-up” watering by hand, on a regu­lar basis. The plants will do best, especially when in plastic trays, if they are supported off the ground to allow good air circulation and prevent waterlogging. As a general rule they should not be watered in late afternoon, lest they remain overly wet through the night.

Plug cell size will also affect the watering schedule. According to Biernbaum and Ver­sluys, “While in larger containers water must be added to thoroughly moisten the entire medium profile, in shallow containers a less than saturating amount of water can be added without detrimental effects to roots since the water will distribute adequately.”(25)

One of the most common problems in green­houses is overwatering. Not only does this contribute to poor plant growth and health, it also encourages the spread of pathogens that thrive in wet conditions. It is especially easy to overwater young plugs; underwatering becomes a problem with older plugs.

There are five major ways to water plugs (25):

  • hand watering
  • stationary sprinklers
  • traveling boom sprinklers
  • fogging
  • sub-irrigation

Of these, hand watering and sub-irrigation are most appropriate for small growers. The biggest problem with hand watering is labor cost. It is also less uniform than mechanized systems. Sub-irrigation eliminates both of these problems but can result in overwatering because of immediate saturation. Sub-irriga­tion is better when growing larger plugs.

Research on Irrigation

A 1999 study at the University of Georgia showed that moisture stress tends to increase aphid populations on New Guinea impatiens and marigolds, but it has little effect on spi­der mite or thrips populations.(26) In the experiment, mite populations were lower on ageratum plants that were irrigated with an ebb-and-flow system, compared to overhead-irrigated plants.

Researchers at North Carolina State Uni­versity found that environmental conditions, rather than plant growth, may dictate irriga­tion practices.(10) Their other findings:

  • Plugs leach fertilizer, sometimes heavily.
  • Plug trays (288s) can take 500 to 1000 ml of water per tray at each irrigation, depending on environ­mental conditions.
  • Winstrip® trays used more water and dried out faster than conventional trays.
  • Plants tended to be larger in Win­strip® trays and finish earlier than in conventional trays.
  • Plants may use less than 2% of the water applied to the tray.
  • Water per tray may be affected more by air humidity than by temperature or plant condition.

Lighting and Growth Regulation

Although supplemental lighting can increase transplant quality, it is generally not needed for most operations. The exception to this rule of thumb is in winter, especially in northern areas of the U.S. Low light levels result in “leggy” plants, making growth regulation techniques more important. Most growers use high intensity discharge (HID) lamps to light their greenhouses. This topic is discussed at length in Styer and Koranski (2) and in Greenhouse GrowerTalks: PlugsII.(6)

Light competition can cause plugs and trans­plants to elongate, resulting in spindly, leggy plants. Conventional growers use chemical growth regulators to combat this problem. There are, however, non-chemical means to control growth. Brushing is a kind of mechanical conditioning whereby the grower uses a tool (like a broomstick, piece of paper, or PVC pipe) to brush the plant tops and reduce their height. According to Garner et al., “Research concerning the effects of mechanical stimulation on floricultural crops is considerably more limited than that on veg­etable crops, probably due to the availability of chemical growth regulators approved for use on bedding plants.”(27)

closeup of pansies

Researchers at Cornell found that brushing is an effective method for controlling pansy plant size.

Researchers at Cornell studied the best ways to brush plants. They used a piece of polysty­rene foam on tomato seedlings and found that ten strokes a day was enough to reduce the seedlings’ ultimate height by about 20%.(28) Using more than about ten strokes a day was ineffective, and there was no difference between applying all the strokes at one time or spreading them out over a longer period. No matter what the seedling height was when brushing began (6, 8, or 10 cm), the reduc­tion in height was 3 mm per day.

In another study, researchers studied the effects of brushing on tomatoes and on four species of bedding plants (geranium, impa­tiens, petunia, and pansy).(27) For the toma­toes, they found that 10 to 20 daily strokes were enough for height control, that there was no difference between morning and afternoon brushing, and that brushing did not affect yield. The best time to begin the brushing treatment was the first or second true leaf stage. Beginning later resulted in leaf damage. However, brushing the bedding plants had different effects. For geranium, impatiens, and petunia, brushing adversely affected the plants and even resulted in sig­nificant damage. The pansy plugs, however, responded to brushing in much the same way as the tomatoes. The authors concluded that brushing is an effective method for control­ling tomato and pansy plant size.

Cucurbits and eggplants respond well to brushing, but peppers are damaged by it. Cole crops respond fairly well and seem to do better if brushing is started at the second or third leaf stage.(29) One study even showed that brushing helped to reduce thrips and mite populations.(26)

Controlling temperatures is another way to control plant height. Cooler temperatures gen­erally slow down growth, and warmer ones speed up growth. “Both root and shoot growth increase linearly with temperature between the general range of 50 to 85°F.”(1)

Recently, research has concentrated on adjust­ing day and night temperatures to control plant “stretching.” A marked difference in day and night temperatures seems to promote internode elongation, and growers have found that keeping the day and night temperatures the same helps to prevent this from happen­ing. Another strategy is to keep day temper­atures below night temperatures. Although many species respond well to this kind of treatment, some react by producing chlorotic (yellow) leaves or stunted growth. For a list of species and their response to this kind of treatment, see the 1995 article by Myster and Moe from Scientia Horticulturae.(30)

In much the same way, adjusting moisture, nutrition, and light can slow down or speed up seedling development. For more infor­mation on these methods, see References 2 and 6.

Scheduling: Holding Plugs

Often, transplants are ready to be planted outdoors before the weather is conducive. This has prompted research to study the effects of storing or “holding” plugs at cool temperatures. An experiment conducted at the University of Georgia examined the effects of storing geranium plugs for 1 to 3 weeks at 5°C. Additionally, the researchers tested the effects of applying nitrogen just before storage. Below is the abstract of their find­ings (31):

Pelargonium x hortorum L.H. Bailey ‘Scarlet Elite’ seedlings were grown in plugs from seed to transplant size. About 14 days before attain­ing transplant size, seedlings were exposed to various fertility or temperature regimes (pre­conditioning treatments), then stored for 1 to 3 weeks at 5°C. Seedlings receiving 150 mg N/liter before storage flowered sooner and required less crop time (days to flower – days in storage) than those receiving 0, 75 or 300 mg. Temperature preconditioning at 10 or 15°C delayed flowering compared to precon­ditioning at 20°C. Final plant height and dry weight were not adversely affected by varying N levels or temperature during precondition­ing. Preconditioning seedlings with 300 mg N/liter resulted in seedling mortality rates up to 16% after 7 days’ storage. Low temperature or fertility were not effective preconditioning treatments. Preconditioning seedlings with 150 mg N/liter attained best results.

Management of humidity, fertility, moisture, and light is important when holding plugs. Lower humidity is good for suppressing dis­ease, but can lead to water stress. Plugs that have been given too much nitrogen tend not to store well. Research at Michigan State Uni­versity showed that light above 5 foot-candles was beneficial for stored plugs.(32)

Researchers at Kansas State University tested several species to determine their holding abil­ities. Their results are presented in Table 4.

Table 4. Recommended cold storage time limits for selected species and cultivars, at 45°F and with light (32)

Species Cultivar

Weeks of Storage

Fibrous begonia Viva


Coleus Multicolor Rainbow


Impatiens Accent Lilac


Accent Red Star


Super Elfin Orange


Marigolds Scarlet Sophia


Pepper Better Bell


Petunia White Flash


Portulaca Double Mix


Salvia Red Hot Sally


Red Pillar


Tomato Better Boy


Verbena Showtime Mix


Vinca Bright Eyes


Grape Cooler


Little Linda


Little Pinkie


Polka Dot


Pretty ‘n’ Rose


Pest Management

For pest management problems, see the ATTRA publications Tipsheet: Organic Pest Management, Organic IPM Field Guide, and Greenhouse IPM: Greenhouse IPM: Sustainable Aphid Control.

An excellent handbook is Integrated Pest Management for Bedding Plants, from Cornell
University.(33) This 112-page manual thor­oughly covers the practical aspects of imple­menting an IPM program for bedding plants. Topics include scouting, developing and implementing control strategies, case studies, use of biological controls, and descriptions and controls of prevalent pests and diseases. See Reference 33 for ordering information.

Top Ten Pitfalls to Plug Production

Mark Bennett of The Ohio State University and Roy Larson of North Carolina State cite the follow­ing as the ten things to watch for when raising plugs.(34)

  1. Economics. Should you grow or buy plugs? Do a cost accounting to make sure you are making the right decision for your operation.
  2. Poor equipment selection. Choose equipment that best fits your growing and economic needs.
  3. Untrained employees. You should not have only one person trained on plug equipment.
  4. Improper tray filling. To achieve uniformity, don’t fill plug trays improperly.
  5. Compaction. Piling filled trays on top of each other leads to this pitfall.
  6. Poor fertilizer practices. A good fertilizer program requires planning.
  7. Poor watering practices. Same as #6.
  8. Waste. Take precautions to avoid wasting seed.
  9. Improper timing. Keep everything on schedule.
  10. Technology gap. Take full advantage of all available technology.


1)  Erwin, John E. 1999. Young plant prefinishers—a new type of grower. Ohio Florists’ Associa­tion. December. p. 3–5.

2)  Styer, Roger, and David Koranski. 1997. Plug and Transplant Production. Ball Publishing, Batavia, IL. 400 p.

3)  Kessler, J., and Bridget Behe. 1998. Pansy Pro­duction and Marketing. ANR-596. Alabama Cooperative Extension. 16 p.

4)  GrowerTalks

5)  Hamrick, Debbie (ed.). 1990. GrowerTalks on Plugs. Ball Publishing, Batavia, IL. 184 p.

6)  Hamrick, Debbie (ed.). 1996. GrowerTalks: Plugs II (2nd ed.)  Ball Publishing, Batavia, IL. 214 p.

7)  Greenhouse Grower

8)  Greenhouse Management

9)  Bedding Plants International
525 SW 5th Ave., Suite A
Des Moines, IA  50309

10)  Fonteno, William C., and Douglas A. Bailey.  1997. How much water do your plugs really want? GrowerTalks. Fall. p. 32, 34, 37–38.

11)  Coleman, Eliot. 1995. The New Organic Grower. Chelsea Green, White River Junction, VT.  340 p.

12)  Armitage, A.M., and P.M. Gross. 1996. Cop­per-treated plug flats influence root growth and flowering of bedding plants. HortScience. October.  p. 941–943.

13)  Nicola, Silvana, and Daniel J. Cantliffe. 1996  Increasing cell size and reducing medium compression enhance lettuce transplant quality and field production. HortScience. April. p. 184–189.

14)  Geneve, R.L., and J.W. Buxton.  1995.  Mari­gold root development during plug production.  ActaHorticulturae.  Vol. 396.  p. 345–350.

15)  NeSmith, D. Scott, and John R. Duval.  1998.  The effect of container size.  HortTechnology.  October-December.  p. 495–498.

16)  Anon.  1995.  Organic transplant production.  New Farmer & Grower.  Autumn.  p. 28–29.

17)  Faust, James E., Royal D. Heins, and Hiroshi Shimizu.  1997.  Quantifying the effect of plug-flat color on medium-surface tempera­tures.  HortTechnology.  October–December.  p. 387–389.

18)  Byczynski, Lynn.  1993.  Growing great trans­plants.  Growing for Market.  February.  p. 1, 4.

19)  Marr, Charles W.  1991.  A planting template for plug flats.  HortTechnology.  October–Decem­ber.  p. 120–121.

20)  Byczynski, Lynn.  1997.  Customers flock to farm for plants.  Growing for Market.  February.  p. 8–10.

21)  Parnes, Robert.  1990.  Fertile Soil.  agAccess Agricultural Booksource, Davis, CA.  190 p.

22)  Van Iersel, M.W. et al.  1998.  Fertilizer effects on the growth of impatiens, petunia, salvia, and vinca plug seedlings.  HortScience.  July.  p. 678–682.

23)  Vavrina, C.S. et al.  1998.  Nitrogen fertilization of Florida-grown tomato transplants: season variation in greenhouse and field performance.  HortScience.  April.  p. 251–254.

24)   Gagnon, Bernard, and Sylvain Berrouard.  1994.  Effects of several organic fertilizers on growth of greenhouse tomato transplants.  Canadian Journal of Plant Science.  Vol. 74, No. 1.  p. 167–168.

25)  Biernbaum, John A., and Natasha Bos Versluys.  1998.  Water management.  HortTechnology.  October–December.  p. 504–509.

26)  Latimer, Joyce, and Ronald D. Oetting.  1999.  Conditioning treatments affect insect and mite populations on bedding plants in the green­house.  HortScience.  April.  p. 235–238.

27)  Garner, Lauren, F. Allen Langton, and Thomas Björkman.  1997.  Commercial adaptations of mechanical stimulation for the control of trans­plant growth.  ActaHorticulturae.  Vol. 435.  p. 219–230.

28)  Garner, Lauren, and Thomas Björkman.  1996.  Mechanical conditioning for controlling exces­sive elongation in tomato transplants: sensitiv­ity to dose, frequency, and timing of brushing.  Journal of American Horticultural Science.  Vol. 125, No. 5.  p. 894–900.

29)  Byczynski, Lynn.  1993.  Keep those transplants in shape—with a broomstick.  Growing for Market.  February.  p. 4.

30)  Myster, J., and R. Moe.  1995.  Effect of diurnal temperature alternations on plant morphology in some greenhouse crops: A mini-review.  Sci­entia Horticulturae.  Vol. 62.  p. 205–215.

31)  Kaczperski, Mark P., Allan M. Armitage, and Pamela N. Lewis.  1996.  Performance of plug-grown geranium seedlings preconditioned with nitrogen fertilizer or low temperature storage.  HortScience.  June.  p. 361–363.

32)  Gast, Karen L.B., and Alan B. Stevens.  1994.  Cold storage for plug production.  MF 1173.  Kansas State University Cooperative Extension Service, Manhattan, KS.  2 p.

33)  Casey, Christine (ed.).  1997.  Integrated Pest Management for Bedding Plants.  IPM No. 407.  Cornell Cooperative Extension, Ithaca, NY.  112 p.

34)  Bennett, Mark, and Roy Larson.  1997.  Top ten pitfalls to plug production.  Greenhouse Grower.  Mid-September.  p. 31.

SARE Farmer/Rancher Research

FNE03-468 Organic plug production: Evaluat­ing growing media, fertilizer, and economic fea­sibility.

In 2003 Brenda Hedges of Greystone Gardens, Water­bury Center, Vermont, began evaluating organic plug production feasibility, also comparing the performance of three media types and two fertilizers in organic produc­tion. The research confirmed that “growing plugs actu­ally produced more income per square foot of production space than growing traditional bedding plants.” Organic germinating mix performed better than did either the organic compost or a peat-based medium. A fertilizer made from liquefied fish protein (2-4-2) performed better than a fertilizer made from liquid fish and seaweed (3-2-2).  In 2005 Greystone Gardens began offering a large selection of organically raised plugs.

Further Reading

Gaston, Michelle (ed.). 1999. Tips on Growing Bed­ding Plants (4th ed.). Ohio Florists’ Association (OFA), Columbus, OH, 43215.  164 p.

Holcomb, E. Jay (ed.). 1994. Bedding Plants IV. Ball Publishing, Batavia, IL. 452 p.

Nau, Jim.  1999. Ball Culture Guide. Ball Publish­ing, Batavia, IL.  248 p.

Pyle, Allen R. 1999. Planting the seeds of success. American Nurseryman. February 1. p. 52-54, 56, 58, 60.

Shores, Sandie. 2003. Growing and Selling Fresh-Cut Herbs [Chap. 9]. Ball Publishing, Batavia, IL. 483 p.


National Organic Rule

Organic Materials Review Institute (OMRI)

Plug Transplants for Processing Tomatoes: Production, Handling, and Stand Establishment, Ontario, Canada

Suppliers of Plug Trays

Blackmore Co.


Growing Systems, Inc.

Landmark Plastic Co.


Summit Plastic Co.

Suppliers of Seeders

Blackmore Co.

Bouldin & Lawson

Gleason Equipment

Growing Systems, Inc.

Seed E-Z Seeder Inc.

Speedy Seeder

Plug and Transplant Production for Organic Systems
By Lane Greer, updated by Katherine L. Adam
NCAT Agriculture Specialist
©NCAT 2005

This publication is produced by the National Center for Appropriate Technology through the ATTRA Sustainable Agriculture program, under a cooperative agreement with USDA Rural Development. ATTRA.NCAT.ORG.