Growing Vegetable Transplants in Plug Trays
This infosheet replaces Factsheet 96-02 originally authored by Jody Bodnar, formerly of OMAFRA Crop Technology Branch, Simcoe, and Ron Garton, formerly of Agriculture & Agri-Food Canada, Harrow.
Table of Contents
In the past, vegetable transplants were either grown in greenhouses using flats or ground beds, or in outdoor ground beds in the southern United States. Now most growers have made the transition to greenhouse-grown transplants using various types of containers, primarily plug trays (Figure 1). With this system, each transplant grows in an indivi=dual cell so there is less competition among plants and greater uniformity. Less labour is required for mixing and sterilizing soil, filling flats and pulling plants. Plug transplants establish better in the field because roots are not damaged in pulling.
Figure 1. A well-grown melon transplant ready for field setting. Note the healthy root development and stocky growth.
The cell size influences the field performance of the transplant, especially earliness. When larger cells are used the plant has more room to grow, so it is possible to produce an older, more mature transplant without it becoming spindly or rootbound. In general, larger transplant trays result in earlier-maturing crops. Larger cells, however, take up more greenhouse space and are more expensive to grow (Figure 2).
Figure 2. Various plug tray sizes for production of vegetable transplants.
Table 1. Specifications of commonly used vegetable transplant trays.
1 Cell volume (cubic centimetres) varies depending on depth and shape of the cell.
Compared to plastic trays, styrofoam transplant trays are more expensive, insulate the root media, delay seedling growth, promote algae and harbour disease. Plastic trays are more commonly used and are preferred. However, darker-coloured trays absorb more heat and tend to produce faster growth than light-coloured trays (e.g., black versus white plastic trays) (Figure 3).
Figure 3. Seeded at the same time, the dark plastic trays are advanced in growth in comparison to the white Styrofoam trays.
A deeper-celled tray has a larger cell volume so more water and fertilizer are available to the plant. Deeper cells tend to promote faster growth and although they will not need watering as frequently as shallow cells, they will require more water to wet the media completely. With deep trays, it is important to water thoroughly and moisten the media to the bottom of the plug to promote root growth there.
For better uniformity, do not mix deep and shallow trays, or different-coloured trays within one lot of transplants.
The optimum age for vegetable transplants depends on both the crop and the cell size to be used. In general, larger cells will enable production of a larger, more mature transplant. Transplants grown in larger cells have been reported to give higher early yields compared to smaller cells: however, the overall yield is largely the same.
Growers must adjust their growing practices and schedules for different crop species and cell sizes. Table 2 lists some general guidelines for transplant production in various cell sizes.
Table 2. Scheduling transplants of various cell sizes, for several vegetable crops.
The growing media or soilless mix is generally made up of a combination of peat, vermiculite and horticultural perlite. Media containing coarser-textured (long-fibred) peat provide better drainage and aeration, therefore promoting better root development.
Some soilless mixes contain fertilizer (nutrient charge), which must be considered when designing a fertility program for transplants (see Table 3). A lower-nutrient charge in the media will allow more control over growth. It is easier to add the fertilizer required than to try to remove what others have added.
Table 3. Chemical analyses of samples of selected plug media. Media vary in nutrient concentration, so fertility and watering practices must be adjusted.
Obtain the best seed money can buy! There is no advantage to compromising on seed cost as this is a low percentage of the total cost of growing the crop. For certain popular varieties, seed should be ordered as much as one year in advance to ensure a supply of transplants. If seed is to be stored for an extended period, proper storage conditions must be provided to maintain viability.
Saving your own seed is not recommended because of the potential for disease contamination, and because hybrid cultivars do not grow true-to-type.
Germination versus Seed Vigour
Germination refers to the percent germination of a lot of seeds under ideal conditions, while seed vigour refers to speed and uniformity of emergence, especially under less-than-ideal conditions. Seed companies have developed processes for enhancing the percent germination, vigour and uniformity of seed lots. The use of these enhanced seeds can help transplant growers maximize output per unit of greenhouse space.
When selecting a seeder, growers should consider a) ease of converting between different tray sizes; b) whether the seeder can handle raw seed as well as pelleted seeds; c) how many trays need to be seeded per day.
Plate-Type Vacuum Seeders
Needle or Nozzle-Type Seeders
Seeding Trays and Covering Seed
The trays are filled with premoistened growing media, then the media is compressed or "dibbled" to make a uniform surface for the seed. The media should be compressed between 1/4" to 3/8" deep. If seeded too shallow, certain crops (especially onions and Cole crops) will tend to push up out of the cell.
The seeded trays are then covered with medium-grade vermiculite. Seed of all vegetable species must be covered after seeding. Vermiculite is preferred because it is easy to apply evenly, allows good aeration, does not support algae growth and does not allow root growth between cells.
The trays are now ready for watering before being placed in the germination or "sweat" chamber. Hot water should be used to raise the temperature of the soilless mix and start the germination process as soon as possible.
Most vegetable crops will benefit from the use of a germination chamber. This is usually an insulated room in which temperature and relative humidity can be maintained at a precise level. The goal is to facilitate the germination process in a confined area to minimize the cost of heating a large greenhouse to germination temperature.
Air circulation is important to ensure uniform temperature and humidity throughout the chamber. There should be a thermostat to maintain the temperature regime. If the temperature goes too high, the variability between the seeds is accentuated, resulting in both uneven germination and transplant development.
Optimum temperature and time for germination vary for different vegetable crops. Germination conditions for the major transplanted vegetables are listed in Table 4. Germination time will vary between seed lots, so growers should check the trays regularly while they are in the germination chamber. The trays should be moved to the greenhouse after the seed coat has cracked and the shoot just starts to emerge. This will prevent excessive elongation. The time in the sweat chamber may only be 2 or 3 days.
Tempered Water: If possible, warm water should be used when watering plants during early growth-stages. Water should be heated to about 21°C. Heating the volume of water required to irrigate a large greenhouse may not be feasible.
Vegetable transplant growers should use a rack system for benching in the greenhouse and for moving transplants to the field. Plug trays are usually handled on racks made of either angle-iron or wood with wire-mesh tops.
Proper placement of racks in the greenhouse is important.
Trays must be elevated off the ground to prevent root growth through the bottom. Trays should be level to prevent water from running into low spots.
The growing-on stage may be defined as the stage of growth of transplants from after emergence until the plants are hardened-off before field setting. During the growing-on stage, environmental conditions (temperature, ventilation, light), watering and fertility, all affect the growth and quality of the transplants.
Trying to do a good job in raising different crops within the same greenhouse environment can be a challenging process. The environmental requirements for one crop may not suit those for another. For example, cabbage transplants require relatively cool temperatures and low fertility levels, while pepper transplants require higher temperatures and more fertility. Wherever possible, crops with dissimilar requirements should be grown in different areas of the greenhouse where they can be managed as required.
Different vegetable crops vary in their response to temperature. The optimum day and night growing temperatures for several crops are listed in Table 4.
Warm-season vegetable crops (tomatoes, peppers, eggplant and vine crops) are susceptible to a disorder known as chilling injury. Chilling occurs when transplants are exposed to temperatures above freezing but below 10°C for an extended period. Chilling causes stunting of growth and can have a long lasting effect on field establishment. For susceptible crops, maintain a minimum greenhouse temperature of 10°C.
The DIF (difference) Method is a method of managing greenhouse day/night temperatures to control plant height. The DIF is determined by subtracting the nighttime from the daytime temperature. A higher day temperature gives a positive DIF and promotes growth while a lower day temperature gives a negative DIF which retards growth.
High temperatures during the first three to four hours after sunrise can cause considerable stretching in vegetable transplants. This stretching can be reduced by keeping the greenhouse temperature cooler during the morning hours compared to the nighttime temperature (negative DIF). When cooling the greenhouse to a negative DIF, be careful to avoid chilling temperatures. Usually, 4-5°C negative DIF will give good height control.
Table 4. Optimum temperature ranges for germination of seeds and growth of various vegetable transplants.
Height control is important because elongated transplants are more susceptible to stresses after field planting. Excessive stem elongation is caused by the following factors: too much heat, overfertilizing, overwatering and low light conditions.
"Pillowing" is uneven growth that results from differences in air circulation and watering between cells within trays. Usually, the outside cells are drier and growth is reduced compared to the plants located in the centre of trays. The problem can become exaggerated when larger plants begin to shade the growing media while the shorter plants are still letting in more sun and allowing more evaporation. As a result, individual flats become dome-shaped or "pillowed".
This problem tends to start either where flats are not placed tightly against one another, allowing for greater air flow and a greater degree of drying, or where the sun warms up one side of the tray disproportionately compared to the other sides such as along a walkway.
Prior to the growing season it is advisable to have a detailed water analysis completed. The greenhouse fertilizer program may have to be adjusted according to the pH, bicarbonate level and nutrient content of the water supply. If high-quality water is not available, an alternative water supply should be obtained.
Complete water analysis should be done every year since water can vary considerably over time. This will be especially true where water is taken from shallow wells or high water-table areas.
The pH of the water used for watering plug transplants should be 5.5 to 6.5. At these levels, micronutrients are more available. Water from ponds and wells is often alkaline (pH higher than 7.0) and should be treated with acid to lower the pH.
Bicarbonates is a measure of water "hardness", and will indicate the amount of acid required. A water sample containing 90 PPM bicarbonates is considered soft, while 350 PPM is considered to be very hard. Both of these samples may have the same pH, for example, but the 350 PPM sample would require more acid to "soften" the water. Note: nitric acid is a more dangerous acid than phosphoric.
The bicarbonate level of the irrigation water is best in the 60-100 PPM range in order to avoid big changes in pH when some types of fertilizers (ammonium) are added. Most often, the complete water test will show a level of 200-350 PPM bicarbonate in the raw water and therefore the need exists to lower it.
Every 60 PPM of bicarbonate to be neutralized requires 7 L of phosphoric acid (85%) per 100,000 L of water or 7 L of nitric acid (67%). The maximum amount of phosphoric acid used should only be 7 L per 100,000 L since more will provide excessive amounts of phosphorus that will cause crop problems (stretching). Nitric acid is the most common choice for neutralizing more than 60 PPM bicarbonate. Each 7 L of nitric acid will contribute 14 PPM of nitrate nitrogen to the irrigation water.
Sample calculation: Using a 1:100 injector to reduce the bicarbonate level by 180 PPM will require a total of 21 L of nitric acid per 100,000 L of water (or 21 ml per 100 L). Therefore, add 21 ml of nitric acid for each litre of water in the concentrate barrel. This acid addition will not only reduce the bicarbonate levels by 180 PPM, but will also provide 42 PPM (3 x 14) of nitrate nitrogen.
Occasionally, a grower will collect rain water for crop irrigation and normally find that the bicarbonates are very low (usually under 10 PPM).
In this situation, add bicarbonates in order to avoid pH fluctuations with the addition of some fertilizers. Potassium bicarbonate should be added to the irrigation water to give about 60 PPM of bicarbonate. The rate is 1 kg per 100,000 L of water.
The EC (electrical conductivity) or Total Salts of water is a measurement of the total nutrition present (combination of all cations and anions). Levels between 1.0 and 2.0 are considered to be ideal. Lower levels will starve plants while higher levels may burn the roots due to high salt contents. Levels for "raw" water are best below 0.6 mmho/cm and when fertilizer is added will result in the 1.5 range.
Nitrate levels can be as high as 50 PPM or more due to ground-water contamination. At these levels, additional nitrogen should be used sparingly. In any event, it is important to know what the starting point is before adding extra nitrogen to the water. Ordinarily, water quality is usually less than 20 PPM nitrate nitrogen.
Note: Water that is high in nitrates should not be used to drink. The health guide suggests maximum levels of 10 PPM!
Sulfates in water commonly range between 150 to 250 PPM A sample of water testing at 1300 PPM, for example, would have a very noticeable sulfur smell. Chlorides ordinarily range between 20 to 400 PPM
The amount and frequency of watering will vary depending on cell type, growing media, greenhouse ventilation and weather conditions. It is important to water thoroughly, and moisten the entire plug, which will promote root growth to the bottom of the plug. If the plug is not watered thoroughly, root growth will be confined to the top of the plug. Allow the plug to dry down before watering, but do not let the plant wilt severely, as this will damage roots.
Plug transplants should be watered thoroughly in the morning, but should not be watered late in the afternoon. If the plants remain wet overnight, disease problems increase.
If an overhead watering boom is used, it is advisable to remove and rearrange the nozzles occasionally to avoid the "streaking" that results from variations in output from different nozzles.
The fertility program used in raising vegetable transplants affects the quality of the finished transplant and its ability to become established in the field. A well-grown transplant will have adequate nutrient reserves to ensure rapid establishment under a wide variety of field conditions.
Vegetable transplants are usually fertilized with a soluble fertilizer which is applied in the irrigation water. Several of the fertilizer analyses that are recommended for transplant production are listed in Table 5. These materials vary in percent nitrogen (N), phosphate (P2O5), and potash (K2O); and in the micronutrient content. Growers should use fertilizers that have most of the nitrogen in nitrate form. Avoid fertilizers with a high concentration of urea.
Table 5. Concentrations of N, P, K and ECs for 100 PPM solutions of various soluble fertilizer materials that are suitable for use in vegetable transplant production.
1Electrical Conductivity of a 100 PPM solution in micromhos. The EC values were determined with a conductivity meter using distilled water. The EC values obtained will vary depending on the background EC of the water source.
A high concentration of phosphate (P205) may promote excessive seedling elongation under certain conditions. Use a fertilizer with a low to medium phosphate concentration. An alternative is to use a fertilizer with no phosphate (such as 14-0-14) for most feedings, and apply a high-phosphate fertilizer periodically (once every four or five feedings) to promote growth. Do not withhold phosphate completely, as this will delay field establishment.
Transplants should be watered as required (see Watering Transplants) and the nutrient solution concentration and application frequency should be modified to promote the desired amount of growth. Fertilizer requirements vary depending on cell size (larger cells require less fertilizer), and the nutrient charge of the growing media (less fertilizer should be used if the media have a high nutrient charge).
Fertility Requirements of Vegetable Crops
Different vegetable crops vary in their response to fertilizer, so the feeding program must be modified for different crops:
Tomatoes are very responsive to fertilizer and excess fertility will reduce transplant quality. If feeding at every watering, use a fertilizer concentration of 50 to 100 PPM N, depending on the stage of plant development. There may be some advantage to feeding less often at a higher concentration. If feeding once every seven days, use a concentration of 250 to 350 PPM N.
Peppers require more fertilizer than tomatoes. If feeding at every watering, use approximately 100 PPM N. and increase the concentration if feeding less often.
Cole Crops do not require as much fertilizer as other crops. One application per week of 100 to 150 PPM N should be sufficient under most conditions.
Vine Crops have a relatively short growing cycle compared to other crops. Two to four applications of fertilizer at weekly intervals, at a 100 to 150 PPM N concentration, should be sufficient to produce good-quality vine crop transplants.
Calculation of Fertilizer Solution Concentration
The concentration of fertilizer solutions is usually expressed in parts-per-million (PPM) of nitrogen. To determine how much fertilizer material is required to produce a solution of a desired concentration, use the following formula:
weight of fertilizer (grams) =
solution concentration (PPM) x solution volume (litres)
10 x (% N of fertilizer material)
For example, to make a 100 PPM solution of 20-10-20 fertilizer in a 500-litre tank, the amount of fertilizer required is
weight of fertilizer = (100 x 500) / (10 x 20)
weight of fertilizer required = 250 grams
If using a fertilizer injector, to determine the amount of fertilizer material required in the stock solution:
The concentration of the final nutrient solution can be checked using Electrical Conductivity (EC). Conductivity meters are available from most greenhouse-supply companies. Approximate ECs for several different fertilizer materials are listed in Table 5. Subtract the background EC of the water from the fertilizer solution EC to compare with the values in Table 5.
Toxicity or Deficiency
More times than not, plants show signs of nutrient deficiencies as opposed to nutrient toxicities. Most growers are very cautious about over-fertilizing in fear of "burning" the roots or stems. Many times, the application of fertilizer is delayed well beyond the time in which plants could use some nutrition.
Transplants that are starved for phosphorous will show very definite purple coloration along the stem and underside of leaves. Plants that have too little nitrogen will have pale-green foliage. However, too much nitrogen will result in very white stems and dark-green leaves.
The primary means of controlling diseases on vegetable transplants is by sanitation and by managing the greenhouse environment to suppress disease development.
Plugs should be watered thoroughly in the morning and "spot watered" in mid-afternoon if necessary. Do not water plug plants in the evening. Foliage should be dry overnight to reduce the development of foliar fungal disease.
Maintain a slow, steady growth rate for the transplants. Plants that are severely held back are more susceptible to many types of disease.
Refer to OMAFRA Publication 838 or 835 for registered products, rates and application information for pest management for vegetable transplants in the greenhouse.
Caution: Do not apply foliar pesticides under high-temperature conditions as this may injure foliage. Unless otherwise noted on the pesticide label, use only sufficient water to wet the foliage, to maintain the product on the foliage and prevent the solution from dripping into the growing media and possibly damaging roots.
Greenhouses should be equipped with a sprayer dedicated to herbicides only, with another sprayer for fungicides and insecticides. Otherwise, contamination may occur and cause serious damage to greenhouse plants.
Weeds outside the greenhouse should not be sprayed during the growing season since herbicide sprays or fumes may drift or be sucked into the greenhouse environment. Transplant trays that are left outside on headlands can be inadvertently sprayed during the growing season. Trays that are stored near herbicides can also absorb fumes. Once contaminated, these trays cannot be cleaned effectively.
Heating: Incomplete Combustion
Very often, expensive greenhouses may be outfitted with used and faulty heating equipment. Natural gas, propane or oil when not completely burned or ventilated properly - will give off gases that are injurious to plants.
Young transplants are especially tender and sensitive to byproducts of incomplete combustion. These plants will usually turn a yellow color and become stunted. Plants often appear very pale yellow in color and the leaf margins and tips can become dried and blackened. Tomato plants are "reliable" indicators of this.
An outside air source should be available to high-efficiency furnaces. Otherwise, when the greenhouse is closed up tight, there may be a shortage of oxygen, especially in the warm end of the greenhouse. Provide fresh air according to the size of furnace, and deliver it to the mouth of the burner.
Fungus Gnats & Shore Flies
These insects were generally regarded as "nuisance pests" until recently. They are now found to cause serious damage to roots while feeding during their larval stage, and have been found to spread Pythium and Rhizoctonia diseases within the soil.
These pests often flourish in damp parts of the greenhouse and where algae and moss are allowed to grow, and will spend part of their lifecycle in the soil beneath the benches. Fungus gnats are more troublesome than shore flies, but both will feed on roots during their larval stage. Young seedling plants have very little root mass and therefore any feeding can potentially be quite serious.
Algae can become a problem on the surface of soilless mixes in that it tends to form a crust, making it difficult to wet the soilless mix. This can happen while plants have already emerged and are growing, or where seeds have yet to germinate or to break through the soil surface. The use of medium-grade vermiculite spread over the surface of newly seeded trays will help prevent algae from forming.
Algae is more of a problem during early cool, damp and overcast weather. Helping seeds emerge quickly will reduce the effect of algae buildup, and help eliminate the possibility of soil crusting before seedlings have the chance to emerge. Therefore, the use of "sweat chambers" will increase the uniformity of seedling emergence and allow for faster plant growth.
The use of hydrated lime spread underneath the benches will prevent algae from growing and eliminate sites for fungus gnats and shore flies to live.
Refer to OMAFRA Publication 835 for recommendations for algae control in the greenhouse.
Hardening-off transplants is important, especially if they are to be planted under stressful, early-season conditions. Vegetable transplants should be hardened-off by:
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