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Transplant Production Approaches


Growers use transplants to grow long-season crops in short-season areas, improve land use efficiency, save costs when growing expensive hybrid seeds, and get early production for early markets. Using transplants can improve water savings, manage early weed problems more efficiently, ensure more uniform production, and assure better stands.

High quality transplants are almost always grown in Production heated greenhouses where growing conditions are carefully managed. To grow quality transplants, it is important to optimize inputs like growing media, temperature, fertilization, water, and spacing needs. Table 1.1 provides seed spacing and temperatures for seed germination and plant growing, and the time required to grow the plant to transplantable size. Quality plants are grown by using the appropriate trays and soil media, controlling germination, temperature and nutrients, and properly conditioning the plants for the field.

Flats, Trays, and Pots

Use new flats and liners for transplant production to avoid pathogens that cause damping-off and other diseases. If old trays or liners are used, they should be thoroughly cleaned. Dip them in 10% chlorine bleach several times, then cover with plastic to keep them wet overnight. The bleach solution should remain below pH 6.8 to effectively kill disease pathogens (make a new bleach solution every 2 hours or whenever it becomes contaminated or diluted). Wash the trays with clean water to eliminate the chlorine, and let the flats dry prior to use. Wash exposed surfaces like benches, frames, and walls in the greenhouse to sterilize them as well. If plastic pots are reused, disinfest them as described above.

Seedling performance depends on cell size. Generally, transplants grown in larger cells (50’s, 72’s) produce earlier yields. Cell size does not affect total yield when growing seasons are long. If earliness is important, use larger cell sizes or bigger pots. While you may grow more plants per unit area of greenhouse in small cells (128’s, 256’s) and keep costs down, these trays may not be appropriate for some vegetables like melons. Transplant production cost depends on the number of plants grown per unit area and the length of time needed to grow the plant to plantable size.

Plant-Growing Mixes

There are many different pre-mixed growing media available and the best are lightweight, disease-free, and made from peat and vermiculite. Most commercial mixes produce quality transplants when used with good management practices. Commercial mixes can vary in composition, particle size, pH, aeration, nutrient content, and water-holding capacity. Most growers find a mix that works well for them and then continue to use it year after year. Avoid fine particle mixes which may hold excessive water and have poor aeration. If switching mixes, have them tested to determine the pH and nutrient levels in the media. Some growers blend their own media to reduce cost and to create a uniform, consistent composition. See Table 1.2 for some simple conventional and organic transplant growing mixes.


Table 1.1. General recommendations for growing transplants from seed.

Crop Seeds / ft2 Seeding
Depth
(inches)
Optimum
Germination
(F)
Germination
Time
(days)
Optimal Growth
Temperature
Grow
Time**
(weeks)
Day
(F)
Night
(F)
 Asparagus 36 1/4 - 1/2  75  8-10  65-70  60 8-12 
Broccoli  48   1/4 85   65-70 60  5-7 
Cabbage  48  1/4  85   65-70 60  5-7 
Other Brassicas  48  1/4  80-85  4-6   65-70 60  6-8 
Cantaloupe  36   1/2 90  75-80  65  3-4 
Cucumber  36  1/2  90  70-75  65  3-4 
Eggplant  36  1/4  85  5-6  75-85  65  6-8 
Endive  60  1/4  70-75  65-70  60  4-6 
Lettuce 60-80 1/4 70-75 3 60-65 45 4-6
Onions 80-100 1/4 75 4-6 65-70 60 8-12
Other Leafy Greens 60-80 1/4 70-75 3-5 60-70 45-60 4-6
Peppers 36 1/4 85 8 75-80 65 7-9
Summer Squash 36 1/2 90 3 70-75 65 3-4
Sweet Potato --- 1/4 --- --- 75-85 65 5-6
Tomato 36 1/4 85 5-6 65-75 60 6-8
Watermelon 36 1/2 90 3 75-80 65 3-4

**Average number of weeks required to grow to transplantable size. Note: Temperature, light levels, nutrients, and other factors can influence grow times.


Transplant mixes for pots, flats, or transplants trays.

Tipi Potting Mix Recipe(Organic) Organic Potting Mix
  • 2 bales sphagnum peat moss (3.8 or 4.0 cubic foot bales)
  • 1 bag coarse vermiculite (4.0 cubic foot bags)
  • 1 bag coarse perlite (4.0 cubic foot bags)
  • 6 quarts of a fertilizing mix comprised of:
  • 15 parts steamed bone meal
  • 10 parts kelp meal
  • 10 parts blood meal
  • 5 to 10 parts dolomitic limestone (80 to 90 mesh)
  • 1 part sphagnum peat
  • 1 part peat humus (short fiber)
  • 1 part compost
  • 1 part sharp sand (builder’s)
  • To every 80 quarts of this add:
    • 1 cup greensand
    • 1 cup colloidal phosphate
    • 1½ to 2 cups crab meal or blood meal
    • ½ cup lime
Standard Vegetable Transplant Mix Organic Soil Blocking Mix
    Equal parts by volume of:
  • Vermiculite
  • Peat moss
  • Perlite
  • (use common liquid feeding program after seedlings emerge)
  • 3 buckets (10-quart bucket) brown peat
  • 1/2 cup lime (mix well)
  • 2 buckets coarse sand or perlite
  • 3 cups base fertilizer (mix equal parts blood meal, colloidal phosphate, and greensand together)
  • 1 bucket good garden soil
  • 2 buckets quality compost
Mix all components thoroughly and moisten to point where blocks hold together.

Seed Germination

Consult Table 1.1 for the optimum temperatures for seed germination. Since vegetables differ in their temperature needs, it is difficult to grow a wide variety of crops in limited greenhouse space due to the different environmental requirements of each.

Seeds that are planted to be "pricked out and repotted" at a later date should be germinated in 100% vermiculite (horticultural grade, coarse sand size) or a high quality commercial plant growing mix. Add fertilizer after the seed leaves (cotyledons) are fully expanded. Use a half-rate of a liquid formulation (Table 1.3). Seedlings can be held for 3 to 4 weeks if fertilization is withheld until 3 to 4 days before "pricking out."

Seed sown directly into trays or pots can be germinated in a mix containing fertilizer. For fast, uniform seedling emergence, germinate and grow seedlings on benches or in a floor-heated greenhouse at the recommended temperature. Research has shown that germinating the seedlings at higher than recommended temperatures for too long results in etiolated (elongated) hypocotyls (stem under the seed leaves). These seedlings tend to be weak and more prone to problems.

Transplants are used to grow long season crops in short season areas, and get early production for early markets.

A germination chamber will better control heat when floor or bench heat is not available. Flats, trays or pots are seeded, watered, and then stacked in the chamber for germination. When using this method be sure to remove the trays from the chamber and un-stack them before the seedlings emerge.

Greenhouse Management

Good greenhouses provide maximum light, have soil heating capabilities, and provide good heating and ventilation systems for effective environmental control. Proper growing temperatures ensure uniform growth throughout the greenhouse.

Properly maintained heating systems ensure energy savings and creates the environmental conditions required for germination and seedling growth. Invest in good heating and ventilation thermostats so that greenhouse temperatures are properly maintained. Heating or ventilation systems that don’t work properly may cause yellowing, stunting, or death of the seedlings.

Transplant Nutrition

There are many different commercial fertilizer formulations available. Supplemental nutrients are needed to augment the fertilizers added to the media. Commercial fertilizers should be 100 percent water soluble and applied 1-2x per week to maintain steady growth. Use additional feeding to accelerate growth. Always rinse the leaves after liquid feeding. Higher amounts of fertilizer added to the irrigation water is not recommended since root “burn” may occur due to fertilizer concentration and salt buildup. When mixing starter solutions for field transplanting, follow recommendation printed on the fertilizer bag.

Common liquid fertilizer formulations and recommended amounts.

Formulation Rate
20-20-20 1-2 oz/5 gal water
15-15-15 2 oz/5 gal water
15-30-15 2 oz/5 gal water

Watering

Keep soil mix moist, but not wet. Water in the morning when possible. This allows the leaves to dry before night and reduces disease. Water less in cloudy weather. Water just enough to ensure some drainage as this helps reduce fertilizer salt buildup. Remember that plants grown in small cells may require several watering each day while plants growing in large pots generally need less frequent irrigations.

Hardening / Conditioning

Special treatments, called hardening/conditioning, are used to slow seedling growth before transplanting. Hardening thickens the cuticle, increases leaf wax, and increases dry matter and carbohydrate levels in the seedlings. Ideally, hardened or conditioned seedlings can take the harsh conditions in the field (temperature extremes, water stress, wind, pests, etc.).

Generally, hardening treatments are imposed about 7 to 10 days before field planting, and include:

  • reducing the amount of water provided to the plant
  • lowering the growing temperatures
  • limiting the amount of fertilizer

When hardening vine crops, tomatoes, peppers, or eggplants, do not lower temperature more than 5°F below the recommended minimum growing temperatures (see Table 1.1). Exposing warm season vegetables to low temperature (less than 45°F) can cause chilling injury, which delays growth after transplanting. Biennial vegetables (cabbage, onion, endive, chard or celery) should only be waterhardened, as cold treatments may induce vernalization and promote premature flowering. Do not over- or under-harden as plant re-growth may be slowed under field conditions.