Lawn and Garden

Growing Vegetable Transplants

By Lewis W. Jett, WVU Extension Service commercial horticulture specialist

Hybrid seeds Transplants Classifying vegetables by season
Trays or containers Germination Lighting Watering and fertilizer

Most vegetables for the garden can be established as transplants. Transplants offer many advantages over direct seeding. Advantages of transplants include: (1) early maturity; (2) reduced seed cost; and (3) greater uniformity. Transplants also eliminate the need for thinning, which can be labor-intensive.

Successful vegetable gardening begins with choosing the best varieties. Gardeners should consult WVU Extension Service recommendations for varieties with high yield and disease tolerance.

Hybrid seeds

Generally speaking, hybrid seed has greater uniformity, vigor, and disease tolerance. Hybrid seed is produced by crossing two plants in the same species with superior traits. Seeds from hybrids do not breed true and thus cannot be collected and saved each year. Heirloom or heritage varieties can often produce excellent yields with good disease tolerance in many areas of West Virginia. There are many heirloom varieties of tomatoes and beans as well as other vegetables. However, there is some variability of performance of heirlooms across regions of West Virginia. Heirloom varieties breed true, and the seed can be saved at the end of the growing season.

Transplants

Good transplants start with good-quality seed. Purchase quality seed from either reputable garden centers or seed companies. Seed left-over from the previous growing season can be used if stored correctly (Table 1). Seed should be stored in airtight containers in cool, dry rooms. As seed is exposed to warm temperatures and moist conditions, it ages and its viability is reduced. Many vegetable seeds can be stored in the freezer.

Table 1. Storage life of vegetable seed under favorable conditions.

Vegetable

Storage life (years)

Bean

3

Beet

4

Broccoli

3

Carrot

3

Sweet Corn

2

Eggplant

4

Leek

2

Lettuce

6

Onion

1

Pea

3

Pepper

2

Pumpkin

4

Spinach

3

Tomato

4

Watermelon

4

Knott’s Handbook for Vegetable Growers, 4th ed.

Classifying vegetables by season

Vegetables are generally classified as warm- or cool-season vegetables.

Cool-season vegetables are hardy or frost-tolerant, and the seeds germinate at lower soil temperatures than do seeds of warm-season vegetables. Cool-season vegetables include many popular vegetables: beets, broccoli, cabbage, lettuce, kale, cauliflower, kohlrabi, leeks, onions, peas, spinach, and radishes. Many cool-season vegetables can be grown in the spring and fall in West Virginia (Table 2). Depending on location in the state and whether season-extending tools (e.g., row covers, low tunnels, or high tunnels) are used, most cool-season vegetable crops are transplanted two to six weeks before the last spring frost.

For fall production, cool-season vegetables are transplanted four to eight weeks before the first fall frost. To determine how late to plant in the fall in your region, there is a simple formula used by growers: Number of days from seeding to harvest + average harvest period + fall factor (2 weeks) + frost sensitivity. So, for example, cucumbers are 55 days from seeding to harvest + 21 days of harvest + 14 days for the lower light/temps of fall + 14 day frost buffer = seeding 104 days before the first frost date (October 15), or approximately July 1.

Warm-season vegetables are not frost-tolerant and must be transplanted after the last frost of the season. Transplanting before the frost-free date requires row covers, hot caps, or low tunnels to prevent damage to the plants. Warm-season vegetables include such crops as beans, tomatoes, melons, peppers, sweet corn, squash, and eggplants.

Table 2. Optimal seeding and transplanting date for vegetable crops in West Virginia.

Vegetable

Seeding date for transplants

Transplant age (weeks)

Transplanting date

Asparagus

February

12

April

Beet

January-February; August

6

March-April; August-September

Broccoli

February-March or July

6-8

April/August

Cabbage

February-March or July

6-8

April/August

Cantaloupe

March-April

6-8

April-May

Cauliflower

February-March or July

6-8

April/August

Corn, sweet

March-May

4

April-June

Cucumber

April or July

3-4

May or July

Eggplant

April-May

6-8

May-June

Kale

February or September

4-6

March or October

Leek

January-February

10-12

April-May

Lettuce

January-May

5-6

April-June; September

Pea

January-February; August

4

February-April; September

Squash

May-June

2-3

May-June

Tomato

March-May

6-8

May-June

Watermelon

April

4-6

May-June

Transplants for spring and fall production.

A number of germination mixes or media can be used to grow vegetable transplants. The medium must be sterile and free of weed seeds and diseases. For small-seeded vegetables such as lettuce, which do not need to be covered, vermiculite can be used. Most media contain a mixture of peat moss and sand or perlite for aeration. Mixes can be purchased or made by the gardener. Many recipes are available for making germination and transplant media from compost, topsoil, and peat moss.

Trays or containers

Many trays or containers for growing the plants are available. Growing trays with individual cells for each seed can be purchased at most garden stores. Germination “flats” or trays also work very well.

Peat pots/pellets and other biodegradable products, such as “CowPots” made from composted cow manure, can also be used. Peat pots tend to lose water more rapidly than plastic or foam transplant trays.

Soil blocks can be made by pressing peat moss into cubes in which the seeds are sown. Using soil blocks saves the cost of buying new containers each year.

Flower pots, milk/juice cartons, newspaper, pots, and pie pans also make good transplant containers. If trays are being reused, make sure they have been properly washed, preferably with a 5% solution of bleach and water.

If using germination flats or trays, broadcast the seeds over the medium and lightly cover and water them. After about two weeks, prick the seedlings out of the flats and transfer them to larger growing containers. Germination flats can be used with most warm- and cool-season vegetables.

For other types of containers, sow individual seeds directly into the germination medium. Small seeds such as lettuce are seeded 1/8 inch deep at most. Larger seed vegetables are seeded about ¼ to ½ inch deep. Seeds should not be planted too deep. Large seeds such as tomatoes and melons are seeded in individual cells or containers. Put two or three seeds in a cell. When they emerge, thin them to the most vigorous seedling. Tomatoes, melons, eggplants, and peppers do very well when grown in relatively large planting containers (Figure 1).

If plants are crowded in the transplant container, they tend to get tall or “leggy,” which results in a weak transplant. After the seeds have been sown, the germination mix should be kept moist but not overwatered. During cloudy weather, less water will be needed.

Germination

Proper temperature is required for good germination. Heating or germination mats will provide bottom heat and accelerate germination (Figure 2). Plastic tops can be placed over the trays to trap heat. Until seedlings emerge, the trays should be placed in the warmest area available. After emergence, the seedlings can be moved to a cooler (65-70F day; 55-60°F night) environment. Bright light will accelerate even growth, so a south-facing window or supplemental lights will be useful.


transplant 1 transplant 1 Figure 1. Warm-season vegetables such as tomatoes should be grown in large cells or trays in order to produce large, stocky transplants.


transplant 1 transplant 1 Figure 2. Heat mats (left) and cold frames (right) can be used to accelerate germination of warm-season vegetables.

Lighting

Supplemental lights can be used for 14 hours per day. Brushing the transplants once or twice daily helps to strengthen the stem. A small fan can be used to blow air on the transplants for an hour or two each day to simulate wind and make the stems stronger.

Watering and fertilizer

Avoid overwatering the seedlings because this will make the plants long and create disease problems. Watering should be done in the morning or afternoon to allow the plants to dry before nighttime. A weak fertilizer solution (1-2 ounces of 20-20-20/5 gallons of water) can be used each week to provide nutrients to the plants. After being fertilized, the leaves should be rinsed with water.

Approximately two weeks before being transplanted in the garden, the plants should be hardened off to reduce the stress of transplanting. Hardening off can be accomplished by placing the young transplants in a cold frame or taking them outside for a couple of hours each day. Reduce applications of fertilizers. Exposing the transplants to outside conditions toughens them up for planting in the garden. Do not expose the transplants to low (< 50°F) temperatures during this time. Low temperatures can injure transplants and delay their establishment after transplanting.

Avoid using old transplants. Transplants that are too old become pot-bound in the planting trays. The young transplants can be carefully pulled out of the tray and roots inspected. The roots should be whitish in color and not wrapped around the rootball. When transplants reach the optimal size and age, they are ready to be transplanted in the garden. The soil should be tilled and mulched in advance of planting. Avoid planting in the morning and midday during hot weather. Establishing transplants in the late afternoon exposes the plants to less stress, giving them a greater chance of survival. Transplants should be planted deeper than the plants were growing in the transplant tray.

Immediately after the plants are transplanted, a starter fertilizer (1 cup/plant) can be applied to each plant. Common starter fertilizers are soluble in water and contain phosphorus, which helps new root establishment. Row covers or hot caps may be needed to provide frost protection. Over time with good care, the vegetables will produce a bountiful yield of nutritious food.