Cooperative Extension Fact Sheet FS787  |  February 2014

Starting Vegetable Seeds Indoors

  • Peter Nitzsche, Agricultural Agent, Morris County
  • Steve Reiners, Extension Specialist in Vegetable Crops

Introduction

The easiest way to start any vegetable garden is direct seeding—wait until the weather warms and plant the seeds directly in the garden. Unfortunately, direct seeding is not practical for some crops. For example, tomatoes and peppers cannot be planted until after the last frost and after the soil has warmed. If seeded directly in the garden at that time, tomatoes and peppers require more than 100 days to produce the first fruit. In addition, newly emerged seedlings are very tender and easily killed by insects and disease or shaded by quicker growing weeds. Transplanting hardier young seedlings started indoors (transplants) allows an earlier start in the garden, which results in earlier yields of certain crops and makes better use of limited garden space. If a few simple guidelines are followed, transplants can be grown with a minimum of problems.

Potting Soil

Choosing the right potting soil is a very important part of starting seedlings indoors. The most convenient type to use is one of the commercial soilless mixes (Jiffy-Mix, Pro-Mix, Redi-Earth, etc.) that contain peat moss, perlite, vermiculite, and usually some fertilizer (see Table 1 for a mix recipe). These are specially formulated for starting seeds indoors. Soilless mixes are sterile and reduce the risk of losing seedlings to damping off, a fatal soilborne disease. Using soil directly from the garden is not recommended, because it may contain insects, weed seeds, or disease organisms that could damage or kill the young transplants. If garden soil is to be used, combine it with equal parts of sand and peat moss to improve drainage and structure. A teaspoon of ground limestone per quart of mix should also be added to obtain the proper pH. The mix must then be pasteurized to eliminate pests by heating to 180oF for 30 minutes. Heated soil gives off a strong smell, so consider doing it outdoors.

Getting Started

Containers, pots, flats, etc., are another essential component for starting transplants. When choosing a container to start your seedlings (Table 2), the most important factor is that the bottom have drainage holes. A lack of drainage can cause the soil to become saturated and can encourage root rot diseases in the transplants.

Before doing any planting, one word of caution: DON'T START PLANTS TOO EARLY. Plants that are started too early become elongated, pale green, and weak. The goal is to produce a stocky, moderately sized plant that will recover quickly when it is planted outdoors.

Table 3 lists how many weeks pass between seeding indoors and transplants are ready for planting in your garden. This period varies widely by crop, ranging from 2 to 12 weeks. Here is an example using Table 3. Suppose tomato is the crop to be grown. Table 3 indicates that tomatoes are a warm season crop, which means they cannot be planted until after the last frost. Assume May 15 is the last frost date for the garden. Because tomatoes take about 6 to 8 weeks from seeding to transplant, seeding indoors should take place between mid-March and April 1. If the crop to be planted were a cool season crop like lettuce, planting in the garden would occur in mid-April. That would mean seeding 5 to 6 weeks before that date, or roughly early to mid-March.

Crops are also listed as being warm or cold season. Warm season crops cannot be transplanted into the garden until all danger of frost is past. In New Jersey, this will range from April 20 in extreme southern New Jersey to June 1 in the colder northwest. Contact the county Rutgers Cooperative Extension office (listed under county government in the phonebook) for the last frost date in the area. Cool season crops withstand frost and may be planted outside much earlier, usually in mid to late April.

Next to each vegetable in Table 3 is listed the ability of that crop to transplant. Some are very easy to establish as transplants, such as tomatoes and lettuce. Others, however, do not do nearly as well. Those listed as poor, such as beans or corn, need great care and a minimum of handling. The roots of these plants are easily damaged. To start these indoors, use peat pots or pellets to minimize root damage. Some vegetables, like carrots and beets and peas, should always be seeded directly in the garden.

Place seeds in containers at the depths recommended in the chart. Plant one or two seeds per individual container or, if using flats, in rows that can be thinned or transplanted into individual containers following germination. Be sure to label the flats to avoid confusion, using a pencil or water resistant marker. Once the seeds are planted, the container should be bottom watered by placing it in a shallow pan of water and waiting until the surface of the mix is moist. This method avoids overhead sprinkling, which can carry away some smaller seeds. The pot should then be removed from the pan and allowed to drain.

Germination is aided by maintaining high moisture levels in the mix and moderately high soil temperatures (Table 3). Sealing the container in a clear, plastic storage bag until seedlings emerge will keep the soil moist. Placing the starting containers in a warm place or on specially designed heating mats, available at some garden centers, can speed up germination. Once the seedlings emerge, remove the plastic bag and give the seedlings as much light as possible by placing them on a sunny window sill or 4 to 6 inches below a fluorescent light. Maintain room temperatures between 60 to 70oF. Higher temperatures or lack of light will promote unwanted, leggy growth. Seedlings can be lightly fertilized by watering with soluble fertilizers, such as Miracle-Gro or Peters, following the directions on the package.

When the seedlings develop their first true leaves, transplant those started in flats to individual containers. Dig the seedlings out of the mix with a small trowel or spoon. When transplanting, always handle the seedlings by the leaves, being careful not to damage the fragile seedling stem or root system.

One week before the seedlings are transplanted, they should be hardened off to better acclimate the transplants to outdoor conditions. Harden plants outdoors in an area protected from wind, and subject them to longer doses of sunlight each day, while cutting back on watering. One exception, harden tomatoes by reducing water only. Tomatoes exposed to cool temperatures may a develop a disorder called catfacing which causes the fruit to be misshapen.

After hardening, the seedlings are ready to be transplanted into the garden at the spacing indicated on the seed packet or the fact sheet Planning a Vegetable Garden FS129). Seedlings should be planted at the same depth at which they were growing indoors, except for tomatoes, which may be planted deeper. Firm the soil around the root ball, and water immediately with a solution of water and starter fertilizer. Try to transplant on a cloudy day to minimize wilting or transplant shock. If it's sunny, provide the plants with some shade.

Table 1. Simple plant growing mix.a
Shredded sphagnum peat moss 10 gallons
No. 2, 3, or 4 domestic or African vermiculiteb
(horticultural grade, dust screened)
10 gallons
Pulverized Limestone
  Dolomitic Lime for mixes with domestic vermiculite
    or
  Calcitic Lime for mixes with African vermiculite
1 1/4 cups
  or
3/4 cups
Superphosphate (20% P)
  or
Triple superphosphate (46%)
1/2 cup
  or
1/4 cup
Fertilizer (5-10-10) 10 gallons 1 cup

aThis mix is used for germination. Supplemental fertilizer will be needed to grow plants to transplant size. Three weeks after seeding, apply soluble fertilizer like 20-20-20, 2 tsp/gallon of water. Apply weekly.

bVermiculite should be pea size and free of dust. Final pH should be 6.0-6.5.

Table 2. Types of containers used for starting vegetable seedlings.
Types Comments
Peat Pots Made from compressed peat moss. These pots are filled with mix and seeded. The whole pot is then planted with the seedling. When planting outside, make sure the entire peat pot is covered with soil to avoid drying out.
Peat Pellets Compressed peat which expands when placed in water. Seeds are placed directly in the pellet after it has expanded. The entire pellet is placed in the soil and covered like peat pots. Peat pots and pellets are recommended for seedlings that transplant poorly since roots remain relatively undisturbed.
Plastic Pots and Flats These are filled with mix and seeded. When planting, carefully slide the seedling out of the container. Plastic flats can be reused if cleaned after use with a 1:10 solution of household bleach and water. Soak them in this solution for 10 minutes. Allow them to thoroughly dry before using. This will eliminate any disease problems
Table 3. Recommendations for starting vegetable seeds indoors.
Vegetable Ability to Transplant Weeks to Grow Depth of Seed (in.) Seasona Optimum Soil Germination Temp. (°F)
Beans, snap poorb 2-3 1 warm 80
Broccoli good 6-7 1/4 cool 80
Brussels Sprouts good 6-7 1/4 cool 80
Cabbage good 6-7 1/4 cool 80
Cauliflower good 6-7 1/4 cool 80
Celery good 10-12 1/8 cool 70
Collards good 5-7 1/4 cool 80
Corn, sweet poorb 2-3 1/2 warm 90
Cucumbers moderate 2-3 1/2 warm 90
Eggplant good 8-10 1/4 warm 85
Kohlrabi good 6-7 1/4 cool 80
Kale good 4-6 1/4 cool 80
Leeks good 10-12 1/8 cool 75
Lettuce good 5-6 1/4 cool 75
Melons moderate 2-3 3/4 warm 90
Okra good 2-3 3/4 warm 90
Onions good 10-12 1/4 cool 75
Peppers good 8-10 1/4 warm 85
Pumpkins moderate 2-3 1 warm 90
Squash moderate 2-3 1 warm 90
Tomatoes good 6-8 1/4 warm 85

aCool season refers to transplants that can tolerate frost. Warm season refers to plants that can not be transplanted until after all danger of frost and soil has warmed.

bTo minimize root disturbance, start these seeds in peat pots or peat pellets.


  1. Rutgers
  2. Executive Dean of Agriculture and Natural Resources
  3. School of Environmental and Biological Sciences
Rutgers New Jersey Agricultural Experiment Station