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Grow Tomatoes from Seed

Tomato seedlings in Cowpots

A crop of tomato seedlings growing in Biodegradable Cowpots.

If you've never bitten into a fragrant, vine-ripened, sun-warmed tomato harvested fresh from your own garden, you haven't tasted a real tomato. And once you do, you'll never again be satisfied with the mealy supermarket impostors. Fortunately, tomato plants are easy to grow and remarkably productive.

Tomatoes are long-season, heat-loving plants that won't tolerate frost, so it's best to set them into the garden as transplants (young plants) after the weather has warmed up in spring. You can purchase tomato transplants, but there's something especially rewarding about starting your own plants indoors. Plus, by growing your own transplants you can choose from among hundreds of tomato varieties that are available as seed but rarely sold as transplants.

Quick to germinate and grow, tomato seeds are best sown indoors about six weeks before your average last frost date. (To determine your last frost date, ask a gardening neighbor or call your Cooperative Extension office.)

Here's what you'll need to start tomato seeds indoors:

Seeds
There are hundreds of tomato varieties available as seed, and choosing a few for your home garden can be a daunting task. Here are a few things to consider:

  • Decide on the type of tomatoes you want; for example, cherry tomatoes, slicers or tomatoes for making sauce or paste.
  • Consider the size of the mature plants. "Determinate" tomatoes grow to about 3 feet tall and are the best choice for containers. "Indeterminate" tomatoes get very large, up to 6 feet tall. (All tomatoes benefit from support with cages or ladders.)
  • Look for disease-resistance. Tomatoes are susceptible to a number of diseases that may or may not be a problem in your region. To be on the safe side, I always grow at least one variety with resistance to verticillium and fusarium (designated by a V or F after the variety name).

For more detailed information, read How to Select the Right Tomato Variety.

Seedstarting Mix
Always use a sterile, soilless mix to start seeds. Avoid garden soil, which often drains poorly and may harbor disease organisms. Look for mixes labeled for seedstarting, such as our Germinating Mix; it has a fine texture that's perfect for delicate seedlings. (Learn more: Choosing the Right Soil for Seedstarting.)

Tomato seedlings in APS

Tomato seedlings in a self-watering APS seedstarter.

Containers
You can start your seeds in just about anything that holds soil and has drainage holes — I've used small yogurt containers and even egg cartons with holes poked in the bottoms and waterproof saucers underneath. However, I've found it more convenient to use purchased pots, such as Biodegradable Cowpots and seedstarting systems, such as APS seedstarters.

Warmth and Light
Seeds germinate best at warm room temperature (70-75F); you can speed germination by providing bottom heat with a heat mat. Once they're up and growing, seedlings grow best in cool room temperature, about 65F.

Although you can start your tomatoes on a sunny windowsill, you'll get better results growing them under some type of grow light. Winter and early spring sunlight isn't nearly as intense as summer sunlight, and there are fewer hours of daylight, too. Insufficient light can lead to weak, spindly plants. A light garden with adjustable fluorescent lights is ideal for seedstarting. Learn more: Gardening Under Lights.

Step-By-Step Seedstarting

Here are the steps to successfully starting your tomato seeds.

  • Thoroughly moisten the seedstarting mix, and then fill the containers to within 1/2″ of the top. Firm the mix but don't compact it.
  • Place two or three seeds into each small container or each cell of an APS. Cover the seed with about 1/4″ of soil and gently firm it over the seeds.
  • Water to ensure good seed-to-mix contact. You can use a plant mister or just dribble a stream of water over the top. You don't need to soak the soil, just moisten the top layer.
  • Place the pots in a warm spot or on top of a heat mat. At this point, the seeds don't need light.
  • Keep the mix moist but not soaking wet. If your seedstarting system has a greenhouse top, use it to help hold moisture. Or, you can lay some plastic kitchen wrap over the tops of the pots.
  • Check pots daily. As soon as you see sprouts remove the covering and place the pots in the sunny window or under grow lights, keeping the lights just an inch or two above the tops of the plants.

Growing On

Continue to keep the soil moist but not saturated. Dry seedstarting mix is lighter in color than moist mix — a good indication that it needs watering. Some gardeners run a fan in the room with their growing seedlings; good air circulation reduces the chances of disease problems, such as damping off.

If you're growing plants on a windowsill, rotate pots daily so plants grow upright instead of leaning toward the light. If you're growing under lights, raise the lights as the plants grow, keeping them just a few inches above the plants.

Thinning out tomato seedlings

Use a scissors to thin out crowded seedlings.

Thinning. For the strongest, healthiest plants you'll want just one seedling per pot or cell. Thinning (removing extra seedlings) is a tough task for many gardeners who hesitate to dispatch the seedlings they've been nurturing. But it has to be done. Select the strongest, healthiest seedling and use a pair of scissors to snip off the others at the soil line. You can try to transplant the extras into different pots, but you risk disturbing the roots of the remaining plant and, realistically, how many tomato plants can your garden accommodate?

Fertilizing. Once the second set of true leaves appears, it's time to begin fertilizing. The first two leaves are called "seed leaves;" subsequent leaves are called "true leaves." Use a soluble fertilizer, such as PHC for Seedlings, diluted to half the recommended rate. Apply it once or twice a week.

Repotting. Your tomatoes may need to be transplanted to larger containers if they outgrow their pots before it's time to set them outdoors. Don't allow the plant to get pot-bound, with the roots filling the container, or growth may be stunted. For step-by-step instructions, read How to Repot a Tomato Seedling.

Transplanting into the garden. Wait to transplant your tomato seedlings into the garden until after the average last spring frost date. Be prepared to protect the seedlings with garden fabric (row cover) if a late frost threatens. If all goes well, you'll be harvesting ripe tomatoes in eight weeks or less.

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