How to Start Tomato Seeds Indoors

1. Select Your Preferred Tomato Seed

   It's fairly easy and inexpensive to start many tomato varieties from seed, so experiment with the types you like best. Here are some factors to take into consideration:

   • Look for certified organic seed if you prefer organic produce.
   • If you know your area is prone to particular plant diseases, choose a hardy tomato variety.
   • If you want your tomatoes all at once to preserve them, look at determinate varieties. ​Indeterminate types bear over a longer period, with some starting later in the season than others.
   • Note the mature size of the plant. In general, determinate plants tend to be smaller than indeterminate ones. Small plants for containers often have names that include "patio" or "pixie."

2. Prepare the Containers for Planting

   It's often more efficient to dampen the potting mix before you put it in the containers. Add some water, and work it through the soil. Keep adding water until the mix stays compressed in your hand but is not dripping wet. It should break apart when you poke it with your finger.

   Then, fill your containers with potting soil. Gently firm the soil, so it's about an inch from the top.

3. Plant the Tomato Seeds

   Make a 1/4-inch furrow in the potting mix. Then, sprinkle two to three seeds into the furrow, and cover them with a sprinkling of potting mix. Gently pat down the mix, so the seeds make good contact with the soil. You can spray the surface with water if it doesn’t feel moist.

   At this point, place your containers somewhere warm. Check them daily to make sure the soil is moist—but not wet—and watch for germination. Tomato seed germination typically occurs in about five to 10 days.

4. Care for the Tomato Seedlings

   Keep your tomato seedlings warm and moist, and provide them with light—preferably grow lights. Rotate the plants if they seem to be leaning in one direction. Once your tomato seedlings have true leaves, it's time to start feeding them. Any good liquid fertilizer can be used once a week, but dilute it to half the label's recommended dose.

   Tomato stems grow sturdier if they are tossed about by the wind. You can simulate this indoors by putting a fan on your plants for an hour a day or by gently running your hand through them each time you pass.

5. Pot the Tomato Seedlings

   When the seedlings are 2 to 3 inches tall and have a couple sets of true leaves, it's time to pot them in larger containers. In general, 3- to 4-inch containers are a good size, though you might have to move them to larger pots later if you can't plant them outdoors.

   Fill the new pots with moist potting mix just as you did when you started the seeds. If more than one seed germinated in the same container, you will need to thin the seedlings. Either gently jiggle entangled roots apart, or simply snip off unwanted seedlings at soil level. This ensures that you won't damage the seedling you want to keep.

   Plant each tomato seedling in its new pot a little deeper than it was in its original container. If it's tall and leggy, you can plant it right up to its top-most leaves. Then, firm the soil gently around the seedling.

6. Transplant the Seedlings Outdoors

   When you're finally ready to plant your tomatoes in the garden, choose a cool or overcast day. Once again, plant them deeper than they were in their pots, so new roots will form along the buried stem.

   You can plant them all the way up to the top couple sets of leaves. This is especially ideal if your plants have gotten too tall indoors, and you want them to become stockier and stronger. If you can’t dig deep enough, you can always plant them sideways in a furrow. The top of the plant will find the sun and grow upright in a few days.

When to Plant Tomato Seeds Indoors

Tomato seeds usually germinate within 10 days. The plants also develop quickly, especially if you keep them warm and give them lots of light, so seeds should be started six to eight weeks before you intend to transplant outdoors. For instance, if you plan to transplant outdoors in early May, start your seeds indoors in mid- to late March.Your transplant timing will depend on your last frost date, which is determined by your USDA hardiness zone and that year's specific weather.

When to Transplant Tomato Seedlings Outdoors

Err on the side of caution when it comes to transplanting outdoors. If you put the plants out too early, frost or a cold spell could easily hinder their growth or kill them. Tomatoes planted a little later in the season will quickly catch up to earlier transplants that have been stunted by cold.

In general, when nighttime temperatures remain steadily above 50 degrees Fahrenheit, it's safe to begin hardening off your seedlings. Your plants should be several inches tall at this point with some branching. One method is to expose them to the outdoors for gradually increasing stretches each day, starting about seven to 10 days before you intend to transplant them into the garden. After this transition period, they should be ready to live in your garden.

Tips to Care for Tomato Plants

Stake your tomato plants right after you plant them, so you don’t disturb their roots later. Then, give them a good drench of water, and be patient. They should start flowering when the days heat up.

Tomatoes are prone to diseases of both the leaves and the fruits. The best defense is to keep your plants healthy and strong. Give them regular water, leave room between plants for good air circulation, and check them daily to catch any problems in the early stages.

Finally, it's easy to get carried away with planting tomato seeds. But note that a family of four can easily feast throughout the summer on just six plants.