Plant tomato seeds indoors 6 to 8 weeks before your last spring frost, or direct sow outdoors after soil reaches 60°F

The timing depends on where you live and whether you start seeds indoors or plant them straight in the ground. Indoor seed starting gives you a head start on the growing season — you'll have seedlings ready to transplant outside when the weather warms. Direct sowing (planting seeds straight into garden soil) is simpler but means a later harvest, since tomato seeds won't germinate in cold soil.

Most home gardeners start seeds indoors under lights in late winter or early spring, then move the seedlings outside after the last frost date passes and nighttime temperatures stay above 50°F. If you prefer to skip the indoor step, you can wait and plant seeds directly in the garden once soil temperature reaches 60°F and frost danger has passed.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date — you can find this date through your local cooperative extension office or by searching your zip code online.
  • Tomato seeds germinate best in soil between 70°F and 80°F, so an indoor seed tray in a warm spot or under a heat mat works better than a cold windowsill.
  • Seedlings need 14 to 16 hours of light daily from a grow light or fluorescent fixture placed 2 to 3 inches above the leaves.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting, so they don't wilt from the shock of sun and wind.
  • If you direct sow seeds outdoors, wait until soil temperature is at least 60°F and nighttime lows stay above 50°F to avoid seed rot.

Finding Your Last Spring Frost Date

Your last spring frost date is the average date of the final frost in your area — the day after which freezing temperatures are unlikely. Tomato plants die at 32°F, so you cannot safely move seedlings outdoors until after this date passes. You can find your frost date through your county's cooperative extension office (search "[your county] cooperative extension"), or by entering your zip code into the USDA Hardiness Zone map at planthardiness.ars.usda.gov.

Once you know the date, count backward 6 to 8 weeks to find when to start seeds indoors. For example, if your last frost date is May 15, you would start seeds indoors around March 20 to April 1. If you live in a warm climate where frost does not occur, you can start seeds indoors in late winter (January or February) to get a harvest before summer heat slows flowering, or direct sow in fall for a winter crop.

Starting Seeds Indoors Under Lights

Indoor seed starting requires a seed tray or small pots, seed-starting mix (a light, sterile blend different from garden soil), and a light source. Fill the tray with moistened seed-starting mix, plant seeds about 1/4 inch deep, and cover loosely with plastic wrap or a humidity dome to keep moisture in while seeds germinate. Place the tray in a warm spot — ideally 70°F to 80°F — such as on top of a refrigerator, on a heat mat, or in a warm closet.

Once seedlings emerge (usually 5 to 10 days), remove the plastic and move the tray under a light source. A grow light or two fluorescent shop lights positioned 2 to 3 inches above the seedlings work well. Keep lights on for 14 to 16 hours daily — a straightforward timer makes this automatic. Water when the soil surface feels dry, but do not let the tray sit in standing water, which causes damping off (a fungal disease that kills seedlings at soil level).

When seedlings develop their first true leaves (the second set of leaves that appear), thin them to one plant per cell or pot, or transplant them into individual containers. This prevents crowding and gives roots room to grow. Continue watering and lighting until transplant time.

Hardening Off Seedlings Before Moving Outdoors

Hardening off means gradually exposing indoor seedlings to outdoor sun, wind, and temperature swings so they toughen up and do not wilt when planted in the garden. Start this process 7 to 10 days before your planned transplant date. On day one, place seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours, then bring them back inside.

Each day, increase the time outdoors and gradually move them into more sun — day two might be 3 hours in dappled shade, day three 4 hours with some morning sun, and so on. By day 7 or 8, seedlings should spend the full day outdoors in partial sun. On the final days, leave them out overnight if nighttime temperatures stay above 50°F. This gradual shift prevents sunscald (pale patches on leaves) and transplant shock.

Transplanting Seedlings Into the Garden

Transplant seedlings outdoors after your last frost date and once nighttime temperatures consistently stay above 50°F. Soil temperature should be at least 60°F — if you are unsure, use a soil thermometer (available at garden centers) to check. Dig a hole slightly larger than the seedling's root ball, bury the stem up to the first true leaves (tomatoes will root along the buried stem for a stronger plant), and water well.

Space plants 24 to 36 inches apart depending on the variety — check the seed packet or plant label for the mature size. If you have a long growing season, you can make successive plantings every 2 to 3 weeks through early summer for staggered harvests, though the last planting may not mature before frost arrives.

Direct Sowing Seeds Outdoors

If you prefer to skip indoor seed starting, you can plant seeds directly in the garden once soil temperature reaches 60°F and frost danger has passed. Prepare the soil by mixing in compost or aged manure, then plant seeds 1/4 inch deep in clusters of 2 to 3 seeds spaced 24 to 36 inches apart. Water gently and keep soil consistently moist until seedlings emerge.

Once seedlings are 2 to 3 inches tall, thin them to one plant per spot by snipping off the weaker seedlings at soil level (pulling them out can damage the roots of the keeper plant). Direct-sown plants will flower and fruit later than transplants, but they often develop stronger root systems since they are never disturbed. In short-season climates, direct sowing may not give you enough time for a full harvest before frost, so indoor starting is usually the better choice.

Adjusting Timing for Your Climate

In cool climates with short growing seasons (northern states, high elevation), start seeds indoors as early as late February to maximize the harvest window. In warm climates where frost does not occur, you have more flexibility — you can start seeds in fall (August or September) for a winter crop, or in late winter (January or February) for a spring crop that finishes before summer heat reduces flowering.

If you live in a very hot climate, tomatoes often stop setting fruit when daytime temperatures exceed 90°F and nighttime temperatures stay above 75°F. In that case, timing your transplants so they flower in spring (before heat arrives) or in fall (as heat breaks) gives better yields. Your local cooperative extension office can provide region-specific recommendations for your area.

Frequently Asked Questions

Can I start tomato seeds in a sunny windowsill instead of under a grow light?

A south-facing windowsill can work in spring when days are long, but seedlings often become leggy (tall and thin) because windowsill light is weaker than a grow light. If you use a windowsill, rotate the tray daily and keep it as close to the glass as possible. A grow light or fluorescent fixture is more reliable and produces sturdier seedlings.

What if I start seeds too early and they get too big before transplant time?

Oversized seedlings become root-bound and stressed. If this happens, you can pot them up into larger containers and continue growing them indoors, or transplant them outdoors earlier if nighttime temperatures are at least 50°F. You can also pinch off the top growth to slow them down, though this delays flowering slightly.

How do I know if my seedlings are ready to transplant?

Seedlings are ready when they have 4 to 6 true leaves (not counting the first seed leaves), are 6 to 8 inches tall, and have been hardened off. They should look stocky and green, not pale or leggy. If they are still small at transplant time, that is fine — they will catch up quickly once in the ground.

Can I plant tomato seeds in fall for a winter harvest?

Yes, in warm climates (zones 9 and warmer). Start seeds in late summer (July or August) so seedlings are ready to transplant in September or October, before frost arrives. They will grow slowly through winter but produce fruit in spring. In cold climates, frost will kill plants before they mature, so fall planting does not work.

What happens if soil is too cold when I plant seeds?

Cold soil (below 60°F) causes seeds to rot before they germinate, or they germinate very slowly and may be attacked by fungi. Wait for soil to warm up, or use a heat mat under an indoor seed tray to speed germination. A soil thermometer takes the guesswork out of knowing when the garden is ready.