Plant tomatoes after your last spring frost, when soil reaches 60°F

The single most important rule is this: tomatoes go in the ground after the danger of frost has passed and the soil has warmed. Frost kills tomato plants outright. Cold soil slows their growth so much that you lose weeks of production. Both matter, but soil temperature matters more—a tomato planted in 50°F soil will sit dormant and may rot before it grows, even if the air feels warm.

Find your last spring frost date by searching "[your city] last frost date" or checking the USDA Hardiness Zone Map. That date is your earliest safe day. Then wait another week or two for the soil to warm. You can measure soil temperature with a straightforward soil thermometer from any garden center—push it 4 inches into the ground in the morning. When it reads 60°F or higher for three days in a row, you are ready to plant.

In most of the United States, this falls between mid-April (southern zones) and late May (northern zones). If you live in a very warm climate where frost never comes, you can plant in late winter instead, aiming for harvest before the hottest part of summer.

Key Takeaways

  • Tomatoes planted before the last frost date will die; check your local frost date before you buy seedlings.
  • Soil temperature matters more than air temperature—wait until soil reaches 60°F, which you can measure with a $5 thermometer.
  • Hardening off seedlings (moving them outside gradually) for 7 to 10 days before planting prevents transplant shock and wilting.
  • In warm climates without frost, plant tomatoes in late winter so they mature before extreme heat arrives in summer.

How to harden off seedlings before planting

If you started tomatoes indoors from seed or bought seedlings from a nursery, they have lived in stable, protected conditions. Moving them directly into the garden shocks them—leaves turn purple or white, growth stops, and some plants die. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days.

Start by placing seedlings outside in a sheltered spot (against a wall, under a porch) for 2 to 3 hours on the first day. Each day, add 2 to 3 more hours and move them into slightly more sun and wind. By day 7 or 8, they should spend the full day outside. On day 9 or 10, leave them out overnight if temperatures stay above 50°F. This toughens the leaves and stems so they handle transplant stress without wilting.

If a cold snap arrives during hardening off, move seedlings back indoors or cover them with a frost cloth. One hard freeze undoes a week of progress.

Planting depth and spacing matter for strong roots

Bury tomato seedlings deeper than they grew in their pot—up to the first true leaves. Tomatoes will sprout roots all along the buried stem, creating a stronger root system that reaches deeper into the soil and finds water during dry spells. This is one of the few times deeper planting actually helps.

Space plants 24 to 36 inches apart, depending on the variety. Determinate (bush) types can go closer; indeterminate (vining) types need more room for air to circulate. Poor air flow traps moisture on leaves and invites fungal disease. If you are planting in rows, space rows 3 to 4 feet apart so you can walk between them to harvest and prune.

Water seedlings well after planting and keep the soil consistently moist (not soggy) for the first two weeks while roots establish. After that, water deeply but less often—this encourages roots to grow deeper rather than staying shallow.

Starting seeds indoors 6 to 8 weeks before planting

If you want to start from seed rather than buy seedlings, sow indoors 6 to 8 weeks before your last frost date. This gives seedlings time to grow large enough to transplant by the time soil warms. Tomato seeds germinate best at 70 to 80°F—a heat mat speeds this up, but a sunny windowsill works if you are patient.

Use seed-starting mix (not garden soil), keep it moist but not waterlogged, and provide 14 to 16 hours of light per day. A grow light or fluorescent shop light hung 2 to 3 inches above seedlings prevents them from becoming leggy and weak. Once seedlings have their first true leaves (the second set to appear), thin them so each one has its own cell or pot.

Fertilize weekly with diluted liquid fertilizer once seedlings have true leaves. They are living on stored energy from the seed coat, and that runs out fast under grow lights.

Adjusting planting time for your climate

In cool northern climates, plant as soon as soil reaches 60°F, even if it is still cool at night. Tomatoes are tough enough to handle cool nights once established; what kills them is frost. Use black plastic mulch or landscape fabric to warm the soil faster in spring.

In hot southern climates, you have two windows. Spring planting works the same way—plant after frost danger passes. But many southern gardeners also plant a second crop in mid-summer (around July) for a fall harvest, because tomatoes stop setting fruit when temperatures stay above 90°F. A July planting matures in September and October when temperatures drop back into the 70s and 80s.

In very hot climates like Arizona or southern California, plant in late winter (January or February) so the main harvest happens in spring before heat arrives. You can plant again in late summer for a winter harvest.

Common mistakes that delay or kill young plants

Planting too early is the most common mistake. Impatience costs weeks—a seedling planted in 50°F soil sits there for 3 weeks while one planted in 65°F soil takes off when ready. The second plant catches up and passes the first by mid-summer. Wait for warm soil.

Skipping hardening off causes wilting and leaf damage that sets plants back 1 to 2 weeks. It takes 10 days but saves you from losing seedlings or watching them struggle all season.

Planting too close together creates humid conditions that invite early blight and other fungal diseases. Space matters. Overcrowding also makes pruning and harvesting harder later on.

Overwatering in the first week is another trap. Seedlings have small root systems and do not need constant moisture. Water when the top inch of soil feels dry, not on a schedule. Soggy soil causes root rot.

Frequently Asked Questions

Can I plant tomatoes before the last frost date if I use row covers?

Row covers and frost cloth slow heat loss and can protect plants from a light frost, but they are not reliable for hard freezes. If your last frost date is May 15 and you plant May 1 under row covers, a 28°F night will still kill the plants. Use covers as insurance for unexpected late frosts after planting, not as permission to plant early.

What if my soil is still cold in late May?

Use black plastic mulch or landscape fabric to warm the soil 5 to 10 degrees faster. Lay it down 1 to 2 weeks before planting and cut holes for seedlings. It also conserves moisture and suppresses weeds. Remove it once summer heat arrives, or leave it in place and water more often.

Is it better to plant seedlings or seeds directly in the ground?

Seedlings are faster and more reliable, especially in short-season climates. Direct seeding takes longer (seeds need 6 to 8 weeks to reach transplant size) and wastes garden space during that time. Start seeds indoors if you want to maximize your harvest window, or buy seedlings from a nursery to skip the indoor step entirely.

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

Seedlings are ready when they are 6 to 8 inches tall, have at least two sets of true leaves (the leaves that look like tomato leaves, not the smooth seed leaves), and have a sturdy stem. Leggy, thin seedlings that are tall but weak will struggle after planting. Stocky seedlings with thick stems do better.

Can I plant tomatoes in containers instead of the ground?

Yes. Use a pot at least 5 gallons for determinate varieties and 10 gallons for indeterminate types. Follow the same planting rules—wait for warm soil, bury the stem deep, and water consistently. Container tomatoes dry out faster, so check soil moisture every 1 to 2 days during hot weather.