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

The single most important rule: tomatoes go in the ground after the danger of frost has passed and the soil has warmed up. Frost kills tomato plants. Cold soil slows their growth and invites disease. Most gardeners plant between late April and late May in the northern United States, and between February and March in the South—but your exact date depends on where you live, not on a calendar.

You need two pieces of information: your area's last spring frost date, and your soil temperature right now. The frost date tells you when freezing nights will stop. The soil temperature tells you whether the plant will actually grow once it's in the ground. A tomato planted in 50°F soil will sit there sulking for weeks. The same plant in 65°F soil will take off.

Key Takeaways

  • Find your last spring frost date by searching "[your city] last frost date" or calling your local cooperative extension office—this date changes year to year.
  • Wait until soil temperature reaches at least 60°F, ideally 65°F or warmer, which usually happens one to three weeks after the last frost date.
  • Check soil temperature with an inexpensive soil thermometer pushed 4 inches into the ground in the morning, not a weather app.
  • Hardening off seedlings for 7 to 10 days before planting prevents transplant shock and helps them survive the move outdoors.
  • If frost threatens after you plant, cover plants with cloth or row cover the night before—plastic touching the leaves will damage them.

Finding Your Last Spring Frost Date

Your last spring frost date is the average date of the final freeze in your area. Search online for "[your city] last frost date" or "[your zip code] frost date." You will find a specific date—for example, May 15 or April 20. This is an average, not a may provide. Some years frost comes later; some years it stops earlier.

If you cannot find it online, call your local cooperative extension office. They maintain frost date records for your county and can tell you the date in one call. The extension office is a free service run by your state's land-grant university and exists to answer exactly this kind of question.

Mark this date on your calendar. Do not plant tomatoes outdoors before it arrives, even if the weather feels warm. A single frost night will kill or severely damage plants that have already gone in the ground.

Measuring Your Soil Temperature

Soil temperature is not the same as air temperature. A sunny day might feel warm while the soil is still cold. Buy a soil thermometer—they cost $8 to $15 and look like a long metal spike with a dial or digital readout. Push it 4 inches into the soil in the morning, wait 30 seconds, and read the temperature.

Check the soil in the spot where you plan to plant, not in a shaded area or near a building. Soil warms unevenly. A bed that gets full sun all day will be warmer than one in partial shade. If you are planting in multiple spots, check each one.

Tomatoes will grow in 60°F soil, but they grow slowly and are more prone to disease. Aim for 65°F or warmer. In most regions, this happens one to three weeks after your last frost date, depending on how much sun your garden gets and whether you have amended the soil with compost or mulch.

Hardening Off Seedlings Before Planting

If you started tomatoes indoors from seed or bought seedlings from a nursery, they have been living in stable, protected conditions. Moving them directly outside is shock to their system. Leaves can burn, growth stops, or the plant dies. Hardening off means gradually exposing seedlings to outdoor conditions over 7 to 10 days.

Start by placing seedlings outside in a sheltered spot—against a wall, under a porch, or in a cold frame—for 2 to 3 hours on the first day. Each day, increase the time outdoors and move them to a slightly more exposed location. By day 7 or 8, they should spend the full day outside. By day 10, they can stay out overnight if temperatures stay above 50°F.

If you skip hardening off and plant directly, expect wilted leaves, stunted growth, or death. The process takes 10 days but saves you from losing plants or setting them back by weeks.

Planting Depth and Spacing

Bury tomato seedlings deeper than they were growing in their pot. Tomatoes can develop roots all along the buried stem, which creates a stronger root system. Dig a hole, remove the lower leaves from the seedling, and plant it so only the top 2 to 3 sets of leaves are above ground. The buried stem will root.

Space plants 24 to 36 inches apart, depending on the variety. Determinate (bush) tomatoes need less space; indeterminate (vining) tomatoes need more. Check the seed packet or plant label for the specific spacing. Good air circulation prevents fungal disease, so do not crowd them.

Water well after planting and keep the soil consistently moist—not waterlogged—for the first two weeks while the roots establish. Mulch around the base with 2 to 3 inches of straw or wood chips to retain moisture and regulate soil temperature.

What to Do If Frost Threatens After Planting

Sometimes frost arrives unexpectedly after you have planted. Weather forecasts can change, or a late cold snap hits. If frost is predicted within a few days of planting, cover your tomatoes the night before with cloth, row cover, or even old sheets. The cover traps heat radiating from the soil and protects plants from freezing air.

Do not use plastic directly on the leaves—it conducts cold and will damage them. Use fabric or cloth that allows air circulation. Remove the cover during the day so the plants do not overheat. If frost does damage the foliage, do not pull the plant out when ready. Tomatoes often recover and send out new growth within a week or two.

Regional Planting Windows

Timing varies widely by region. In the upper Midwest and Northeast, the last frost date typically falls between May 10 and May 25, so outdoor planting happens in late May. In the mid-Atlantic and upper South, it is usually April 20 to May 10, so planting happens in early to mid-May. In the lower South and Gulf Coast, it can be as early as March 15 to April 1, so planting happens in March or early April.

In warm climates where frost does not occur, the limiting factor is soil temperature. Even in Florida or southern California, soil may not reach 60°F until late winter or early spring. Check your soil temperature regardless of your region.

If you live in an area with a short growing season, start seeds indoors 6 to 8 weeks before your last frost date so seedlings are ready to plant as soon as conditions allow. If you live in a long-season area, you can direct-seed or wait to buy nursery seedlings closer to planting time.

Frequently Asked Questions

Can I plant tomatoes if the weather forecast shows warm days but cold nights?

Wait for nighttime temperatures to stay consistently above 50°F. A few warm days followed by cold nights will stall growth and stress the plant. Soil temperature matters more than air temperature anyway—check that first.

What if I plant too early and frost kills my seedlings?

If the damage is light—just the top leaves are blackened—the plant may recover and send out new growth. If the stem is frozen solid or the entire plant is blackened, it will not recover. You will need to replant. This is why waiting for the frost date is worth the patience.

Do I need to harden off seedlings I bought from a nursery?

Yes. Nursery seedlings have been in a greenhouse or shade house, not outdoors. They need the same gradual exposure to sun, wind, and temperature swings as seedlings you started yourself. Hardening off takes 7 to 10 days and prevents transplant shock.

Can I plant tomatoes in a container instead of the ground?

Yes. Container soil warms faster than ground soil, so you can plant a week or two earlier. Use a pot at least 5 gallons for determinate varieties and 10 gallons for indeterminate types. Fill it with potting mix, not garden soil, and water more frequently since containers dry out faster.

What if my soil is still cold but my last frost date has passed?

Wait for the soil to warm. The frost date is only one condition. Cold soil will cause the plant to sit dormant, invite root rot, and delay harvest by weeks. Warm the soil by laying black plastic over the bed for a week or two before planting, or straightforward wait another week and check the temperature again.