Plant seedlings outside when nighttime temperatures stay above 50°F and your soil has warmed to at least 60°F

The single biggest mistake home growers make is moving seedlings outside too early. Tomatoes are tropical plants—they stop growing, turn purple, and sometimes die when exposed to cold soil and air. You need two conditions met at the same time: warm nights and warm soil. Checking a thermometer is faster and more reliable than guessing by calendar date.

In most of the United States, this happens between mid-May and early June, but the exact timing depends entirely on where you live. A gardener in zone 5 (Minnesota, upstate New York) may not plant until late May. A gardener in zone 9 (coastal South Carolina, southern Texas) can plant in April. The only way to know your window is to check your local frost date and then wait an additional week or two after that date passes.

Key Takeaways

  • Nighttime temperatures must stay consistently above 50°F—a single cold night can stunt seedlings or kill them.
  • Soil temperature matters as much as air temperature; cold soil slows root growth and invites root rot.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before planting them permanently.
  • Your local frost date plus one to two weeks gives you a safe planting window, but checking soil temperature with a thermometer is the most reliable method.

Find your last spring frost date

Your last spring frost date is the average date of the final frost in your area. This is not the same as the first warm day—it is the statistical midpoint of when freezing temperatures stop happening. Search "last frost date [your city]" or visit the USDA Plant Hardiness Zone Map and enter your zip code. The map will show your zone and link to frost date tables.

Write down the date you find. Tomato seedlings can tolerate a light frost (32°F) for a few hours if they are hardened off, but they cannot survive a hard freeze. Do not plant until at least one week after your frost date, and two weeks is safer. If an unexpected frost is forecast after you have planted, cover seedlings with row cloth or cardboard overnight.

Check soil temperature before planting

Soil temperature is more important than air temperature. Tomato roots grow slowly in soil below 60°F and stop growing entirely below 50°F. Cold soil also creates conditions for damping-off disease and root rot. Buy a soil thermometer (they cost $8 to $15 at garden centers) and measure the soil temperature in the morning, when it is most stable.

Push the probe 4 inches into the soil where you plan to plant. Wait 2 to 3 minutes for an accurate reading. If the soil is still below 60°F, wait another week and test again. Soil warms faster in raised beds and containers than in ground beds, so you may be able to plant in one location but not another in your yard.

Harden off seedlings for 7 to 10 days

Seedlings grown indoors under lights are soft—they have not experienced wind, direct sun, or temperature swings. Planting them straight outside causes sunburn (white or papery patches on leaves), wilting, and sometimes death. Hardening off means gradually exposing them to outdoor conditions so they build toughness.

Start 7 to 10 days before your planned planting date. On day one, place seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours. Each day, increase the time outside and move them into brighter light. By day 7 or 8, they should spend the full day outside in dappled shade. On the final days, expose them to a few hours of direct morning sun. Bring them inside at night until nighttime temperatures stay above 50°F.

Water seedlings more often during hardening off—wind and sun dry them out faster than indoor conditions. Check soil moisture every few hours on hot days. If a seedling wilts, move it back to shade and water it when ready.

Watch the weather forecast before planting day

Even if soil and air temperatures are right, do not plant if a cold snap is forecast within the next week. Check a 10-day forecast the morning you plan to plant. If temperatures are expected to drop below 50°F at night, wait. A single cold night can set seedlings back by weeks or kill them outright.

If you have already planted and a frost is forecast, cover seedlings with row cloth, old sheets, or cardboard boxes before sunset. Remove the cover the next morning when temperatures rise. This straightforward step can save your entire crop.

Space seedlings correctly at planting time

Crowded plants compete for light, water, and air circulation, which invites disease. Determinate varieties (bush types) need 24 inches between plants. Indeterminate varieties (vining types) need 30 to 36 inches. If you are planting in rows, space rows 3 to 4 feet apart so you can walk between them and reach fruit.

Dig a hole slightly deeper than the seedling's root ball. Tomatoes can be planted deeper than they grew in the pot—bury the stem up to the lowest set of true leaves. Roots will form along the buried stem, creating a stronger root system. Water thoroughly after planting and keep soil consistently moist (not soggy) for the first two weeks while roots establish.

Protect young plants from wind and intense sun

Newly planted seedlings are fragile. Strong wind can snap stems or dry them out in hours. If your garden is windy, install stakes or cages at planting time and tie seedlings loosely to them with soft cloth strips. This also prevents the root ball from rocking in the soil, which damages new roots.

If you live in a very hot climate (zone 9 or warmer), afternoon shade cloth for the first two weeks can prevent sunburn and heat stress. Use 30 to 50 percent shade cloth and remove it once plants are established and growing vigorously. In cooler climates, full sun is fine from day one.

Frequently Asked Questions

What if my seedlings are already tall and leggy indoors?

Leggy seedlings (tall and thin) are a sign of insufficient light. Before planting outside, bury them deeper than normal—up to the lowest leaves—to create a stronger root system. Leggy seedlings are more fragile, so harden them off slowly and protect them from wind for the first few weeks.

Can I plant seedlings in a cold frame to extend the season?

Yes. A cold frame (a low box with a transparent lid) can warm soil and air by 10 to 15 degrees on sunny days. You can plant seedlings in a cold frame 2 to 3 weeks earlier than in open ground. Open the lid on warm days to prevent overheating, and close it at night. Remove the lid entirely once nighttime temperatures stay above 50°F.

What happens if I plant too early and seedlings get cold?

Cold-stressed seedlings turn purple or dark red, stop growing, and may develop yellow leaves. Growth resumes once soil warms, but the plant has lost weeks of development. In severe cases (soil below 50°F for more than a few days), seedlings may not recover. It is always better to wait.

Do I need to wait the same amount of time for cherry tomatoes and large slicing varieties?

Yes. All tomato varieties have the same temperature requirements for root growth and survival. Cherry tomatoes are not more cold-hardy than beefsteaks. Wait until soil reaches 60°F and nighttime temperatures stay above 50°F, regardless of variety.

Can I plant seedlings in containers instead of the ground?

Yes, and containers warm up faster than ground soil in spring. Use a container at least 5 gallons for determinate varieties and 10 gallons for indeterminate types. Fill with potting soil (not garden soil) and check soil temperature the same way. Containers dry out faster, so water more often once plants are established.