Move tomato plants outside after the last frost date in your area, when soil temperature reaches at least 60°F and nighttime air temperature stays above 50°F

The timing depends entirely on where you live. Tomato plants are killed by frost, so moving them too early is the most common mistake. Check your local frost date — the date when the last frost typically occurs in spring — by searching "[your city] last frost date" or calling your county extension office. They have the actual historical data for your area.

Even after the frost date passes, wait for soil to warm. Tomatoes planted in cold soil sit dormant and often rot before they grow. Soil thermometers cost $10 to $20 and take the guesswork out. If you do not have one, wait two weeks after your frost date as a rough rule, but soil temperature matters more than the calendar.

Key Takeaways

  • Check your local last frost date by searching online or calling your county extension office — this is the single most important date for your area.
  • Soil temperature must reach 60°F or higher; a soil thermometer removes doubt and costs less than a single plant.
  • Nighttime temperatures should stay above 50°F consistently; a few cold nights after planting can stunt growth or kill young plants.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the ground.
  • Plant in the evening or on a cloudy day to reduce transplant shock, and water deeply after planting.

How to Find Your Last Frost Date

Your last frost date is the average date of the final frost in your area. Search "[your city] last frost date" or "[your state] frost date map" — the USDA has an interactive map at planthardiness.ars.usda.gov. Your county extension office also has this information and can tell you if the current year is running early or late.

The date is not a hard cutoff. Frost can occur a week or two after the average date, especially in cooler years. Many gardeners wait an extra week or two beyond the official date to be safe. If you plant on the frost date itself and frost hits, your plants will die.

Checking Soil Temperature Before Planting

Tomatoes need soil at least 60°F, ideally 65°F or warmer. Cold soil slows root growth and can cause the roots to rot before they establish. A soil thermometer is the only reliable way to know. Push it 4 inches into the soil in the morning, wait 2 minutes, and read the temperature. Check in the spot where you plan to plant.

If soil is still cold on your frost date, wait another week or two and check again. Soil warms faster in raised beds and containers than in ground beds. Dark mulch or black plastic laid over the soil for a week or two before planting can speed warming.

Hardening Off Seedlings Before Moving Them Outside

Seedlings started indoors are soft and tender. Moving them directly from a warm, still indoor environment to outdoor sun and wind will shock them and stunt their growth or kill them. Hardening off means gradually exposing them to outdoor conditions over 7 to 10 days.

Start by placing seedlings in a sheltered spot outdoors — a porch or against a wall — for 1 to 2 hours on the first day. Each day, increase the time outside and expose them to more wind and direct sun. By day 7 or 8, they should be outside all day. Leave them outside overnight only after nighttime temperatures stay above 50°F. This process toughens the stems and leaves so they can handle the real outdoors.

Planting Depth and Spacing in the Ground

Bury the stem deeper than it was in the pot — tomatoes can root along the buried stem, which strengthens the plant. Bury it up to the first true leaves if the seedling is tall, or up to the lowest leaves if it is short. Space plants 24 to 36 inches apart depending on the variety; check the seed packet or plant label for the exact spacing.

Plant in the evening or on a cloudy day to reduce transplant shock. Water deeply after planting so the soil settles around the roots. If the plant wilts the next day, it is normal — it will recover as roots establish. Do not fertilize for the first week; the soil has enough nutrients for now.

Protecting Young Plants from Late Cold Snaps

Even after planting, a cold snap can damage or kill young tomato plants. If frost is forecast within a week of planting, cover plants with row covers, blankets, or cloches at night. Remove the cover during the day so the plant gets sun and air. A straightforward frost cloth costs $15 to $30 and can save a whole planting.

If temperatures drop below 50°F at night repeatedly after planting, growth slows dramatically. The plant will not die, but it will sit still until it warms up. This is why waiting for consistent warm nights is worth the delay.

Container Tomatoes and Timing

Tomatoes in containers can go outside a few days earlier than ground-planted ones because you can move them to shelter if frost threatens. However, container soil warms faster than ground soil, so check the soil temperature in the pot the same way. Containers also dry out faster in sun and heat, so plan to water daily once temperatures climb above 75°F.

Large containers (at least 5 gallons) hold moisture better than small ones and give roots more room. A tomato plant in a small pot will need water twice a day in hot weather, which is impractical for most gardeners.

Frequently Asked Questions

What if my area does not have a clear last frost date?

Tropical and subtropical areas with no frost can plant tomatoes year-round, but they still need warm soil and air. Plant when nighttime temperatures stay above 60°F. If you live in a place with unpredictable frost, ask at a local garden center or nursery — they know the actual timing for your microclimate.

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

You can plant a week or two early if you cover plants every night and uncover them during the day. This is extra work and only saves a week or two. Most gardeners find it simpler to wait for warm soil and air.

What happens if I plant tomatoes in cold soil?

Cold soil slows root growth and can cause roots to rot before they establish. The plant sits stunted and may never recover, even after soil warms. It is better to wait two weeks and plant in warm soil than to plant early and lose the plant.

How do I know if my seedlings are ready to go outside?

Seedlings are ready when they have at least two sets of true leaves (the leaves that look like tomato leaves, not the first round seed leaves) and are 4 to 6 inches tall. They should be sturdy and not leggy or pale. Start hardening them off at this stage.

Can I plant tomatoes in a greenhouse or cold frame earlier?

Yes. A greenhouse or cold frame traps heat and protects from frost, so you can plant 2 to 4 weeks earlier than in open ground. Vent the greenhouse on warm days so it does not overheat, and water regularly because the enclosed space dries out fast.