Plant seedlings outdoors after your last frost date, when soil reaches 60°F

The single most important rule for tomato seedlings is this: wait until the danger of frost has passed and the soil is warm enough. Tomatoes are frost-sensitive plants that will die or stop growing if exposed to temperatures below 50°F. Your last frost date is the average date of the final frost in spring for your area — this is the earliest safe day to move seedlings from indoors to the garden.

You can find your last frost date by entering your zip code into the USDA Hardiness Zone map at planthardiness.ars.usda.gov, or by calling your local cooperative extension office. The date varies dramatically by region: gardeners in southern Florida may never experience frost, while those in northern Minnesota might not plant until late May. Soil temperature matters as much as air temperature. Tomato roots grow slowly in cold soil and seedlings will sit dormant rather than establish. Wait until soil reaches at least 60°F, ideally 65°F or warmer.

Key Takeaways

  • Plant seedlings outdoors only after your last frost date has passed and soil temperature reaches 60°F or higher.
  • Start seeds indoors 6 to 8 weeks before your last frost date so seedlings are the right size when outdoor conditions are ready.
  • Seedlings should be 6 to 8 inches tall with at least two sets of true leaves before transplanting to the garden.
  • Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the ground.
  • Plant seedlings deeper than they grew in their containers — tomatoes will root along buried stems and develop stronger root systems.

Count backward from your last frost date to time indoor seed starting

Tomato seeds take 6 to 8 weeks to grow into transplant-ready seedlings indoors. If your last frost date is May 15, you should start seeds indoors around March 15 to March 20. If your last frost date is April 1, start seeds in early to mid-February. Starting too early means seedlings will be overgrown and leggy by planting time; starting too late means you will have small, weak plants when conditions are finally warm enough to plant.

The exact timing depends on the variety. Determinate tomatoes (bush types that fruit all at once) and early-maturing varieties can be started closer to 6 weeks before the frost date. Indeterminate tomatoes (vining types that fruit all season) and long-season varieties benefit from the full 8 weeks. Check your seed packet for the days to maturity and work backward from there.

Recognize when seedlings are ready to transplant

Seedlings are ready to move outdoors when they are 6 to 8 inches tall and have developed at least two sets of true leaves — the leaves that look like actual tomato leaves, not the smooth oval cotyledons (seed leaves) that emerged first. At this stage, the stem should be sturdy enough to support itself without flopping over, and the root system should be established enough to survive the shock of transplanting.

Do not wait for seedlings to become tall and spindly in hopes of getting a head start. Leggy seedlings with weak stems are more prone to breaking during transplanting and take longer to recover in the garden. A stocky, compact seedling with good leaf development will outperform a tall, thin one every time. If your seedlings are getting too tall before outdoor conditions are ready, move them under a grow light or closer to a sunny window, and reduce watering slightly to slow their growth.

Harden off seedlings before planting them outside

Hardening off is the process of gradually exposing indoor-grown seedlings to outdoor conditions — wind, direct sun, and temperature fluctuations — so they can adjust without wilting or dying. Seedlings grown indoors under lights or in a sheltered window have not experienced these stresses and will go into shock if moved directly to the garden. Plan for 7 to 10 days of hardening off.

Start by placing seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours on the first day. Each day, increase the time outdoors and gradually move them into more direct sunlight. By day 7 or 8, seedlings should be outside all day. Leave them outside overnight only once temperatures stay above 50°F. On the final day or two before planting, leave them out 24 hours. If a cold snap is forecast during hardening off, bring seedlings back indoors — one hard frost will kill them.

Plant seedlings deeper than they grew in containers

When you dig the planting hole, bury the seedling deeper than it was growing in its pot. Tomatoes have the ability to root along any part of the stem that is buried underground. Plant so that only the top two sets of leaves are above soil level. This deeper planting creates a stronger, more extensive root system that can reach deeper water and nutrients, making the plant more drought-tolerant and stable as it grows tall.

If a seedling is particularly tall or leggy, you can bury it at an angle — the buried portion of the stem will straighten as it grows and roots develop along the buried section. Space seedlings 24 to 36 inches apart depending on the variety; indeterminate types need more space than determinate ones. Water thoroughly after planting and keep soil consistently moist (not waterlogged) for the first two weeks while roots establish.

Adjust timing if you live in a very hot climate

In regions where summer temperatures regularly exceed 95°F, the usual spring planting timeline may not work. Tomatoes stop setting fruit when nighttime temperatures stay above 75°F, and daytime heat above 95°F can cause blossom drop and sunscald. In these areas, you may need to plant seedlings earlier than the traditional last frost date so they can flower and set fruit before extreme heat arrives, or plan a second planting for late summer that will produce fruit in fall.

Consult your local cooperative extension office for the best planting windows in your specific area. They can tell you whether spring planting or a fall crop makes more sense, and whether shade cloth or other heat-management strategies are necessary. Some gardeners in hot climates start seeds even earlier — 10 to 12 weeks before the last frost date — to get seedlings in the ground as soon as frost danger passes, giving them a head start before heat peaks.

Frequently Asked Questions

Can I plant seedlings before the last frost date if I use row covers or cloches?

Yes, row covers and cloches can protect seedlings from light frost, but only if temperatures do not drop more than a few degrees below freezing. If a hard frost is forecast, the protection may not be enough. It is safer to wait until the frost danger has truly passed, unless you are willing to monitor the forecast closely and be ready to cover plants quickly.

What if my seedlings are ready but the soil is still cold?

Wait for soil to warm up. You can speed the process by laying black plastic or landscape fabric over the planting area for a week or two before planting — it absorbs heat and warms the soil underneath. Planting in cold soil will only slow growth and increase the risk of root rot. A few extra weeks of waiting is worth it.

Can I plant seedlings that are still small if I am running out of time?

Small, healthy seedlings with good leaf development will catch up faster than you expect once they are in warm soil with full sun. Leggy or weak seedlings are riskier. If you must plant early, choose the sturdiest seedlings and baby them with consistent watering and protection from wind for the first week.

How do I know if my seedlings are getting too much or too little light indoors?

Seedlings that are tall, thin, and pale with large spaces between leaves are not getting enough light — move them closer to a window or under a grow light. Seedlings that are short, dark green, and compact are getting good light. Grow lights should be 2 to 3 inches above the seedlings and on for 14 to 16 hours per day.

What temperature should I keep seedlings at indoors before planting?

Tomato seedlings grow best at 65°F to 75°F during the day and 55°F to 65°F at night. Cooler nighttime temperatures actually help produce stockier, stronger seedlings. Avoid placing seedlings on a heat mat or in a warm spot unless they are still germinating — once they have sprouted, cooler conditions are better.