Plant seedlings outdoors after the last frost date, when soil is warm and they have their first true leaves

Tomato seedlings are ready to move into soil when they are 6 to 8 weeks old and have developed their first set of true leaves — the second set of leaves that appear after the initial seed leaves. Before you plant, check your local frost date. Tomatoes cannot survive freezing temperatures, so planting too early is the most common mistake home growers make. If you plant seedlings before the soil reaches 60°F (ideally 65°F or warmer), they will sit dormant and may rot instead of growing.

The soil temperature matters more than the air temperature. A soil thermometer costs $8 to $15 and takes the guesswork out of timing. Stick it 4 inches into the ground in the morning. If it reads below 60°F, wait another week or two before planting. Once soil is warm enough and your seedlings are sturdy, you can harden them off — a 7 to 10 day process of gradually exposing them to outdoor sun and wind — before planting them permanently.

Key Takeaways

  • Plant seedlings outdoors only after your last frost date has passed and soil temperature reaches at least 60°F, ideally 65°F or warmer.
  • Seedlings need their first true leaves (the second set) and should be 6 to 8 weeks old before moving to the garden or containers.
  • Harden off seedlings over 7 to 10 days by gradually exposing them to outdoor conditions before planting them in their final location.
  • Bury the stem deeper than it grew in the pot — tomatoes will root along the buried stem and develop a stronger root system.
  • Space seedlings 24 to 36 inches apart in the garden, or use one seedling per 5-gallon container, and water deeply after planting.

Harden off seedlings before planting them outside

Seedlings grown indoors under lights are tender and will wilt or burn if you move them directly into full sun and wind. Hardening off gradually acclimates them to outdoor conditions over 7 to 10 days. Start by placing seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours on the first day. Each day, increase their time outside and move them into more direct sun. By day 7 or 8, they should be in full sun for the entire day. By day 10, they can stay outside overnight if temperatures stay above 50°F.

Watch the soil moisture during hardening off — outdoor air dries pots faster than indoor air. Check seedlings twice a day and water whenever the top of the soil feels dry to the touch. If a seedling wilts during hardening off, move it back to shade for a few hours, water it, and slow down the process. There is no penalty for taking 12 or 14 days instead of 10 — a slow hardening produces sturdier plants than a rushed one.

Choose a planting location with at least 6 to 8 hours of direct sun

Tomatoes need full sun to produce fruit and to resist disease. A location that receives 6 hours of direct sun will grow tomatoes, but 8 hours or more produces heavier yields and sweeter fruit. If your garden has afternoon shade, that is actually helpful in very hot climates — afternoon shade in zones 9 and 10 can prevent sunscald on fruit. In cooler zones, choose the sunniest spot available.

Soil should drain well. If water pools in your planting area after rain, either amend the soil with compost or plant in containers instead. Tomatoes rot quickly in waterlogged soil. If you are planting in the ground, mix 2 to 3 inches of compost or aged manure into the top 8 inches of soil before planting. This improves drainage, adds nutrients, and gives roots something to grow into. For containers, use a potting mix (not garden soil), which is lighter and drains better.

Dig a hole deeper than the seedling's root ball and bury the stem

Tomatoes have a unique ability: they will root along any buried stem. This means you can plant a seedling deeper than it grew in its pot, and the buried stem will develop additional roots. A stronger root system means the plant can access water and nutrients more reliably, especially during dry spells. Dig a hole 1.5 to 2 times as deep as the seedling's root ball. If your seedling is 6 inches tall, dig a hole 8 to 10 inches deep.

Gently remove the seedling from its pot by tipping the pot on its side and sliding the plant out. If roots are circling the bottom of the pot (root-bound), loosen them with your fingers before planting. Place the seedling in the hole so that only the top 2 to 3 leaves are above soil level. Bury the rest of the stem. Firm the soil around the base and water deeply until water drains from the bottom of the hole. The seedling will look short and buried, but new roots will form along the stem within 2 to 3 weeks.

Space seedlings correctly and stake or cage them early

Tomato plants grow large and need room for air circulation, which reduces disease. Space seedlings 24 to 36 inches apart in the garden — determinate (bush) varieties can be closer at 24 inches, while indeterminate (vining) varieties need the full 36 inches. In containers, use one seedling per 5-gallon bucket minimum. A 10-gallon container can hold one large plant comfortably.

Install a stake, cage, or trellis at planting time, not later. Waiting until the plant is large risks damaging roots when you drive a stake into the ground. A 6-foot stake works for indeterminate varieties; a 4-foot cage suits determinate types. If you are using string or clips to tie the plant to a stake, wait until the plant is 12 inches tall before tying, then tie loosely so the stem can thicken.

Water deeply after planting and maintain consistent moisture

Water the seedling when ready after planting so soil settles around the roots and eliminates air pockets. Water until it drains from the bottom of the hole or container. For the first week, check soil moisture daily — newly planted seedlings in warm weather may need water every day. Once the plant is established (after 2 to 3 weeks), water deeply but less frequently. Tomatoes prefer deep, infrequent watering over shallow daily sprinkling, which encourages shallow roots.

Aim for 1 to 1.5 inches of water per week, including rain. Water at the base of the plant, not on the leaves, to reduce fungal disease. Early morning is the best time to water. If you cannot water by hand, drip irrigation or soaker hoses deliver water directly to the soil and are more efficient than overhead sprinklers. Mulch around the base with 2 to 3 inches of compost or straw to retain moisture and regulate soil temperature.

Protect seedlings from pests and harsh weather in the first week

Newly planted seedlings are vulnerable to cutworms, which sever stems at soil level. Create a physical barrier by wrapping a 2-inch collar of cardboard or plastic around the stem, pushing it 1 inch into the soil and leaving 1 inch above. Remove the collar after 3 to 4 weeks when the stem hardens. If a seedling is damaged by cutworms, it is usually too late to save it — prevention is easier than recovery.

If a late frost or unexpected cold snap is forecast within a week of planting, cover seedlings with row covers, cloches, or even overturned buckets on the coldest nights. Remove covers during the day so the plant can access sunlight. Once seedlings are 3 to 4 weeks old and have grown new leaves, they are more resilient and less likely to be killed by a brief cold spell. If a seedling is damaged by frost, prune off the blackened leaves and let the plant recover — it will often branch and regrow.

Frequently Asked Questions

Can I plant seedlings that are taller than 8 inches?

Yes. Tall, leggy seedlings are actually easier to plant because you can bury more of the stem. Bury all but the top 2 to 3 leaves, and the extra buried stem will root. Tall seedlings often produce stronger plants than short, stocky ones because they have more stem to work with.

What if my seedlings are still small at the frost date?

Wait. Small seedlings planted in cold soil will rot or sit dormant for weeks. It is better to wait 2 to 3 weeks for soil to warm and seedlings to grow than to plant too early. You can always plant later in the season — tomatoes planted in late May or early June will still produce a full harvest.

Do I need to remove the seed leaves before planting?

No. Seed leaves (cotyledons) will eventually yellow and fall off on their own. Leave them on the seedling — they still photosynthesize and feed the plant. Remove them only if they are diseased or touching the soil after planting.

How long until a planted seedling starts growing new leaves?

Expect a 3 to 5 day pause while the plant recovers from transplant shock and roots establish. After that, new leaves should appear within 1 to 2 weeks. If a seedling shows no new growth after 3 weeks, check soil moisture and sunlight — it may need more of one or the other.

Can I plant seedlings in containers on a patio instead of the ground?

Yes. Containers work well for tomatoes and give you control over soil quality and drainage. Use a 5-gallon container minimum, fill it with potting mix, and follow the same planting depth and watering rules as in-ground planting. Containers dry out faster, so check moisture more often during hot weather.