Start seeds indoors 6 to 8 weeks before your last spring frost

The single most important date for tomato seeds is your local last frost date — the average day when freezing temperatures stop in spring. Tomato seeds should go into soil indoors 6 to 8 weeks before that date. If your last frost date is May 15, start seeds indoors around March 20 to April 1. If it's June 1, start them around mid-April.

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 by hundreds of miles — a gardener in Georgia and one in Minnesota are working from completely different calendars.

Starting indoors gives seedlings time to develop true leaves and a small root system before they face outdoor soil and weather. Seeds started too early will outgrow their containers and become leggy (tall and weak). Seeds started too late won't be ready to transplant when soil warms up, and you'll lose weeks of growing season.

Key Takeaways

  • Count back 6 to 8 weeks from your last spring frost date to find when to start seeds indoors in pots or trays.
  • Your last frost date is specific to your location and can be found through the USDA Hardiness Zone Map or your local extension office.
  • Tomato seedlings need 12 to 16 hours of light per day indoors, either from a sunny window or a grow light positioned a few inches above the plants.
  • Transplant seedlings outdoors only after soil temperature reaches 60°F and nighttime air temperature stays above 50°F, usually 1 to 2 weeks after your last frost date.
  • Direct seeding tomatoes outdoors (planting seeds straight in the ground) works in warm climates but takes longer and is less reliable than starting indoors first.

How to count backward from your last frost date

Write down your last frost date, then subtract 6 to 8 weeks. A straightforward way: count back 42 to 56 days on a calendar, or use an online date calculator. If you're unsure whether to use 6 weeks or 8 weeks, use 7 weeks — that's the middle ground and works for most tomato varieties.

Determinate varieties (bush tomatoes that ripen all at once) can go on the shorter end, around 6 weeks. Indeterminate varieties (vining tomatoes that produce all season) benefit from the full 8 weeks because they need more time to establish before they start flowering. If you're new to growing tomatoes, 7 weeks is your safest bet.

Mark the start date on a calendar or set a phone reminder. Tomato seeds germinate in 5 to 10 days at room temperature (around 70°F), so you'll see sprouts within two weeks of planting.

Indoor light and temperature for seedlings

Once seeds sprout, seedlings need 12 to 16 hours of light per day. A sunny south-facing window works if it gets direct sun most of the day, but most indoor growers use a grow light or shop light with LED or fluorescent bulbs positioned 2 to 3 inches above the seedlings. Raise the light as plants grow to keep that distance steady.

Keep soil temperature between 65°F and 75°F during the day. Room temperature is usually fine. At night, seedlings can tolerate cooler conditions (down to 55°F), which actually strengthens them. Water when the soil surface feels dry to the touch, but don't let pots sit in standing water — soggy soil causes damping off, a fungal disease that kills seedlings at the soil line.

Once seedlings develop their first true leaves (the second set of leaves that appear, which look like actual tomato leaves), thin them out or transplant them into individual small pots. Crowded seedlings compete for light and air, and they'll become weak.

Hardening off seedlings before moving them outside

Hardening off means gradually exposing indoor seedlings 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 a mild day. Each day, increase the time and exposure to wind and direct sun. By day 7 or 8, seedlings can stay outside all day and night if temperatures stay above 50°F.

This step matters because seedlings grown indoors under lights are tender — moving them straight from a warm, still, bright room to outdoor sun and wind will shock them and stunt their growth. Hardening off takes time but prevents setbacks.

If a frost is forecast during hardening off, bring seedlings back indoors. Even a light frost will kill tender tomato plants.

Soil temperature and outdoor planting timing

Transplant seedlings into the garden only when soil temperature reaches at least 60°F and nighttime air temperature stays above 50°F. This usually happens 1 to 2 weeks after your last frost date, not on the frost date itself. A soil thermometer (available at any garden center for under $10) takes the guesswork out — push it 4 inches into the soil in the morning to check.

Tomatoes are warm-season plants. Planting them in cold soil slows root growth and invites root rot. If you plant too early, seedlings will sit in the ground barely growing for weeks while they wait for soil to warm up. It's better to wait a week or two and have seedlings take off when ready.

Space transplants 24 to 36 inches apart, depending on the variety. Indeterminate types need more room because they grow tall and wide. Bury the stem deeper than it was in the pot — tomatoes will root along the buried stem and develop a stronger root system.

Direct seeding outdoors in warm climates

If you live in a warm climate where soil reaches 70°F by late spring and you have a long growing season (120+ days until first fall frost), you can plant tomato seeds directly in the garden instead of starting indoors. Plant seeds 1/4 inch deep, 12 inches apart, then thin seedlings to final spacing once they have true leaves.

Direct seeding takes longer — seeds germinate in 5 to 10 days, then seedlings need 6 to 8 more weeks to reach transplant size. By that time, an indoor-started seedling is already flowering. Direct seeding works best for gardeners who don't mind waiting or who are growing a large number of plants and don't have space for trays indoors.

In cooler climates with short growing seasons, direct seeding usually doesn't work because plants won't mature before frost arrives. Start indoors instead.

Common timing mistakes and how to avoid them

Starting seeds too early is the most common mistake. Seedlings started in January for a May frost date will be overgrown, leggy, and stressed by transplant time. They may flower indoors before they go in the ground, which wastes energy and delays fruit production. Stick to the 6 to 8 week window.

Starting seeds too late means seedlings won't be ready when soil warms up. You'll either plant small, weak seedlings that take weeks to establish, or you'll miss the early part of the growing season. If you miss your target date, it's better to buy nursery seedlings than to plant your own late.

Planting seedlings outdoors before soil and air warm up is the second most common mistake. Cold soil stops root growth and invites disease. If you're eager to plant, wait — the extra week or two of patience pays off in faster growth and healthier plants.

Frequently Asked Questions

Can I start tomato seeds in February if my last frost date is in May?

No. February to May is 12 to 13 weeks, which is too long. Seedlings will become overgrown and leggy before transplant time. Start seeds in late March or early April instead, which gives you the 6 to 8 weeks you need.

What if I don't know my last frost date?

Enter your zip code into the USDA Hardiness Zone Map at planthardiness.ars.usda.gov. If you can't find it online, call your local cooperative extension office — they have the exact date for your area and can answer other gardening questions too.

Can I start seeds outdoors in a cold frame or greenhouse?

Yes. A cold frame or unheated greenhouse extends the season and protects seedlings from wind and frost. You can start seeds a few weeks earlier outdoors in a protected structure than you can indoors, but you still need to harden them off before moving them to open garden soil.

What happens if I plant seedlings outside and a frost comes?

Tomato plants die at 32°F or below. If frost is forecast after you've planted, cover plants with blankets, cardboard, or frost cloth overnight. Remove the cover during the day so plants get sun and air. If a hard frost hits uncovered plants, they're lost — you'll need to replant or buy new seedlings.

Do all tomato varieties need the same start time?

Most varieties need 6 to 8 weeks indoors before transplant. Cherry tomatoes and determinate types can use the shorter timeline (6 weeks). Large beefsteak and indeterminate varieties benefit from the full 8 weeks. Check the seed packet — many list days to maturity, which helps you work backward from your first frost date if you're planting late in the season.