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

The timing for starting tomato plants depends on your location's frost date and whether you are starting from seed or buying transplants. If you are growing from seed indoors, count backward 6 to 8 weeks from the date of your area's last spring frost. That frost date is the average day when freezing temperatures stop for the season in your region — tomatoes cannot survive frost, so you need to know this number before you plant anything outside.

You can find your last spring frost date by searching "[your city] last frost date" or by checking the USDA Hardiness Zone Map, which lists frost dates by region. Once you have that date, mark it on a calendar and count back 6 to 8 weeks. That is when you should plant seeds in seed trays indoors under grow lights or in a sunny window.

If you are buying transplants from a nursery instead of starting from seed, you can wait until 1 to 2 weeks before your last frost date to purchase them. Nursery transplants are already several weeks old, so they do not need the full 6 to 8 week head start that seeds require.

Key Takeaways

  • Find your last spring frost date by searching online or checking the USDA Hardiness Zone Map, because tomatoes cannot go outside until after that date.
  • If starting from seed indoors, plant 6 to 8 weeks before your last frost date so seedlings are ready to transplant outside at the right time.
  • If buying transplants from a nursery, purchase them 1 to 2 weeks before your last frost date, when they are ready to move into the garden.
  • Seedlings started too early will become leggy and weak; seeds started too late will not have time to grow before transplanting outdoors.
  • Keep indoor seedlings under grow lights for 14 to 16 hours per day and maintain soil temperature between 70 and 80 degrees Fahrenheit for best germination.

Why timing matters: too early and too late both cause problems

Starting seeds at the wrong time creates two opposite problems. If you start seeds too early — more than 8 weeks before your frost date — the seedlings will grow tall and spindly (called "leggy") while waiting in your home for warm weather outside. Leggy seedlings are weak and often do not recover well after transplanting, even if you bury them deeper in the soil.

If you start seeds too late — less than 6 weeks before your frost date — the seedlings will not be large enough to transplant outside when the weather is warm enough. Small seedlings are fragile and are more likely to be damaged by wind, pests, or inconsistent watering once they are in the garden. They also take longer to produce fruit because they spend their first weeks in the garden just catching up in size.

The 6 to 8 week window is the sweet spot: seedlings will be sturdy, have their first true leaves, and be ready to move outside right when soil temperatures warm up and frost danger passes.

Adjust timing based on your growing season length

Some regions have short growing seasons, and others have long ones. If you live in a cold climate where summer is short — such as the northern United States or Canada — you may need to start seeds closer to the 8-week mark to give plants enough time to flower and produce fruit before fall frost arrives. If you live in a warm climate with a long growing season, you can start seeds closer to the 6-week mark.

You can also plant a second round of seeds in mid-summer (about 8 weeks before your first fall frost) to grow a fall crop in regions where temperatures cool down but do not freeze until late in the year. Fall tomatoes often have better flavor because they mature during cooler weather, though they produce fewer fruits than spring plantings.

Indoor seed starting: temperature, light, and moisture

Once you have planted seeds indoors, keep the soil consistently moist but not waterlogged. Tomato seeds germinate best in soil that is between 70 and 80 degrees Fahrenheit — a heat mat under seed trays speeds up germination if your home is cooler. Seeds usually sprout within 5 to 10 days at that temperature.

As soon as seedlings emerge, move them under grow lights positioned 2 to 3 inches above the tops of the plants. Keep lights on for 14 to 16 hours per day. Without enough light, seedlings become leggy even if you started them at the right time. A basic fluorescent shop light or LED grow light works well and costs far less than specialty grow light systems.

Water seedlings from below by setting trays in a shallow pan of water and letting soil absorb moisture from the bottom, or water gently from above without splashing the leaves. Once seedlings develop their first true leaves (the second set of leaves that appear), you can thin them out or transplant them into individual small pots to give roots more room to grow.

Hardening off: the bridge between indoors and outdoors

About 1 to 2 weeks before your last frost date, seedlings need to adjust to outdoor conditions in a process called hardening off. Start by placing seedlings outside in a sheltered, shaded spot for 1 to 2 hours on a mild day. Each day, gradually increase the time outside and move them into more sunlight, adding an hour or two per day over the course of a week.

Hardening off prevents transplant shock and sunburn, which can kill seedlings that go straight from an indoor windowsill to full outdoor sun. By the end of the hardening-off period, seedlings should be able to stay outside all day and night without damage, and they are ready to plant in the garden.

Transplanting outdoors: soil temperature and spacing

Plant seedlings or nursery transplants outside only after your last frost date has passed and soil temperature is at least 60 degrees Fahrenheit — ideally 65 to 70 degrees. Cold soil slows growth and can cause seedlings to sit dormant or rot. You can check soil temperature with an inexpensive soil thermometer, or wait until nighttime temperatures stay above 50 degrees Fahrenheit consistently, which usually means soil is warm enough.

Space transplants 24 to 36 inches apart depending on the variety — determinate (bush) types need less space, while indeterminate (vining) types need more. Bury seedlings deeper than they were growing in their pots; tomatoes can develop roots along buried stems, which strengthens the plant. Water thoroughly after planting and keep soil consistently moist for the first 2 to 3 weeks while roots establish.

Regional frost date examples and what they mean for your schedule

Your last frost date varies widely by location. In the southern United States, it may be in March or early April, meaning you would start seeds indoors in January or early February. In the northern United States and Canada, it may be in May or early June, meaning you would start seeds in March or April. In mild climates with no frost, you can plant tomatoes almost year-round, though summer heat can reduce fruit set in some regions.

Once you know your frost date, write it down and use it as your anchor point for all tomato timing decisions. The same frost date also tells you when to plant other warm-season crops like peppers, eggplants, and basil, so finding it once saves time every year.

Frequently Asked Questions

Can I start tomato seeds directly in the garden instead of indoors?

You can, but it delays harvest significantly. Direct-seeded tomatoes take 2 to 3 weeks longer to produce fruit than transplants because seeds germinate slowly in cool spring soil and seedlings grow slowly outdoors. In short growing seasons, direct seeding may not give plants enough time to mature before fall frost. Starting indoors is faster and more reliable in most climates.

What if I miss my planting window and it is already too late?

If your last frost date has passed and you have not started seeds yet, buy transplants from a nursery instead. Nursery transplants are already weeks old and will produce fruit much sooner than seeds you plant now. You can also plant seeds directly in the garden at this point, though harvest will be later than with transplants.

Do I need grow lights, or can seedlings grow in a window?

A sunny south-facing window works if you have one, but grow lights are more reliable. Window light is often too weak and too angled, causing seedlings to lean toward the light and become leggy. Grow lights positioned close to plants provide consistent, strong light for 14 to 16 hours daily, which produces stockier, healthier seedlings.

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

Seedlings are ready when they are 6 to 8 inches tall, have at least two sets of true leaves, and have been hardened off for a week. They should look sturdy and green, not pale or spindly. If they are still small or leggy, wait another week before hardening them off.

Can I start tomato seeds in fall for a winter crop?

In warm climates where winter temperatures stay above freezing, yes. Count backward 6 to 8 weeks from your first fall frost date and start seeds then. In cold climates, tomatoes cannot survive winter outdoors, so fall seeding does not work.