Start seeds indoors 6 to 8 weeks before your last spring frost
The best time to plant tomato seeds indoors is 6 to 8 weeks before the last frost date in your area. This timing gives seedlings enough time to grow strong roots and develop their first true leaves before you move them outside. If you plant too early, seedlings become leggy and weak while waiting for warm soil. If you plant too late, you'll transplant small, fragile plants into the garden when they're barely ready.
To find your last frost date, search online for "[your city] last frost date" or check with your local cooperative extension office. This date varies widely by region — it might be April 15 in one state and June 1 in another. Once you know the date, count backward 6 to 8 weeks and mark that day on your calendar. That's when you fill your seed trays and water them in.
Key Takeaways
- Plant seeds indoors 6 to 8 weeks before your region's last spring frost date, which you can find by searching online or calling your local extension office.
- Tomato seeds germinate best in soil that stays between 70 and 80 degrees Fahrenheit, so use a heat mat if your home is cooler than that.
- Move seedlings outdoors only after nighttime temperatures stay above 50 degrees and the soil has warmed to at least 60 degrees.
- Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the garden.
- Direct seeding outdoors (planting seeds straight in the ground) works only in warm climates where the growing season is long enough for fruit to mature.
Soil temperature matters more than air temperature
Tomato seeds will sit dormant in cold soil and rot if the soil stays wet and cool. They need soil that is consistently warm — between 70 and 80 degrees Fahrenheit — to germinate reliably. A seed starting heat mat costs $15 to $40 and makes a real difference, especially if you're starting seeds in a basement, garage, or cool bedroom.
Without a heat mat, place your seed trays in the warmest spot in your home: on top of the refrigerator, near (but not touching) a heating vent, or on a south-facing windowsill. Check the soil temperature with a soil thermometer, not just the air temperature. Soil that feels warm to your hand may still be in the 60s, which is too cold for reliable germination.
Transplant seedlings outdoors only when conditions are right
Moving seedlings from your warm indoor setup to the garden is the riskiest moment in the tomato-growing season. Cold soil and cold nights kill young plants or stunt them so badly they never recover. Wait until nighttime temperatures stay above 50 degrees Fahrenheit consistently — not just for one warm night, but for a full week. Soil temperature should be at least 60 degrees, ideally 65 or warmer.
Even if the calendar says it's time, check the weather forecast for the next two weeks. If a cold snap is coming, wait another week or two. Tomatoes are not in a hurry once they're in the ground; a plant that goes in at the right time will always outpace one that went in too early and spent weeks struggling.
Harden off seedlings before moving them outside
Seedlings grown indoors under lights have never felt wind, direct sun, or temperature swings. Moving them straight from your shelf to the garden shocks them and causes wilting, leaf burn, or death. Instead, spend 7 to 10 days gradually introducing them to outdoor conditions — a process called hardening off.
Start by placing seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours on the first day. Each day, increase the time outside and move them into brighter light. By day 7 or 8, they should be in full sun for most of the day. On the final days, leave them out overnight if temperatures stay above 50 degrees. This gradual shift toughens the leaves and stems so they can handle the real world.
Direct seeding works only in warm climates with long growing seasons
In some regions, you can skip the indoor seed-starting step and plant seeds directly in the garden soil. This works only if your growing season is long enough for tomatoes to mature before the first fall frost — usually at least 70 to 85 days from planting to harvest, depending on the variety. Direct seeding also requires soil that is already warm (at least 65 degrees) and stays that way.
If you live in a place where the last frost date is in late May or June, or where fall frost arrives in early September, direct seeding is risky. The plants may not have enough time to produce fruit before cold weather stops growth. Starting seeds indoors gives you a head start and makes the most of your short growing season.
Timing by region and tomato variety
Early-maturing varieties (60 to 70 days to harvest) can be started later than long-season types (80+ days). If you're growing a variety that takes 85 days to produce ripe fruit, and your first frost arrives September 15, you need to transplant by late May at the latest. A faster variety gives you more flexibility.
Heirloom and specialty tomatoes often take longer to mature than standard hybrids, so they need an earlier start. Check the seed packet or the seed company's website for the days-to-maturity number, then work backward from your first fall frost date. This calculation is more reliable than a calendar date, because it accounts for your actual growing season length.
Common mistakes that delay harvest or kill seedlings
Planting too early indoors creates leggy seedlings that are weak and prone to disease. They sit under lights for weeks waiting for transplant time, stretching toward the light and using up energy. Planting too late means you transplant small seedlings into warm soil, and they spend the first month just catching up instead of growing fruit.
Skipping the hardening-off step causes sudden wilting or leaf burn when seedlings hit direct sun and wind. Transplanting into cold soil shocks the roots and slows growth for weeks, even if the plant survives. Ignoring the soil thermometer and relying only on the calendar leads to seed rot or failed germination if the soil is still cold.
Frequently Asked Questions
Can I start tomato seeds in a sunny windowsill instead of under grow lights?
A south-facing windowsill works if it gets at least 12 to 14 hours of direct sun daily, but seedlings often become leggy because the light is weaker than grow lights. Grow lights are inexpensive ($20 to $50 for a basic setup) and produce stockier, stronger plants. If you use a windowsill, keep the lights on for 14 to 16 hours per day and position the tray as close to the glass as possible.
What if I miss my planting date and it's already too late?
If your last frost date has passed and you haven't started seeds yet, buy transplants from a garden center instead. They're already the right size for transplanting and will produce fruit on roughly the same schedule as seedlings you started at the right time. Starting seeds now would mean transplanting very small plants into warm soil, which delays flowering and harvest.
Do I need to use seed-starting mix, or can I use regular potting soil?
Seed-starting mix is lighter and fluffier than potting soil, which helps tiny roots push through more easily. Regular potting soil is heavier and can compact, making germination harder. Seed-starting mix costs about the same and is worth using for the first 6 to 8 weeks. Once seedlings have true leaves, you can transplant them into regular potting soil.
How do I know if my seedlings are ready to transplant outside?
Seedlings are ready when they have at least two sets of true leaves (the leaves that look like tomato leaves, not the smooth seed leaves that emerged first) and are 3 to 4 inches tall. They should be sturdy and green, not pale or stretching toward the light. Even if they look ready, wait for soil and air temperatures to reach the thresholds above — size alone doesn't mean they'll survive outdoors.
Can I plant tomato seeds in the fall for a winter crop?
In warm climates where winter temperatures stay above 50 degrees and frost never arrives, you can plant seeds in late summer for a fall and winter harvest. In cold climates, tomatoes stop growing when temperatures drop below 50 degrees at night, so a fall planting produces nothing. Check your region's first fall frost date; if it arrives before mid-October, a fall crop won't work.