Start seeds indoors 6 to 8 weeks before your last spring frost
The timing for starting tomatoes depends almost entirely on when frost stops in your area. Count backward from your last frost date — the day when nighttime temperatures reliably stay above 50°F — and start seeds indoors 6 to 8 weeks before that date. This gives seedlings time to grow sturdy enough to transplant without being so old that they become root-bound or leggy in their containers.
Your last frost date varies by location. If you do not know yours, search your zip code plus "last frost date" or contact your local cooperative extension office. They maintain frost date maps for your county and can tell you the average date within a few days of accuracy.
Starting too early is a common mistake. Seedlings started in January for a May frost date will outgrow their pots and become weak and pale by transplant time. Starting too late means you transplant small, fragile seedlings that take weeks to recover and produce fruit later in the season.
Key Takeaways
- Count backward 6 to 8 weeks from your last spring frost date to find when to start seeds indoors.
- Seedlings need 12 to 16 hours of light daily from a grow light or sunny window, or they will become tall and spindly.
- Transplant seedlings outdoors only after nighttime temperatures stay above 50°F and the soil has warmed to at least 60°F.
- Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting them in the ground.
- Determinate varieties (bush types) and indeterminate varieties (vining types) have the same frost-date timing but different space and support needs.
Indoor seedling care from seed to transplant size
Once seeds sprout, seedlings need strong light when ready. A sunny south-facing window works if it gets at least 12 hours of direct sun daily, but most indoor spaces need a grow light positioned 2 to 3 inches above the seedlings. Without enough light, seedlings stretch toward the window and become tall, thin, and weak — a condition called leggy growth. Leggy seedlings transplant poorly and recover slowly.
Water seedlings when the soil surface feels dry to your finger, but do not let them sit in standing water. Overwatering causes damping-off disease, a fungal infection that kills seedlings at the soil line. Use a spray bottle or water from below by setting pots in a tray of water and letting them absorb moisture from underneath.
Keep soil temperature between 70°F and 80°F for fastest germination and growth. A seedling heat mat under your trays speeds germination and keeps roots warm. Once seedlings emerge, you can remove the heat mat or leave it on — either works, but warmth does not hurt once they are growing.
Recognize when seedlings are ready to transplant
Seedlings are ready to move to larger containers when they develop their first true leaves — the second set of leaves that appear after the initial seed leaves. At this stage, gently separate seedlings and pot them into individual 4-inch containers filled with potting soil. Tomato seedlings can be buried deeper than they were growing; they will develop roots along the buried stem, creating a stronger root system.
Continue growing seedlings indoors under lights until they are 6 to 8 inches tall with thick stems and several sets of true leaves. This usually takes 4 to 6 weeks from germination. At this point, they are ready for hardening off — the process of gradually introducing them to outdoor conditions.
Harden off seedlings before moving them outside
Seedlings grown indoors are tender and will wilt or die if moved directly into full sun and wind. Hardening off takes 7 to 10 days and means gradually exposing seedlings to outdoor conditions. Start by placing them in a sheltered, shaded spot outdoors for 2 to 3 hours on the first day. Each day, increase their time outside and gradually move them into more sunlight.
By day 7 or 8, seedlings should spend the full day outside in partial sun. By day 9 or 10, they can handle full sun and stay out overnight if nighttime temperatures stay above 50°F. If a cold night is forecast, bring them back inside. This gradual transition toughens the stems and leaves so they do not shock when planted in the ground.
Wait for warm soil and nighttime temperatures before planting
Transplant seedlings into the garden only after two conditions are met: nighttime temperatures stay above 50°F consistently, and soil temperature is at least 60°F. Tomatoes planted into cold soil sit dormant and often rot before they grow. A soil thermometer costs a few dollars and tells you the actual soil temperature — do not guess based on air temperature alone.
In most regions, this happens 1 to 3 weeks after the last frost date. If you transplant too early and a cold snap arrives, cover plants with row covers or cloches at night to protect them. Once soil warms and nighttime lows stay above 50°F, remove covers and let plants grow freely.
Adjust timing for determinate and indeterminate varieties
Determinate tomatoes (bush types that grow to a fixed size and produce most fruit at once) and indeterminate tomatoes (vining types that grow all season and produce continuously) follow the same frost-date timing for starting seeds and transplanting. The difference is in how you space and support them in the garden, not when you start them.
Both types need the same 6 to 8 weeks indoors and the same warm soil conditions before transplanting. Choose the variety based on your garden space and how you want to harvest — determinate for canning or sauce-making all at once, indeterminate for fresh eating throughout the season.
Plan ahead if your frost date is very late or very early
If your last frost date is in June or later, starting seeds indoors in early March means seedlings will be very large by transplant time. In this case, start seeds later — in late March or early April — so seedlings are the right size when soil finally warms. Oversized seedlings are harder to harden off and transplant successfully.
If your last frost date is in March or early April, you have a short window. Start seeds in late January or early February, but be prepared to keep seedlings under lights and in containers longer while you wait for soil to warm. Some gardeners in very short-season areas start seeds even earlier and use row covers or cloches to warm soil before transplanting.
Frequently Asked Questions
Can I start tomato seeds directly in the garden instead of indoors?
You can, but it delays harvest by 4 to 6 weeks. Direct seeding works only if your growing season is long enough — at least 70 to 85 frost-free days depending on variety. Starting indoors lets you use your full season and harvest ripe fruit before fall frost arrives. In short-season areas, direct seeding often does not produce mature fruit.
What if I started seeds too early and seedlings are already large?
Pot them into larger containers and keep them under lights until soil warms enough to transplant. Do not plant them outside early — cold soil will stunt them or cause rot. If they become very tall and spindly, you can bury them deeper when you do transplant, which encourages root growth along the buried stem.
Do I need a grow light, or will a window work?
A sunny south-facing window with 12+ hours of direct sun daily works, but most indoor locations do not get enough light. Seedlings in dim windows become leggy and weak. A basic LED grow light costs $20 to $50 and produces much stronger seedlings. Position it 2 to 3 inches above seedlings and run it 14 to 16 hours daily.
What temperature should the room be for seedlings?
Seedlings grow best between 65°F and 75°F during the day. Nighttime temperatures can drop to 55°F to 60°F without harm. Avoid placing seedlings near heating vents or cold drafts. Consistent, moderate warmth produces sturdier plants than fluctuating temperatures.
Can I transplant seedlings if nighttime temperatures are still dropping to 45°F?
No. Tomatoes planted into soil below 60°F or exposed to nighttime temperatures below 50°F will sit dormant and often rot. Wait until both conditions are met: soil at least 60°F and nighttime lows consistently above 50°F. If you transplant early and cold weather arrives, cover plants with row covers at night to protect them.