When to start tomato seeds indoors is 6–8 weeks before your average last frost date. Tomatoes germinate in 5–10 days in soil at 70–80°F, and seedlings need strong light and warm nights to grow stocky. Transplant outdoors only after the last frost, when nights stay above 50°F and the soil has warmed to at least 60°F.
Your last frost date is the anchor — not the calendar
“When to start tomato seeds indoors” has no fixed calendar date. Every sowing date counts back from your local average last frost date — the date when there is a 50% probability of no further frost, based on long-term weather data. Find yours from the USDA Plant Hardiness Zone Map, your state cooperative extension service, or the Old Farmer’s Almanac frost date calculator (enter your ZIP code). Microclimates matter: elevation, nearby water and urban heat can shift your real date by up to 10 days, so treat the published date as a guide and watch your local forecast.
Zone-by-zone sowing and transplant dates
Count back 6–8 weeks from your last frost for sowing, and transplant only when nights stay above 50°F and soil is 60°F or warmer.
| USDA Zone | Average last frost | Sow indoors | Transplant outdoors |
|---|---|---|---|
| Zone 4 | Late May | Early–mid April (6–8 weeks before) | Late May–early June |
| Zone 5 | Mid May | Late March–early April | Mid–late May |
| Zone 6 | Late April–early May | Mid–late March | Early–mid May |
| Zone 7 | Late March–mid April | Early–mid March | Mid April–early May |
| Zone 8 | Mid–late March | Late Jan–early Feb | Early–mid April |
| Zone 9–10 | Rare / no frost | Jan–Feb, or direct-sow outdoors | When soil reaches 65°F |
Fast varieties and cherry tomatoes can go at 6 weeks; large beefsteak and paste types do better with the full 8 weeks. Peppers, by contrast, need 8–10 weeks — start them 2 weeks before your tomatoes. See our zone-by-zone pepper timing guide for the full comparison.
How to start tomato seeds indoors (step by step)
- Find your last frost date and count back 6–8 weeks. Write the sowing date on your calendar.
- Use a fine, sterile seed-starting mix, not garden soil. A light mix drains well and prevents damping off — see our DIY seed starting mix recipe.
- Sow ¼ inch deep, 2 seeds per cell, and mist the surface. Tomato seeds need warmth more than light to germinate.
- Keep soil at 70–80°F. A heat mat helps, but a warm spot works too — below 60°F germination slows sharply. Check the germination temperature chart for exact ranges.
- Give 14–16 hours of bright light a day once seedlings emerge, with the light 2–4 inches above the tops. A sunny windowsill is rarely enough — leggy seedlings are almost always a light problem.
- Thin to one seedling per cell after the first true leaves appear, and feed only lightly after true leaves show.
- Harden off for 7–10 days before transplanting: an hour of outdoor shade first, adding time, sun and breeze daily, with nights out only at the end when it stays above 50°F.
- Transplant on a cloudy afternoon after frost danger has passed and soil is 60°F or warmer. Bury the stem up to the first true leaves — tomatoes root along the buried stem.
Started too early or too late? How to tell
| Problem | Cause | Fix |
|---|---|---|
| Leggy, pale, stretched seedlings | Too little light, or started too early | Move lights to 2–4 inches above tops; 14–16 hours a day |
| Seeds never sprout | Soil too cold (below 60°F) or old seed | Warmth at 70–80°F; test old seed first with the 10-seed viability test |
| Seedlings stall after sprouting | Cold nights, or overwatering | Warmth above 60°F; water only when the surface dries |
| Root-bound, flowering in the pot | Started far too early | Transplant as soon as conditions allow; pinch early flowers |
| No fruit after transplanting | Transplanted before the last frost, or into cold soil | Wait for soil at 60°F+ and nights above 50°F |
Starting too early is the more common mistake. A tomato started 10 weeks before frost in a small pot will be stressed, root-bound and slow to fruit — a 6-week seedling transplanted into warm soil will overtake it. If your tomatoes are struggling to sprout at all, see why tomato seeds fail to germinate.
Tomatoes vs peppers — why the timing is different
Tomatoes and peppers are often started together, but tomatoes need 2 fewer weeks indoors. Tomatoes germinate faster (5–10 days vs 7–21 for peppers), grow faster as seedlings, and tolerate cool nights better. If you start both on the same day, your peppers will be behind when transplant time comes. Start peppers first, then tomatoes 2 weeks later. See starting seeds indoors for the full indoor setup.
Frequently Asked Questions
When should I start tomato seeds indoors?
Start tomato seeds indoors 6–8 weeks before your average last frost date — for example, early to mid March in Zone 7, or late March to early April in Zone 5. Cherry types can go at 6 weeks; large beefsteak types do better with 8.
How long do tomato seeds take to germinate?
5–10 days in soil at 70–80°F. Below 60°F germination slows sharply or fails. A heat mat is the most useful tool if your room is cool.
Can I start tomato seeds too early?
Yes. Seedlings started more than 8 weeks before transplanting become leggy, root-bound and slow to fruit. A 6-week seedling moved into warm soil will usually overtake an overgrown early start.
What temperature do tomato seeds need to germinate?
Soil at 70–80°F (21–27°C). Night temperatures after transplanting should stay above 50°F, and soil should be at least 60°F before you plant out.
Do tomato seeds need light to germinate?
No — warmth matters more than light for germination. Once seedlings emerge, give 14–16 hours of bright light a day with the light 2–4 inches above the tops.
