Sow tomato seeds indoors 6 to 8 weeks before your average last spring frost, 1/4 inch deep in a sterile seed-starting mix kept at 70 to 80°F. Seedlings usually appear in 5 to 10 days and need 14 to 16 hours of bright light. Move them outside after hardening off, once nights stay above 50°F.
Most tomato trouble starts in the first 8 weeks, not in August. Seeds sown too early grow tall, pale and leggy by the windowsill; seeds sown too late give you transplants that fruit just as the season ends. Generic sowing advice treats tomatoes like any other seed, but they are a warm-season crop with a long runway, so the timing has to run backward from your frost date.
The 13 steps run on a simple sowing-to-seedling clock: the date you sow, the day the seedlings show, the day they get potted up, and the day they go outside. Before you buy anything, find your average last frost date by ZIP code and read the days to maturity on the seed packet.
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13 tomato seed-sowing steps from packet to planting
1. Hold the germination temperature steady
Tomato seeds sprout fastest when the mix stays between 70 and 80°F. At 60°F they can take 2 weeks or more, and below 50°F many never come up. A cool basement or a drafty windowsill is the most common reason for a slow, patchy tray.
A seedling heat mat, which usually runs $15 to $30, raises the mix about 10 to 20°F above room temperature. Take the tray off the mat once most seedlings are up, since warm roots plus weak light make leggy stems.
2. Keep the seedbed evenly moist
The mix should feel like a wrung-out sponge, never soggy. Moisten it before sowing, then cover the tray with a clear dome or plastic wrap to hold humidity until the first seedlings appear.
After that, water from below: set the cell tray in a shallow tray of water for 10 to 20 minutes, then drain. If the surface stays dark and wet for days or grows green film, you are watering too often.
3. Give emerging seedlings strong light
The day the first loops push through, light becomes the main job. A sunny window is rarely enough in late winter; seedlings lean and stretch within a few days. Hang a shop light or LED grow light 2 to 4 inches above the leaves and run it 14 to 16 hours a day on a timer.
Raise the light as the plants grow. Stems thicker than a pencil lead and leaves that stay deep green mean the light is right.
4. Thin seedlings before they crowd
Sow 2 seeds per cell, then thin to the strongest one when the first true leaves appear, usually 10 to 14 days after sowing. Snip the extra at soil level with small scissors rather than pulling, which tears the roots of the keeper.
Crowded tomato seedlings fight for light, stretch, and pass disease around. Keep it to one plant per cell.
5. Label the variety and sowing date
Tomatoes look alike for the first 6 weeks. Push a label into every cell or row with the variety name and sowing date the moment the seed goes in. A cherry, a paste tomato and a beefsteak can differ by 3 to 4 weeks in days to maturity, and you will want to know which is which at planting time.
6. Plan for root disturbance
Tomatoes handle transplanting better than most crops, so you can start them small and pot them up. When seedlings have 2 sets of true leaves, move each one into a 3 to 4 inch pot. You can bury the stem up to the first leaves; tomatoes grow new roots along buried stems.
Skip this step only if you sowed straight into 3 to 4 inch pots, which works but takes more space under the lights.
7. Harden seedlings off gradually
Indoor seedlings burn and wilt if they go straight into full sun and wind. Over 7 to 10 days, set them outside in a sheltered, shaded spot for 1 to 2 hours, then add a few hours of sun each day. Cut back watering slightly, without letting them wilt.
Bring them in any night forecast below 50°F. White or papery patches on leaves mean too much sun too soon; give them a day of shade.
8. Transplant at the right stage
The ideal transplant is about 6 to 10 inches tall, stocky, dark green and 6 to 8 weeks old, with no open flowers yet if you can manage it. Older plants that are already flowering and root-bound in small pots tend to stall after planting.
Wait for the weather as well as the plant: nights steadily above 50°F and soil around 60°F, usually 1 to 2 weeks after your last frost date.
9. Watch young seedlings for damping-off
Damping-off is a fungal problem that makes seedlings pinch at the soil line and topple, often overnight. It thrives in cool, wet, still conditions. Use fresh sterile mix and clean trays, water from below and run a small fan on low a few feet away for gentle air movement.
Once a seedling has collapsed, it will not recover. Remove it, let the surface dry slightly and improve airflow for the rest.
10. Keep one backup sowing
Sow a second small batch about 1 to 2 weeks after the first, even just 4 to 6 cells. If the first batch damps off, gets frost-bitten or stretches badly, the backup is still young enough to catch up. If everything goes well, you have extras to share. Honestly, a few seeds and a corner of the shelf is cheap insurance.
11. Record days to emergence
Write down the day the first seedling appears and when most of the tray is up. At 75°F, tomatoes usually emerge in 5 to 7 days. If yours took 12 days, the mix was too cool, the seed was old, or both.
Tomato seed usually stays good for about 4 years when kept cool and dry, so older packets may sprout more slowly and less evenly.
12. Adjust the next batch
Fix one thing at a time, using your notes. Leggy seedlings: lower the light or shorten the time on the heat mat. Transplants too big: sow 1 week later next year. Patchy emergence: test the mix temperature with a soil thermometer before blaming the seed.
13. Read the seed packet before sowing
The packet tells you the variety type and days to maturity, which is counted from transplanting, not from sowing. A 70-day tomato needs its 6 to 8 weeks indoors plus about 70 days in the garden, so roughly 4 months in all. In a short-season area, choose varieties listed at 55 to 70 days.
Set the tomato sowing window from local conditions
The sowing-to-seedling clock starts from one date: your average last spring frost. Count back 6 to 8 weeks for your indoor sowing date. If your last frost is around April 15, sow from mid to late February; if it is May 20, sow from late March to early April.
The University of Minnesota Extension guide to growing tomatoes covers indoor starting and why tomatoes should wait for warm soil before planting out. In warm southern areas with long seasons, you can sow a second round in midsummer for a fall crop.
Direct sowing outdoors is possible only where the season is long, roughly 150 or more frost-free days, and even then it delays the harvest by several weeks. In most of the US, indoor starting or buying transplants is the practical choice.
| Clock point | Timing from last frost |
|---|---|
| Sow indoors | 6 to 8 weeks before |
| First seedlings up | 5 to 10 days after sowing |
| Pot up to 3 to 4 inch pots | 2 to 3 weeks after sowing |
| Start hardening off | 1 to 2 weeks before planting |
| Plant outside | 1 to 2 weeks after, nights above 50°F |
Control depth, moisture, temperature and light
Sow tomato seeds about 1/4 inch deep. Deeper seeds use up their energy before reaching the light; seeds on the surface dry out. Firm the mix lightly over them and mist so it settles.
Use a sterile seed-starting mix rather than garden soil or regular potting mix. It is light, drains well and carries fewer disease spores; an 8 to 16 quart bag usually runs $8 to $15 and fills several trays.
Once seedlings are up, grow them a little cooler: around 65 to 70°F during the day and 60 to 65°F at night. That, plus 14 to 16 hours of light a few inches above the leaves, gives short, sturdy stems. Brush your hand across the tops for about 10 seconds a day; the gentle motion helps thicken the stems.
Start feeding with a liquid fertilizer at half strength once the first true leaves open, about once a week, since seed-starting mix holds very few nutrients. Follow the label on the bottle.
If you are starting other crops at the same time, lettuce works on a much cooler, shorter clock; when and how to sow lettuce seeds shows how to fit both on one shelf.
Move tomato seedlings without rushing them
The urge to plant on the first warm weekend is strong, but a cold snap after planting can stall tomatoes for 2 to 3 weeks. Watch the 10-day forecast: when nights stay above 50°F and daytime highs reach the 70s, plant.
Water seedlings an hour before moving them. Plant them deep, with the soil up to the first set of leaves, removing any leaves that would sit below the surface. Water in with about a gallon per plant.
Pet note: the ASPCA lists tomato plants as toxic to dogs, cats and horses because of the leaves and green stems. Keep seedling trays and trimmings where pets cannot chew them.
Cucumbers usually go out about the same time as tomatoes but are best sown much later; the timing is in when and how to sow cucumber seeds.
Record emergence and improve the next batch
Keep one line per batch: variety, sowing date, mix temperature, days to first seedling, date potted up, date hardened off and date planted. That is seven fields, and it fits on a label or a notebook page.
After one season, the record shows where the clock slipped. If seedlings were 12 inches tall and flowering at planting, sow 1 to 2 weeks later. If they were still small and spindly, sow earlier or add light. Marigolds share a similar warm-season clock if you want companions ready at the same time; see when and how to sow marigold seeds.
FAQ
When should I sow tomato seeds?
Sow tomato seeds indoors 6 to 8 weeks before your average last spring frost. Look up that date by ZIP code and count back. In long-season areas, a second sowing in early summer can give a fall crop, but always tie the timing to frost dates rather than a single calendar month.
Should I start tomato seeds indoors?
Yes, in most of the US. Tomatoes need a long warm season, and indoor starting gives them a 6 to 8 week head start. Direct sowing only makes sense where you have roughly 150 or more frost-free days, and the harvest will still come later.
How deep should I sow tomato seeds?
Sow them about 1/4 inch deep in moist, sterile seed-starting mix, two seeds per cell. Firm the surface lightly and cover with a dome until seedlings appear. If nothing emerges after 14 days, check the mix temperature before sowing again.
What temperature helps tomato seeds germinate?
A mix temperature of 70 to 80°F gives the fastest, most even germination, usually 5 to 7 days. Below 60°F it slows to 2 weeks or more. Use a heat mat to reach that range, then remove it once seedlings appear.
How much light do tomato seedlings need?
Indoor seedlings need about 14 to 16 hours a day from grow lights or shop lights kept 2 to 4 inches above the leaves. A windowsill alone usually produces leggy plants in late winter. Outdoors, mature tomatoes need at least 6 to 8 hours of direct sun.
When can I move tomato seedlings outdoors?
Move them out after a 7 to 10 day hardening-off period, once nights stay above 50°F and the soil is around 60°F. That is usually 1 to 2 weeks after your last frost date. The seedlings should be 6 to 10 inches tall and stocky.
Final thoughts
Tomatoes ask for patience twice: once for a warm, bright start indoors, and again while you wait for warm nights. Get the frost date right and most of the rest follows.
Today, look up your average last frost date, count back 6 weeks and write that sowing date on your first tomato label. For regional timing and planting tips, keep the University of Minnesota Extension tomato guide on hand.
Last update on 2026-10-02 / Affiliate links / Images from Amazon Product Advertising API