Growing Tomatoes From Seed
Count back from your last frost, raise sturdy seedlings, and grow any variety you want from a single seed packet.
Starting your own tomatoes from seed is the single best upgrade most home gardeners can make. A six-pack of transplants gives you a few plants of whatever the store happened to grow. A single seed packet gives you dozens of plants of exactly the variety you want, for a fraction of the price.
This guide walks the whole journey, from choosing a sowing date with the interactive calculator below, through germination warmth and light, to potting on, hardening off, and transplanting. Get the timing and the warmth right and tomato seed is one of the easiest things to grow.
Why grow tomatoes from seed
- Variety choice. Garden centers stock a handful of common types. From seed you can grow heirlooms like Cherokee Purple and Brandywine, the sweetest cherries like Sungold and Sun Sugar, and paste tomatoes like San Marzano for sauce. See our how to grow tomatoes guide and its variety picker for ideas.
- Cost. One packet of 20 to 50 seeds costs about the same as a couple of transplants, and tomato seed stays viable for years if stored cool and dry.
- Stronger plants. You control the light, warmth, and watering from day one, so you raise sturdy seedlings rather than leggy, root-bound store plants, and you avoid bringing home pests or disease.
- Seed saving. Grow an open-pollinated variety and you can save seed from your best fruit for next year.
Seed-starting timing calculator
Tell us your last frost (a region or month is fine) and we will count back to your start-indoors date and forward to potting on, hardening off, and transplanting.
Florida and the frost-free South have no real winter freeze. Start seed in late summer (around August) and grow tomatoes through the cool dry season, skipping the brutal summer heat.
Step 1: Seed-starting mix
Sow into a proper seed-starting mix, not garden soil or general potting soil. Seed-starting mix is fine, light, and low in nutrients, which is exactly what tiny roots want, and it drains freely so seedlings do not sit wet. Garden soil compacts, carries disease, and is far too rich and heavy for germinating seed.
Fill clean cells, trays, or pots, firm the mix lightly, and water it before sowing. Sow seed about a quarter inch deep, cover lightly, and label every variety. Sow two seeds per cell and thin to the stronger one later.
Step 2: Warmth for germination
Warmth is the biggest single factor in good germination. Tomato seed comes up best at a soil temperature of about 70 to 80F. In that range it germinates evenly in 6 to 10 days. In cold mix it is slow, patchy, and far more likely to rot before it sprouts.
Indoors in late winter, a windowsill is often too cold at night. A seedling heat mat under the tray gives the steady bottom warmth tomato seed loves and is the most reliable way to get an even strike. The top of the refrigerator also works. Keep the mix moist but never soggy until the seedlings emerge.
Step 3: Light (and fixing leggy seedlings)
The moment seedlings emerge they need bright light, fast. Tomato seedlings that get too little light stretch toward the window with long, weak, pale stems. These leggy seedlings flop over and are slow to establish.
A windowsill is rarely bright enough in late winter. If your seedlings are stretching, give them more light:
- Move them to the brightest possible spot and turn the tray daily so they grow evenly.
- Better still, use a grow light 2 to 4 inches above the seedlings for 14 to 16 hours a day, raising it as they grow.
- Drop the temperature a little once they are up. Warm, dim conditions are what cause stretching.
Step 4: Potting on
Once seedlings have their first one or two sets of true leaves (the second pair, which look like real tomato leaves rather than the rounded seed leaves), pot them up individually into 3 to 4 inch pots. This gives the roots room to develop into strong, stocky plants.
Handle seedlings by a leaf, never the fragile stem, and lift them out with a dibber or the handle of a spoon. Plant each one deeper than it was, burying the stem up to the lowest leaves, and water gently. Move them into individual pots of regular potting soil now that they need more nutrients than seed-starting mix provides.
Step 5: Preventing damping off
Damping off is the heartbreak of seed starting. Healthy seedlings suddenly keel over, pinched and rotted at the soil line, often overnight. It is caused by soil-borne fungi that thrive in cool, wet, still, crowded conditions. You cannot cure it, only prevent it:
- Use fresh, sterile seed-starting mix and clean containers. Reused mix and dirty pots carry the fungus.
- Sow thinly so seedlings are not crowded, and thin early.
- Water from the bottom by setting the tray in shallow water, and let the surface dry slightly between waterings.
- Give good airflow. A small fan on low keeps the surface drier and stems stronger.
- Remove humidity domes as soon as seeds germinate, and never let the mix stay soggy.
Step 6: Hardening off
Seedlings raised indoors are soft and have never felt wind, full sun, or cold. Setting them straight out in the garden shocks them badly. Harden them off over 7 to 10 days by gradually exposing them to outdoor conditions.
Start by putting them outside in a sheltered, partly shaded spot for an hour or two, then bring them back in. Each day leave them out a little longer and in slightly more sun and wind, until after a week or so they are spending all day and night outside. By the end they are toughened up and ready to plant.
Step 7: Transplanting deep
Transplant after your last frost, once nights stay above about 50F and the soil is warm, and seedlings are 6 to 8 inches tall. Cold soil and cold nights stall tomatoes for weeks, so there is no rush.
The golden rule is to plant tomatoes deep. Bury the stem up to the first set of leaves, or lay a leggy plant on its side in a shallow trench with just the top poking out. Every part of the buried stem grows new roots, giving you a much stronger, more drought-resilient plant. Water in well and mulch, leaving a small gap around the stem.
| Stage | Timing | Key conditions |
|---|---|---|
| Start indoors | 6 to 8 weeks before last frost | Seed-starting mix, 1/4 inch deep, 70 to 80F soil |
| Germination | 6 to 10 days after sowing | Warm, moist, domed until sprouted |
| Pot on | At 1 to 2 sets of true leaves | Individual pots, potting soil, plant deeper |
| Harden off | 7 to 10 days before transplanting | Increasing time outdoors in sun and wind |
| Transplant | After last frost, nights above 50F | Full sun, plant deep, water in, mulch |
Track your seedlings from seed to harvest
Add tomatoes to your garden in the Planting Season app and get reminders for starting seed, potting on, hardening off, transplanting, and harvest, all tuned to your region and zone.
Open the App →Common questions
When should I start tomato seeds?
Start tomato seeds indoors about 6 to 8 weeks before your last frost. In the cold North that means March to early April, in the Midwest and Northeast late February to March, and in the Mid-Atlantic and Upper South February. In the frost-free South and Florida, start seed in late summer for the cool growing season. Use the calculator on this page to work out your dates from your last frost.
Why grow tomatoes from seed instead of buying transplants?
Seed gives you a huge choice of varieties that garden centers never stock, including heirlooms and the sweetest cherries, and a single packet costs less than a few transplants. You also control the growing conditions from day one, so you raise stronger, healthier plants and avoid bringing home pests or disease.
What temperature do tomato seeds need to germinate?
Tomato seeds germinate best at a soil temperature of about 70 to 80F. At that warmth they come up in 6 to 10 days. In cold soil they are slow and prone to rotting. A seedling heat mat or a warm spot indoors gives the most reliable, even germination.
Why are my tomato seedlings tall and leggy?
Leggy seedlings are stretching toward weak light. A bright windowsill is rarely enough in late winter. Give seedlings as much direct light as possible, or use a grow light 2 to 4 inches above the plants for 14 to 16 hours a day. Keep them cooler once germinated, and bury leggy stems deep when you pot on, since tomatoes grow roots along buried stems.
How do I stop damping off?
Damping off is a fungal rot that topples seedlings at the soil line. Prevent it by using fresh sterile seed-starting mix and clean containers, sowing thinly, watering from the bottom, keeping good airflow with a small fan, and never letting the mix stay soggy. Avoid overwatering and overheating once seedlings are up.
When do I transplant tomato seedlings outside?
Transplant after your last frost, once nights stay above about 50F and the soil is warm, and seedlings are 6 to 8 inches tall with a few sets of true leaves. Harden them off over 7 to 10 days first, then plant deep, burying the stem up to the lowest leaves.
Source: university extension seed-starting and tomato growing guidance.
When to plant in your region
Pick your region to see exactly when to plant tomatoes where you garden.
See also: Tomatoes in the plant library →