Starting tomatoes indoors from seed gives you the strongest plants and the widest choice of varieties

You can start tomatoes from seed indoors 6 to 8 weeks before your last spring frost, or buy seedlings from a nursery and plant them outside after frost danger passes. Starting from seed costs less and lets you grow heirloom and specialty varieties you won't find at a garden center. Seedlings are faster if you're short on time or indoor space. Both methods produce healthy plants — the choice depends on your schedule and how many tomatoes you want to grow.

The timing matters because tomato seeds need warmth to sprout and light to grow strong, and seedlings need time to harden off before facing outdoor conditions. If you start too early, seedlings become overgrown and root-bound before planting time. If you start too late, you'll be planting small, weak plants that take longer to produce fruit. Getting the timing right means knowing your frost date and counting backward from there.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date, which you can find by searching your zip code and "frost date".
  • Seeds need warm soil (70 to 80 degrees Fahrenheit), consistent moisture, and bright light within a few days of sprouting or they will become leggy and weak.
  • Harden off seedlings by moving them outdoors for a few hours each day for one week before planting them in the garden, so they adjust to wind and direct sun.
  • Plant seedlings outside only after all frost danger has passed and soil has warmed to at least 60 degrees Fahrenheit, usually 2 to 3 weeks after your frost date.
  • Nursery seedlings skip the indoor stage but cost more and offer fewer varieties than seeds.

What you need to start seeds indoors

Gather seed-starting mix (a sterile, lightweight blend made for seeds, not garden soil), small pots or seed trays with drainage holes, a heat mat or warm location, and a light source. A heat mat keeps soil at the right temperature without heating your whole house. A grow light, shop light with LED or fluorescent bulbs, or a sunny south-facing window works for light — the key is keeping the light close to the seedlings (2 to 4 inches above) so they don't stretch toward it.

You'll also need a spray bottle or watering can with a fine rose (the sprinkler head) to keep soil moist without washing seeds away. A label and pencil to mark varieties is straightforward to forget but saves confusion later. If you're starting more than a few pots, a seedling heat mat costs $20 to $40 and makes the difference between seeds that sprout in 5 days and seeds that take 14. Seed-starting mix runs $8 to $15 per large bag and lasts for multiple seasons if stored dry.

Sowing seeds and the first two weeks

Fill pots or trays with seed-starting mix, dampen it thoroughly, and press seeds into the surface about one-quarter inch deep. Tomato seeds are small, so you can place one seed per cell or pot, or sow 2 to 3 seeds per pot and thin to the strongest seedling later. Cover the pot loosely with a humidity dome or plastic wrap to hold moisture, and place it on a heat mat set to 75 to 80 degrees Fahrenheit.

Check the soil daily — it should feel like a wrung-out sponge, not soggy or bone-dry. Seeds sprout in 5 to 10 days at the right temperature. The moment you see green, remove the cover and move the pot under a light source. Keep the light on for 14 to 16 hours a day. If you use a window, rotate the pot every few days so seedlings grow straight instead of leaning toward the glass. Without enough light, seedlings become tall and spindly (called "leggy") and won't recover even after you plant them outside.

Watering and feeding seedlings

Water from below by setting pots in a tray of water and letting them drink it up, or water gently from above with a spray bottle. The soil should stay moist but not waterlogged — overwatering causes damping off, a fungal disease that kills seedlings at the soil line. If you see white fuzz on the soil, improve air circulation by running a small fan on low nearby.

Once seedlings have their first true leaves (the second set that appears, which look like tiny tomato leaves), they've used up the nutrients in the seed-starting mix. Feed them every 7 to 10 days with a diluted liquid fertilizer mixed at half strength — tomato seedlings are tender and strong fertilizer burns them. Stop feeding one week before you move them outside.

Potting up and growing larger

When seedlings are 2 to 3 inches tall with at least two sets of true leaves, move them to larger pots (4-inch pots work well). Tomatoes can be buried deeper than they were growing — bury them up to their first true leaves, and they'll grow roots along the buried stem. This makes a stronger root system that supports larger plants and heavier fruit production.

Keep the light close and the temperature between 65 and 75 degrees Fahrenheit during the day. At night, a drop to 55 to 60 degrees is fine and actually strengthens seedlings. Continue watering when the top of the soil feels dry and feeding every 7 to 10 days. By 4 to 6 weeks after sowing, you should have stocky plants 6 to 8 inches tall with thick stems and no sign of legginess.

Hardening off before planting outside

One week before your last frost date, begin hardening off — moving seedlings outdoors for a few hours each day to adjust to sun, wind, and temperature swings. Start with a sheltered spot in dappled shade for 2 to 3 hours, then gradually increase time and sun exposure over 7 to 10 days. By the end, seedlings should spend a full day and night outside. This toughens the stems and leaves so they don't wilt or sunburn when planted in the garden.

If frost threatens during hardening off, bring seedlings back inside. Don't rush this step — seedlings hardened off properly establish faster and produce earlier than those planted soft. Watch the weather forecast during this week and be ready to move pots to shelter if temperatures drop below 50 degrees Fahrenheit.

Planting seedlings in the garden

Plant seedlings outside after your last frost date has passed and soil temperature reaches at least 60 degrees Fahrenheit (a soil thermometer costs $5 to $10 and takes the guesswork out). Space determinate varieties (bush types) 24 to 36 inches apart and indeterminate varieties (vining types) 36 to 48 inches apart. Dig a hole slightly deeper than the pot, bury the seedling up to its first true leaves, and firm soil around it gently.

Water thoroughly after planting and keep soil consistently moist for the first two weeks while roots establish. Mulch around plants with 2 to 3 inches of straw or shredded leaves to hold moisture and regulate soil temperature. Avoid mulching right against the stem, which can trap moisture and cause rot. Stake or cage indeterminate varieties early so you don't damage roots later.

Using nursery seedlings instead

If you buy seedlings from a garden center or nursery, choose stocky plants with thick stems and dark green leaves. Avoid tall, spindly plants or those with flowers already blooming — they've been stressed and won't perform as well. Harden off nursery seedlings the same way you would homegrown ones, even though they may look more mature. Plant them at the same depth and spacing as homegrown seedlings, and water well after planting.

Nursery seedlings are ready to plant 2 to 3 weeks after your frost date, so you don't need to start seeds as early. The trade-off is cost — a six-pack of seedlings costs $4 to $8, while a packet of seeds costs $2 to $4 and produces dozens of plants. Nursery seedlings also limit you to whatever varieties the local garden center stocks, which is usually a handful of popular types rather than the hundreds available as seeds.

Frequently Asked Questions

Can I start tomato seeds on a windowsill without a grow light?

A bright south-facing window works if you rotate pots daily and accept that seedlings may be slightly leggy. Grow lights are better because they stay the right distance and don't move, but a sunny window is free and many gardeners use it successfully. If seedlings stretch more than 2 inches between leaf sets, they need more light.

What does damping off look like and how do I stop it?

Damping off appears as a thin, dark pinch at the soil line where the seedling topples over. It's caused by fungal disease in wet soil. Prevent it by watering only when soil is dry to the touch, using sterile seed-starting mix, and running a small fan to keep air moving. If you see it, remove affected seedlings and let the soil dry out slightly between waterings.

Why are my seedlings tall and spindly?

Seedlings stretch toward light when it's too far away or too dim. Move your light source to 2 to 4 inches above the seedlings and keep it on for 14 to 16 hours daily. If you're using a window, rotate pots every few days. Leggy seedlings can be buried deeper when you pot them up, which helps, but prevention is easier than fixing it.

How do I know when my last frost date is?

Search your zip code and "last spring frost date" online — the National Weather Service and local extension offices publish this for every area. It's the average date of the last freezing temperature in spring. Planting before this date risks losing seedlings to frost, so it's worth waiting those extra weeks.

Can I start tomatoes directly in the garden instead of indoors?

You can, but tomatoes need a long growing season and direct seeding outdoors delays harvest by 6 to 8 weeks compared to starting indoors. In short-season areas, direct seeding may not give you ripe tomatoes before fall frost. Starting indoors is the standard method for most home gardeners.