Start tomato seeds indoors 6 to 8 weeks before your last spring frost date

The timing depends entirely on when frost stops in your area, not on the calendar. If your last frost date is May 15, you would start seeds indoors around March 20 to April 1. If it's June 1, you'd start around mid-April. Seeds need that 6- to 8-week window to grow into sturdy transplants ready to move outside without shock.

You can find your last frost date by searching your zip code on the National Weather Service website or asking your local cooperative extension office. Many gardeners also ask neighbors or check what's already in the ground at local garden centers—that's often a reliable local indicator.

Starting too early means seedlings get leggy and weak while waiting for warm soil outdoors. Starting too late means you'll transplant tiny seedlings that take weeks to catch up, and you'll lose the head start that indoor growing gives you. Six to eight weeks is the sweet spot.

Key Takeaways

  • Count backward from your last frost date: start seeds 6 to 8 weeks before that date arrives in your area.
  • Seedlings need 12 to 16 hours of light daily indoors, either from a grow light or a sunny south-facing window, or they will become tall and spindly.
  • Keep soil temperature between 70 and 80 degrees Fahrenheit for germination; seeds will sprout in 5 to 10 days at that range.
  • Transplant seedlings into larger containers once they develop their first true leaves (the second set of leaves that appear), usually 3 to 4 weeks after germination.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before planting them in the garden.

How to find your exact frost date

Your last spring frost date is the average date when the last freeze of the season occurs in your specific location. This varies by hundreds of miles—even neighboring counties can differ by two weeks. The USDA Plant Hardiness Zone Map and the National Weather Service both have searchable databases where you enter your zip code and get a specific date.

If you prefer not to search online, call your county's cooperative extension office. They have this information memorized and can tell you in one conversation. You can also ask at a local garden center or nursery—staff there work with this date every spring and usually know it by heart.

Write the date down and keep it somewhere visible. You'll use it every year you start seeds indoors, and it's the single most important number for timing your whole spring garden.

Setting up light and warmth for germination

Tomato seeds germinate best in warm soil, ideally between 70 and 80 degrees Fahrenheit. A heating mat placed under your seed trays will speed germination to 5 to 10 days. Without heat, germination takes 14 to 21 days and is less reliable. Heating mats are inexpensive—usually $15 to $30—and last for years.

Once seedlings emerge, they need strong light when ready. A sunny south-facing window works if you have one, but most indoor growers use a grow light positioned 2 to 3 inches above the seedlings. Seedlings need 12 to 16 hours of light daily. Without enough light, they stretch toward the window and become weak and tall—this is called "leggy" growth, and it's hard to fix.

You can use an inexpensive LED shop light from a hardware store, or a dedicated grow light. Set it on a timer so it turns on and off automatically. Adjust the light height as seedlings grow, keeping it close enough that they stay compact and dark green.

Watering and soil for indoor seedlings

Use a seed-starting mix, not garden soil or potting soil. Seed-starting mix is lighter and fluffier, which helps seeds germinate and roots develop. It also drains well and holds moisture evenly. Garden soil compacts and can harbor fungal diseases that kill seedlings.

Keep the soil moist but not waterlogged. Water from below by setting trays in a shallow pan of water and letting the soil wick moisture up, or water gently from above with a spray bottle. Overwatering is the most common mistake—it causes damping off, a fungal disease that kills seedlings at the soil line. If the soil surface feels dry to your finger, water. If it feels damp, wait another day.

Once seedlings have their first true leaves, you can start feeding them with a diluted liquid fertilizer every two weeks. Seed-starting mix has almost no nutrients, so seedlings need supplemental feeding to stay healthy and green.

Transplanting seedlings into larger containers

When seedlings develop their first true leaves—the second set of leaves that look like actual tomato leaves, not the smooth rounded seed leaves—they're ready to move into larger containers. This usually happens 3 to 4 weeks after germination. At this point, roots are crowded in the seed tray and seedlings need more room to grow.

Use 4-inch pots or small cell packs filled with potting soil. Gently separate seedlings and plant each one into its own container, burying the stem up to the first true leaves. Tomato stems can develop roots all along their length, so burying them deeper actually strengthens the root system. Water gently after transplanting and keep the soil consistently moist.

Continue providing 12 to 16 hours of light daily and keep temperatures between 65 and 75 degrees Fahrenheit. Seedlings grow more slowly at this stage, which is fine—you want them to stay compact and sturdy, not tall and weak.

Hardening off before moving seedlings outdoors

Seedlings grown indoors are tender and will wilt or die if moved directly into the garden. They need 7 to 10 days of gradual exposure to outdoor conditions—wind, direct sun, and temperature swings. This process is called hardening off.

Start by placing seedlings in a sheltered spot outdoors for 1 to 2 hours on the first day, in dappled shade or morning sun. Each day, increase the time outdoors and the amount of direct sun they receive. By day 7 or 8, they should be outside all day. Bring them back inside at night until nighttime temperatures stay above 50 degrees Fahrenheit consistently.

Water seedlings more frequently during hardening off because outdoor air dries soil faster than indoor air. Check soil moisture daily and water when the surface feels dry. Seedlings are most vulnerable during this transition, so don't rush it.

Common mistakes that weaken seedlings

Starting seeds too early is the most common error. Seedlings started in January for a May frost date will be overgrown and root-bound by planting time. They'll struggle to recover when transplanted and often produce fewer flowers. Stick to the 6- to 8-week window.

Insufficient light creates leggy seedlings that fall over and never catch up. If you don't have a grow light, a sunny window is better than nothing, but move seedlings to the brightest window available and rotate the tray daily so they don't lean toward the light.

Overwatering kills more seedlings than underwatering. Soggy soil invites fungal disease and root rot. Let the soil surface dry slightly between waterings. If seedlings start to wilt, water when ready, but wilting from dry soil is less damaging than wilting from fungal disease.

Skipping the hardening-off step causes transplant shock and stunted growth. Even if it seems tedious, those 7 to 10 days of gradual outdoor exposure make the difference between seedlings that thrive and seedlings that barely survive.

Frequently Asked Questions

Can I start tomato seeds earlier if I have a grow light?

A grow light helps seedlings stay compact and healthy, but it doesn't change when you should plant them outdoors. Starting too early just means seedlings sit in pots longer and become root-bound. Stick to 6 to 8 weeks before your frost date, regardless of your lighting setup.

What if I don't know my exact frost date?

Search your zip code on the National Weather Service website or call your county cooperative extension office. If neither is available, ask at a local garden center or nursery—they use this date every spring. You can also ask neighbors what they plant and when.

Can I start seeds in a regular potting soil instead of seed-starting mix?

Potting soil is heavier and compacts more easily than seed-starting mix, which makes germination slower and less reliable. Seed-starting mix is inexpensive and worth the small difference. If you only have potting soil, mix it half-and-half with perlite or vermiculite to lighten it.

How do I know if my seedlings are getting enough light?

Seedlings getting enough light are short, stocky, and dark green. Seedlings not getting enough light are tall, thin, pale green, and lean toward the light source. If yours look stretched, move them closer to a light source or add a grow light when ready.

What temperature should the room be for seedlings?

During germination, soil temperature matters most—70 to 80 degrees Fahrenheit. After seedlings emerge, air temperature between 65 and 75 degrees Fahrenheit is ideal. Cooler temperatures slow growth but produce sturdier plants. Avoid placing seedlings near heating vents or cold windows.