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

The right time to plant tomato seeds indoors depends on when frost stops in your area. Count backward 6 to 8 weeks from your last spring frost date — that is when you should plant seeds in trays or pots under lights indoors. If your last frost date is May 15, for example, start seeds around March 20 to April 1. Starting too early means seedlings grow leggy and weak while waiting for warm soil outdoors; starting too late means small plants that take longer to produce fruit.

You can find your last frost date by searching your zip code on the National Weather Service website or asking at a local garden center. The date varies by region — it might be April 1 in the South, May 15 in the Midwest, or June 1 in cold climates. Mark this date on your calendar now so you know exactly when to sow.

Key Takeaways

  • Plant tomato seeds indoors 6 to 8 weeks before your area's last spring frost date, which you can find by searching your zip code online.
  • Seeds need soil temperature between 70 and 80 degrees Fahrenheit to germinate reliably, so use a heat mat under seed trays if your home is cooler.
  • Seedlings need 14 to 16 hours of bright light daily from a grow light or sunny window, or they will become tall and spindly.
  • Harden off seedlings by moving them outdoors for a few hours each day for one week before transplanting into the garden.
  • Transplant seedlings into the garden only after soil temperature reaches 60 degrees Fahrenheit and all frost danger has passed.

Check your soil temperature before planting seeds

Tomato seeds germinate best in warm soil — between 70 and 80 degrees Fahrenheit. If your home is cool, seeds will sit dormant in the soil for weeks and may rot before sprouting. A seed heat mat placed under your seed trays raises soil temperature reliably and costs $15 to $40. Plug it in when you plant seeds and leave it on until seedlings emerge, usually 5 to 10 days.

If you do not use a heat mat, place seed trays on top of the refrigerator, on a shelf above a heating vent, or in the warmest room in your house. Check soil temperature with a straightforward thermometer pushed into the soil. Cold soil is the most common reason seeds fail to sprout, so this step matters more than the exact calendar date.

Set up lights and a watering schedule before seeds arrive

Seedlings need 14 to 16 hours of bright light each day to grow stocky and strong. A sunny south-facing window works if you have one, but most home windows do not provide enough light. A grow light — a fluorescent or LED fixture hung 2 to 3 inches above seedlings — is more reliable and costs $20 to $60. Set it on a timer so lights turn on at 6 a.m. and off at 10 p.m., or adjust the hours to fit your schedule as long as seedlings get 14 to 16 hours total.

Water seedlings from below by setting pots in a tray of water and letting soil wick moisture up, or water gently from above with a spray bottle. Keep soil moist but not waterlogged — soggy soil causes damping off, a fungal disease that kills seedlings at the soil line. Check soil daily and water when the top feels dry to your finger. Most seedlings need water every 1 to 3 days depending on heat and light.

Thin seedlings and pot them up as they grow

Most seed packets contain more seeds than you need. When seedlings have their first true leaves — the second set of leaves that look like real tomato leaves — thin them by snipping off the weakest seedlings with scissors. Leave one strong seedling per cell or pot. Removing seedlings this way is less disruptive than pulling them out by hand.

When seedlings are 2 to 3 inches tall and have 2 to 3 sets of true leaves, move them into larger pots filled with potting soil. Tomato seedlings can be buried deeper than they were growing — bury them up to their first true leaves, and roots will form along the buried stem. This creates a stronger root system. Keep lights at the same height above the new pots, raising the light fixture as seedlings grow.

Harden off seedlings one week before moving them outdoors

Hardening off means gradually exposing seedlings to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start one week before you plan to transplant. On day one, place seedlings in a sheltered spot outdoors in shade for 2 to 3 hours, then bring them back inside. Each day, leave them out a bit longer and move them into more direct light. By day seven, seedlings can stay outside all day and night if temperatures stay above 50 degrees Fahrenheit.

Skipping this step causes seedlings to wilt, turn purple, or stop growing when planted in the garden. The process takes only a week and makes a real difference in how quickly seedlings recover and start growing in the garden.

Transplant seedlings into the garden at the right time

Move seedlings into the garden only when two conditions are met: soil temperature is at least 60 degrees Fahrenheit, and your area has passed the last frost date. Tomatoes planted in cold soil sit dormant and may rot. Use a soil thermometer pushed 4 inches into garden soil to check temperature in the morning. If soil is still cold, wait another week and check again.

Plant seedlings deeper than they grew in pots — bury them up to their first true leaves or even up to the lowest leaves if the stem is tall. Tomatoes root along buried stems, so deeper planting creates a stronger plant. Space plants 24 to 36 inches apart depending on the variety. Water well after planting and keep soil moist for the first two weeks while roots establish.

Common mistakes that delay growth

Starting seeds too early is the most common mistake. Seedlings started in January or early February grow too large and leggy before soil warms enough to plant them outside. They also outgrow pots and become root-bound, which stresses the plant. Stick to the 6 to 8 week window and you will have seedlings ready exactly when you need them.

Insufficient light is the second mistake. Seedlings grown in dim windows stretch toward the light, becoming tall and weak. A grow light solves this problem and costs less than replacing failed seedlings. Overwatering is the third mistake — soggy soil kills more seedlings than dry soil does. Water when soil feels dry to your finger, not on a fixed schedule.

Frequently Asked Questions

Can I plant tomato seeds directly in the garden instead of starting them indoors?

Yes, but you will get fruit much later. Direct seeding works in warm climates where the growing season is long, but in most areas starting indoors gives you a 6 to 8 week head start. Indoor-started plants produce fruit by mid-summer; direct-seeded plants often do not produce until fall.

What if I missed the planting window and it is already too late?

You can still plant seeds, but seedlings will be smaller when transplanted and produce fruit later in the season. Plant them now and adjust your transplant date forward — instead of waiting 6 to 8 weeks, transplant when seedlings are large enough to handle, even if it is only 4 to 5 weeks after planting.

Do I need to use seed-starting mix, or can I use regular potting soil?

Seed-starting mix is lighter and drains better than potting soil, which makes it easier to keep moisture balanced. Regular potting soil works but tends to stay wetter and increases the risk of damping off. Seed-starting mix costs only a few dollars more and is worth the investment.

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

Seedlings with enough light are short, stocky, and dark green. Seedlings with too little light are tall, thin, pale, and lean toward the light source. If seedlings are stretching, move the light closer or add more hours of light per day.

Can I reuse soil from last year's seedlings?

Reusing old soil can introduce disease and pests. Fresh seed-starting mix is inexpensive and eliminates this risk. If you must reuse soil, sterilize it by baking it in a 180-degree oven for 30 minutes, then let it cool completely before use.