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

The right time to plant tomato seeds indoors depends on when your area's last frost date falls. Count backward 6 to 8 weeks from that date — that is your indoor planting window. If your last frost is May 15, for example, start seeds indoors around March 20 to April 1. Seeds started too early will outgrow their containers and become leggy (tall and weak) before transplant time. Seeds started too late will still be small seedlings when soil outdoors is warm enough to plant.

Your local frost date is the average date when the last freeze of spring occurs in your area. The USDA Plant Hardiness Zone Map and your county extension office both publish this date. Search "[your county] last frost date" to find it quickly. Some gardeners also use online tools like almanac.com or their state's cooperative extension website, which let you enter your zip code.

Key Takeaways

  • Count back 6 to 8 weeks from your last spring frost date to find when to plant tomato seeds indoors.
  • Seeds started before this window will become overgrown and weak before transplant time arrives.
  • Your county extension office or USDA zone map will tell you your exact last frost date.
  • Seedlings need 12 to 16 hours of light daily indoors, or they will stretch and weaken before moving outside.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before planting in the garden.

Why 6 to 8 weeks matters

Tomato seedlings reach transplant size — roughly 6 to 8 inches tall with 2 to 3 sets of true leaves — in about 6 to 8 weeks under proper indoor conditions. This timing lets you move them outdoors right when soil has warmed and frost danger has passed. If you start earlier, seedlings will be ready weeks before the soil is warm enough, and they will suffer in small pots waiting for planting day.

Starting too late means seedlings will still be small and fragile when you must plant them outdoors. Young plants struggle to establish roots in the garden and produce fruit later in the season. The 6 to 8 week window is a balance: seedlings are mature enough to handle transplanting but not so mature that they outgrow their containers or become stressed from waiting.

How to find your last frost date

The USDA Plant Hardiness Zone Map divides the United States into zones based on average annual minimum winter temperatures. Each zone has a corresponding last frost date. Visit planthardiness.ars.usda.gov, enter your zip code, and the map will show your zone and frost date. This is the most reliable source because it is based on decades of weather data.

Your county's cooperative extension office also publishes this information and can answer questions about your specific location. Search "[your state] cooperative extension" plus your county name to find the office's website or phone number. Many extension offices have printable frost date calendars or can tell you the date in one call. Some gardeners also check the Farmer's Almanac or local garden centers, which often post frost dates for their area.

Indoor growing conditions seedlings need

Tomato seedlings need 12 to 16 hours of bright light daily to grow strong and compact. A sunny south-facing window may work if it receives direct sun most of the day, but most indoor growers use fluorescent or LED grow lights placed 2 to 3 inches above the seedlings. Without enough light, seedlings stretch toward the light source and become tall, thin, and weak — a condition called "leggy." Leggy seedlings often fail after transplanting.

Keep soil consistently moist but not waterlogged. Tomato seeds germinate best at soil temperatures between 70 and 80 degrees Fahrenheit. Once seedlings emerge, they can tolerate slightly cooler temperatures (65 to 70 degrees), but growth slows below 60 degrees. Use a heat mat under seed trays if your home is cool, or place trays in a warm spot like the top of a refrigerator or near a heat source.

Hardening off seedlings before moving them outside

Seedlings grown indoors are tender and will wilt or die if moved directly into outdoor sun and wind. Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting in the garden. Start by placing seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase the time outdoors and gradually move them into more sunlight and wind.

By day 7 or 8, seedlings should spend most of the day outside in dappled shade or morning sun. By day 10, they can handle full sun for several hours. This gradual transition toughens the plants and reduces transplant shock. If frost threatens during hardening off, bring seedlings back indoors until the danger passes.

What happens if you plant too early or too late

Planting seeds too early means seedlings will be large and root-bound by transplant time. Overgrown seedlings are harder to transplant without damaging roots, and they often struggle to establish in the garden. They may also flower and set fruit while still in containers, which diverts energy from root growth and weakens the plant after transplanting.

Planting seeds too late means seedlings will be small and fragile when soil outdoors is ready. Small plants take longer to establish roots and produce fruit later in the season. In short growing regions, late-planted tomatoes may not mature before fall frost arrives. The 6 to 8 week window gives seedlings time to develop without becoming overgrown or too young.

Adjusting timing for your growing region

Gardeners in cold climates with short growing seasons should start seeds at the earlier end of the 6 to 8 week window — around 8 weeks before the last frost. This gives plants maximum time to grow and produce fruit before fall frost. Gardeners in warm climates with long growing seasons can start seeds closer to 6 weeks before the last frost.

If you live in a region where tomatoes are grown year-round or where frost never occurs, you can plant seeds indoors whenever you want to harvest. Many southern gardeners start a spring crop in late winter and a fall crop in midsummer. Check with your local extension office or experienced gardeners in your area for timing that works best for your climate.

Frequently Asked Questions

Can I start tomato seeds indoors earlier than 8 weeks before the last frost?

You can, but seedlings will outgrow their containers and become leggy or root-bound before transplant time. If you must start early, use larger containers or transplant seedlings into bigger pots as they grow. Provide plenty of light and keep temperatures cool to slow growth and prevent legginess.

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

Use the USDA Plant Hardiness Zone Map or contact your county extension office — both can give you the date in minutes. If you cannot find the exact date, ask local gardeners or garden centers when they typically plant tomatoes outdoors. Most gardeners in your area will have a reliable estimate based on years of experience.

Can I plant seedlings outdoors before the last frost date if they are large enough?

No. Tomato seedlings will die or stop growing if exposed to frost or soil below 50 degrees Fahrenheit. Wait until after the last frost date and when soil has warmed to at least 60 degrees (ideally 65 to 70 degrees) before planting outdoors. Planting too early wastes the seedlings and delays your harvest.

How do I know if my seedlings are ready to transplant outdoors?

Seedlings are ready when they are 6 to 8 inches tall, have 2 to 3 sets of true leaves (the leaves that look like tomato leaves, not the first round seed leaves), and have a sturdy stem. They should also have been hardened off for 7 to 10 days. If seedlings are smaller or still very tender, wait a few more days before planting.

What if I start seeds indoors too late and seedlings are still small at transplant time?

Plant them outdoors anyway once the last frost date has passed and soil is warm. Small seedlings will grow more slowly at first, but they often catch up to larger plants by mid-summer. Provide consistent water, warmth, and sunlight to help them establish quickly. Your harvest will be later, but the plants should still produce fruit before fall frost.