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

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 containers under lights or in a sunny window. If your last frost date is May 15, for example, you would start seeds around March 20 to April 1.

Tomato seedlings need that much time to grow sturdy enough to transplant outdoors. If you start too early, they become leggy and weak while waiting for warm soil. If you start too late, they are still small when the weather turns hot enough to plant them out, and you lose weeks of growing season.

You can find your local last frost date through the USDA Hardiness Zone Map or by searching "[your city] last frost date." The date varies widely — it might be April 1 in the South or June 1 in the North. Some gardeners also check with their county extension office, which often has frost date data specific to your area.

Key Takeaways

  • Count backward 6 to 8 weeks from your last spring frost date to find when to plant seeds indoors.
  • Seedlings started too early become weak and stretched; those started too late stay small when transplant time arrives.
  • You can find your last frost date through the USDA Hardiness Zone Map or by contacting your county extension office.
  • Seedlings are ready to move outdoors when they have their first true leaves and soil temperature stays above 60°F at night.
  • Harden off seedlings by gradually exposing them to outdoor conditions for 7 to 10 days before planting them in the garden.

What your seedlings need while growing indoors

Tomato seeds germinate in 5 to 10 days at room temperature (around 70°F), but seedlings grow best under bright light. A sunny south-facing window works if you have one, 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 window and become thin and weak.

Keep soil moist but not waterlogged — soggy soil invites damping-off disease, which kills seedlings at the soil line. Water from below by setting containers in a tray of water and letting them soak it up, or water gently at the base. Once seedlings have their first true leaves (the second set of leaves that appear), you can begin feeding them with diluted liquid fertilizer every two weeks.

Seedlings also benefit from air movement. A small fan on low speed, running a few hours a day, strengthens stems and reduces disease. Keep the growing area between 65°F and 75°F during the day and slightly cooler at night.

Recognizing when seedlings are ready to transplant

Seedlings are ready to move to larger containers when they develop their first true leaves — the leaves that look like actual tomato leaves, not the smooth oval seed leaves that emerged first. At this stage, you can pot them up into 4-inch containers filled with potting soil. Bury the stem deeper than it was growing; tomatoes will root along the buried stem and develop a stronger root system.

Continue growing them indoors under lights until outdoor soil temperature stays above 60°F at night and all danger of frost has passed. In most regions, this means moving seedlings outside after your last frost date. Seedlings are usually ready at 6 to 8 weeks old, which is why the timing from seed-starting date matters.

Hardening off before moving seedlings outdoors

Seedlings grown indoors are tender and will wilt or die if moved directly into sun and wind. Harden them off by gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing them in a sheltered, shaded spot outdoors for an hour or two, then bring them back inside.

Each day, increase the time outside and the amount of direct sun they receive. By day 7 or 8, they should be able to spend the full day outside. On the final night, if temperatures stay above 50°F, you can leave them out overnight. This gradual transition toughens the stems and leaves so they can handle garden conditions.

Adjusting timing for different tomato varieties

Determinate tomatoes (bush types that ripen all at once) and indeterminate tomatoes (vining types that produce all season) both follow the same 6 to 8 week indoor timeline. However, heirloom and specialty varieties sometimes take longer to mature in the garden, so some gardeners start them a week or two earlier to give them a head start.

If you are growing a variety that takes 85 days or more from transplant to first fruit, starting seeds on the earlier end of the 6 to 8 week window gives you a longer harvest window before fall frost. Check the seed packet for days to maturity and adjust your start date if needed. A variety that takes 90 days to mature needs to be in the ground earlier than one that produces fruit in 65 days.

Common mistakes that set seedlings back

Starting seeds too early is the most common error. Seedlings that sit indoors for 12 or 14 weeks become root-bound, leggy, and stressed before they ever go outside. They recover slowly in the garden and may not produce as much fruit. Stick to the 6 to 8 week window unless you have a very long growing season.

Overwatering is the second major problem. Seedlings need moist soil, not soggy soil. If the soil stays wet, roots rot and seedlings collapse. Water when the top of the soil feels dry to the touch, and always use containers with drainage holes.

Skipping the hardening-off step causes wilting and sunscald (pale, papery patches on leaves). Even if seedlings look sturdy indoors, they need that gradual transition to handle outdoor sun and wind. Do not skip it, even if it takes extra time.

Frequently Asked Questions

Can I start tomato seeds directly in the garden instead of indoors?

You can, but you will lose 6 to 8 weeks of growing time. Tomatoes need a long season to produce fruit before frost arrives. Direct seeding works in warm climates with 120+ frost-free days, but in most regions, starting indoors gives you a much larger harvest.

What if I missed the ideal start date?

If you are a few weeks late, start seeds anyway — you will still get tomatoes, just fewer of them. If it is already late spring and soil is warm, you can buy transplants from a nursery instead of starting from seed. Nursery plants are already hardened off and ready to plant.

Do I need grow lights, or is a window enough?

A bright south-facing window can work, but seedlings often stretch toward the light and become weak. Grow lights are inexpensive and give much better results. If you use a window, rotate seedlings daily and keep them as close to the glass as possible.

How deep should I bury the seedling when I transplant it to the garden?

Bury it up to the first true leaves. Tomatoes root along buried stems, so deeper planting creates a stronger root system. This is one of the advantages of starting indoors — you can bury leggy seedlings deeper than you could with direct-seeded plants.

What temperature kills tomato seedlings?

Frost (32°F or below) kills seedlings. Even temperatures in the 40s slow growth and stress plants. Wait until nighttime temperatures stay above 50°F consistently before moving seedlings outside, and do not plant them in the garden until soil is at least 60°F and frost danger has passed.