Plant tomato seeds indoors 6 to 8 weeks before your last spring frost, or direct-sow them outdoors after soil reaches 60°F

The timing depends on whether you start seeds indoors under lights or plant them straight into the garden. Indoor seed starting gives you a head start—you'll have sturdy seedlings ready to transplant when the soil warms up. Direct sowing (planting seeds straight into garden soil) is simpler but takes longer to produce fruit because seeds germinate slowly in cool soil.

To find your last spring frost date, search "[your city] last frost date" or check the USDA Plant Hardiness Zone Map. Once you know that date, count backward 6 to 8 weeks to find when to start seeds indoors. For example, if your last frost is May 15, start seeds indoors around March 20 to April 1. If you prefer direct sowing, wait until soil temperature reaches at least 60°F—ideally 65°F or warmer—which usually happens 1 to 2 weeks after your last frost date.

Key Takeaways

  • Start seeds indoors 6 to 8 weeks before your last spring frost date so seedlings are ready when soil warms.
  • Direct-sow seeds outdoors only after soil temperature reaches 60°F, which typically occurs 1 to 2 weeks after your last frost date.
  • Indoor-started seedlings need grow lights, consistent moisture, and hardening off (gradual outdoor exposure) before transplanting.
  • Seeds planted too early indoors become leggy and weak; seeds planted too early outdoors rot in cold, wet soil.

Starting Seeds Indoors Under Lights

Indoor seed starting gives you the longest growing season and the strongest plants. Fill a seed-starting tray or small pots with moist seed-starting mix (not garden soil—it compacts and drains poorly). Plant one seed per cell, push it down about 1/4 inch, and cover lightly with mix. Keep the soil consistently moist but not waterlogged.

Place the tray under grow lights positioned 2 to 3 inches above the soil surface. Keep lights on for 14 to 16 hours daily. Seedlings should emerge in 5 to 10 days at a soil temperature of 70°F to 75°F. Once they sprout, they need bright light when ready—without it, they stretch tall and weak (called "leggy"). Lower the light fixture as the seedlings grow to maintain that 2- to 3-inch distance.

When seedlings develop their first true leaves (the second set of leaves that appear), thin them to one plant per cell or transplant them into larger individual pots. Water from below by setting pots in a tray of water for 10 minutes, then removing them—this prevents damping off, a fungal disease that kills seedlings at soil level.

Hardening Off Before Transplanting Outdoors

About 7 to 10 days before your last frost date, begin hardening off your seedlings. This means gradually exposing them to outdoor conditions so they adjust to wind, direct sun, and temperature swings. Start by placing seedlings in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase their time outside and move them into brighter light.

By day 7 or 8, seedlings should spend the full day outside in dappled shade or morning sun. On the final days, leave them in full sun for several hours. Watch the weather—if frost threatens, bring them back inside. Once nighttime temperatures stay above 50°F and soil is workable, transplant seedlings into the garden, spacing them 24 to 36 inches apart depending on the variety.

Direct Sowing Seeds Outdoors

If you skip indoor starting, direct-sow seeds into the garden after the soil has warmed to at least 60°F. Plant seeds 1/4 inch deep in loose, well-draining soil enriched with compost. Space seeds 6 inches apart in rows or clusters. Water gently so the soil stays moist but not soggy.

Seeds germinate in 5 to 10 days in warm soil, but germination slows dramatically below 60°F and stops below 50°F. Planting too early wastes seeds—they sit in cold, wet soil and rot before sprouting. Once seedlings have 2 to 3 true leaves, thin them to 24 to 36 inches apart by removing the weaker plants.

Direct sowing delays your harvest by 3 to 4 weeks compared to transplanting seedlings, because seeds need time to germinate and grow before they flower. In short-season climates (frost arrives in August or September), direct sowing may not give you enough time to ripen fruit before cold weather stops growth.

Regional Timing Examples

Timing varies widely by location. In the upper Midwest (Minnesota, Wisconsin), the last frost is typically May 20 to June 1, so start seeds indoors in early April and transplant in late May. In the mid-Atlantic (Pennsylvania, Maryland), the last frost is around May 10, so start seeds in late March and transplant in mid-May. In the South (Georgia, South Carolina), the last frost is April 1 to April 15, so start seeds in February and transplant in late March or early April.

In warm climates (southern Florida, southern Texas, southern California), frost may not occur at all, or it arrives very late. You can start seeds indoors in late summer or early fall to plant seedlings in fall and winter, giving you a winter and spring harvest. Check your local frost date first—it is the anchor for all other timing decisions.

Common Mistakes That Delay or Kill Seedlings

Planting seeds indoors too early creates leggy, weak seedlings that struggle after transplanting. If you start 10 weeks before your last frost instead of 6 to 8, seedlings outgrow their containers and become root-bound before they can go outside. They also stretch under insufficient light, becoming tall and fragile.

Planting seeds outdoors in cold soil is the most common direct-sowing mistake. Soil at 50°F feels warm to your hand but is too cold for tomato seeds. They sit dormant, rot, or germinate so slowly that weeds overtake them. Use a soil thermometer to check temperature at planting depth—do not guess based on air temperature.

Skipping the hardening-off step causes transplant shock. Seedlings grown indoors under lights are soft and tender. Moving them directly into full sun and wind causes wilting, leaf scald (bleached patches), and stunted growth. Even if they survive, they recover slowly and produce less fruit.

Frequently Asked Questions

Can I plant tomato seeds in February if I have grow lights?

You can, but seedlings will be overgrown and root-bound by transplant time. Count backward from your last frost date to find the right start date. If you start too early, seedlings become leggy and weak even under lights. If you have extra seedlings, you can pot them up into larger containers to delay transplanting, but this uses space and resources.

What if my last frost date has already passed and I haven't started seeds?

Direct-sow seeds outdoors if soil temperature is 60°F or warmer. You will harvest later than gardeners who transplanted seedlings, but you will still get fruit before fall frost in most climates. In short-season areas, choose early-maturing varieties (60 to 70 days to maturity) rather than long-season types (80+ days).

Do I need grow lights, or can I start seeds on a sunny windowsill?

A south-facing windowsill works if you have 6+ hours of direct sun daily, but seedlings often stretch toward the light and become leggy. Grow lights are inexpensive and give consistent, intense light that produces stocky, compact seedlings. If using a windowsill, rotate trays daily and keep lights on for 14 to 16 hours.

How long can seedlings stay in their seed trays before transplanting?

Seedlings can stay in seed trays for 2 to 3 weeks after sprouting, but they grow better if transplanted into larger individual pots once they develop true leaves. Leaving them crowded in seed trays for too long stunts growth and makes hardening off harder because roots are tangled and stressed.

Can I plant tomato seeds in summer for a fall harvest?

Yes, in climates where frost arrives in October or later. Count backward from your first fall frost date to find when to start seeds. In most areas, this means starting seeds in June or July for transplanting in July or August. Fall tomatoes often produce less fruit because days shorten and temperatures cool, but you can still harvest before frost.