Plant tomatoes after the last frost date in your area, when soil temperature reaches at least 60°F
The single most important rule for tomato planting is this: soil temperature matters more than calendar date. Tomatoes are warm-season plants. If you plant them in cold soil, the seeds will sit dormant or rot before they germinate. If you transplant seedlings into cold ground, they will stall and may die.
Your last frost date — the average date of the final frost in spring — is the earliest you should plant. But that date alone is not enough. Wait until soil temperature reaches at least 60°F, ideally 65°F or higher. You can check soil temperature with a straightforward soil thermometer from any garden center. Soil warms more slowly than air, so even if daytime air temperatures are warm, the ground may still be cold.
In most of the United States, this means planting between mid-April and late May, depending on your region. In the South, planting may happen in March. In the North, it may not happen until June. Check your local extension office or a frost date calculator online for your specific zip code.
Key Takeaways
- Soil temperature must reach 60°F or higher before you plant tomatoes, regardless of the calendar date or air temperature.
- Transplants (seedlings started indoors) go in the ground 6 to 8 weeks after your last frost date, when soil is warm enough.
- Direct seeding tomatoes outdoors is slower and riskier than starting seeds indoors and transplanting, so most home gardeners use transplants instead.
- Harden off seedlings by exposing them to outdoor conditions for 7 to 10 days before planting them in the garden.
- Plant seedlings deeper than they grew in their pots — bury the stem up to the first true leaves to encourage a stronger root system.
Starting seeds indoors 6 to 8 weeks before your last frost date
Most home gardeners do not direct-seed tomatoes outdoors. Instead, they start seeds indoors under lights in late winter or early spring, then transplant the seedlings into the garden after the last frost date and when soil is warm.
Count backward from your last frost date. If your last frost date is May 15, start seeds indoors around March 20 to April 1. Tomato seeds germinate in 5 to 10 days at room temperature (around 70°F) and grow into transplant-ready seedlings in 6 to 8 weeks. This timing gets you sturdy, leafy plants ready to go into the ground at exactly the right moment.
Use seed-starting mix (not garden soil) in small pots or trays. Keep the soil moist but not waterlogged. Once seedlings sprout, move them under grow lights or in a very bright window. Without enough light, seedlings become tall, weak, and leggy — they will not perform well in the garden.
Hardening off seedlings before transplanting
Seedlings started indoors are tender. They have never felt wind, direct sun, or temperature swings. If you move them straight from your warm house to the garden, they will wilt, turn purple, or die.
Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days. Start by placing them in a sheltered, shaded spot outdoors for 1 to 2 hours. Each day, increase the time outside and the amount of direct sun they receive. By day 7 or 8, they should be outside all day. By day 10, they can stay out overnight if temperatures stay above 50°F.
This process toughens the plant tissue and prepares the seedlings for the real garden. Skip this step and you risk losing most of your plants to transplant shock.
Planting depth and spacing in the garden
When you plant a tomato seedling, bury it deeper than it grew in its pot. Bury the stem up to the first set of true leaves (not the cotyledons, which are the first tiny leaves that emerged from the seed). Tomatoes will grow roots all along the buried stem, creating a stronger, deeper root system that reaches more water and nutrients.
Space plants 24 to 36 inches apart, depending on the variety. Determinate (bush) types can be closer; indeterminate (vining) types need more room. Good spacing allows air to move around the foliage, which reduces disease. Crowded plants are more prone to fungal problems like early blight and powdery mildew.
Dig a hole slightly larger than the root ball, set the seedling in place, and backfill with soil. Water gently but thoroughly. Do not fertilize at planting time — a balanced fertilizer or compost mixed into the soil before planting is enough for the first few weeks.
Soil preparation before planting
Tomatoes grow best in soil rich in organic matter with a pH between 6.0 and 6.8. Before planting, work compost, aged manure, or other organic material into the top 8 to 10 inches of soil. This improves drainage, adds nutrients, and helps the soil hold moisture during dry spells.
If your soil is heavy clay, add extra compost or peat moss to improve structure. If it is sandy and drains too fast, organic matter will help it retain water. A soil test from your local extension office will tell you exactly what your soil needs — nitrogen, phosphorus, potassium, or pH adjustment.
Avoid planting tomatoes in the same spot year after year. Rotate to a different bed every 2 to 3 years to reduce soil-borne disease pressure.
Watering and mulching after planting
Water seedlings thoroughly after planting, then keep the soil consistently moist (not soggy) for the first 2 to 3 weeks while they establish roots. Once plants are growing, water deeply but less often — this encourages roots to grow deeper rather than staying shallow.
explore 2 to 3 inches of mulch around each plant, keeping it a few inches away from the stem. Mulch keeps soil temperature steady, reduces water loss, and prevents soil from splashing onto leaves (which spreads disease). Straw, shredded leaves, or wood chips all work well.
Inconsistent watering — wet one week, dry the next — causes problems like blossom-end rot and fruit cracking. Drip irrigation or soaker hoses deliver water directly to the root zone and are more reliable than overhead watering.
Direct seeding outdoors (less common, higher risk)
You can direct-seed tomatoes outdoors, but it is slower and riskier than using transplants. Seeds need soil temperature of at least 60°F to germinate reliably, and they take 5 to 10 days to sprout. Once they do, seedlings are vulnerable to pests, disease, and weather.
If you choose to direct-seed, wait until soil is at least 65°F and all frost danger has passed. Plant seeds 1/4 inch deep, 2 to 3 inches apart. Thin seedlings to final spacing once they have 2 to 3 true leaves. Direct-seeded plants will flower and fruit 2 to 3 weeks later than transplants, which shortens your harvest window in cooler climates.
Most gardeners in northern regions do not have time for direct seeding. In warmer zones with a long growing season, it is more practical.
Frequently Asked Questions
Can I plant tomatoes before the last frost date if the soil is warm?
No. Even if soil temperature is 65°F, a late frost will kill or severely damage young plants. Wait until after your last frost date, then check soil temperature. Both conditions must be met. If a frost is forecast after you plant, cover plants with row covers or blankets overnight.
What if I plant too early and the seedlings turn purple?
Purple discoloration is a sign of cold stress, usually from soil or air temperatures below 50°F. The plants are not dead, but they are stressed and growth has slowed. Move them indoors or cover them if frost threatens. Once soil and air warm up, they will recover and turn green again.
How do I know if my seedlings are ready to transplant?
Seedlings are ready when they are 6 to 8 inches tall, have several sets of true leaves, and have been hardened off. They should look sturdy and green, not tall and spindly. If they are leggy, they did not get enough light indoors.
Can I plant tomatoes in containers instead of the ground?
Yes. Use a pot at least 5 gallons for determinate varieties and 10 gallons or larger for indeterminate types. Fill with potting mix (not garden soil), plant at the same depth as in-ground seedlings, and water consistently. Container plants dry out faster than ground plants, so check soil moisture daily during hot weather.
What should I do if I miss the planting window?
If you miss the ideal window, you can still plant, but your harvest will be later. Plant as soon as soil reaches 60°F, even if it is June. Choose early-maturing varieties (60 to 70 days) rather than long-season types (80+ days) to maximize your chances of fruit before frost arrives in fall.