Start tomatoes after your last spring frost date, indoors 6 to 8 weeks before that date if you want a head start

The single most important date for tomato planting is your last spring frost date—the average day when frost stops happening in your area. Tomato plants die in frost. You can start seeds indoors 6 to 8 weeks before that date, but you cannot move seedlings outside until after frost danger has passed. If you plant too early, a late frost will kill them. If you wait until after frost, you are straightforward planting later than you could have.

Find your frost date by searching "last frost date" plus your zip code, or by visiting the National Weather Service website and entering your location. The date varies by hundreds of miles—a place 50 miles north might have a frost date two weeks later. Once you know it, you can count backward to decide when to start seeds indoors, or forward to decide when to direct-sow.

Most home gardeners start seeds indoors because it gives plants a 6 to 8 week head start before transplanting, which means earlier harvests. Starting seeds outdoors directly in the garden is simpler but delays fruiting by that same amount.

Key Takeaways

  • Your last spring frost date is the day you can safely move tomato seedlings outside; find it by searching your zip code online.
  • Start seeds indoors 6 to 8 weeks before your frost date if you want mature plants ready to transplant on that day.
  • Direct-sow seeds outdoors after your frost date if you prefer not to start seeds indoors, but expect to harvest 6 to 8 weeks later.
  • Tomato seedlings need 12 to 16 hours of light per day indoors; a sunny windowsill works, but a grow light produces sturdier plants.
  • Harden off seedlings by moving them outside for a few hours each day for one week before transplanting, so they adjust to outdoor conditions.

How to find your last frost date

The easiest method is to search "last frost date [your zip code]" in any search engine. The National Weather Service maintains frost date data by county, and many gardening sites republish it. You will see a single date—for example, May 15 or April 20—which is the statistical average for your area.

That date is an average, not a may provide. A frost can occur a week or two after the average date in some years. Many gardeners plant a few days or a week after the official date to add a safety margin, or they keep frost cloth or row covers on hand to protect young plants if a late frost is forecast.

If you live in a warm climate where frost rarely or never occurs, you have more flexibility. In USDA zones 9 and warmer, you can plant tomatoes in late winter or early spring and again in mid-to-late summer for a fall crop. Check your local extension office for the best timing in your specific zone.

Starting seeds indoors 6 to 8 weeks before your frost date

Sow seeds indoors in seed-starting mix (a light, sterile blend, not garden soil) in small containers with drainage holes. Keep the soil moist but not waterlogged, and maintain a temperature between 70 and 80 degrees Fahrenheit. Seeds usually sprout in 5 to 10 days.

Once seedlings emerge, they need 12 to 16 hours of light per day. A south-facing windowsill can work, but seedlings grown in window light often become leggy (tall and thin) because they stretch toward the light. A grow light positioned 2 to 3 inches above the seedlings produces stockier, stronger plants. Raise the light as the seedlings grow.

When seedlings develop their first true leaves (the second set of leaves that appear), you can thin them or transplant them into larger individual containers. Keep the soil consistently moist and the air temperature between 65 and 75 degrees during the day.

About one week before your frost date, begin hardening off the seedlings: move them outside to a sheltered, shaded spot for a few hours, then bring them back inside. Each day, leave them out a bit longer and gradually expose them to more direct sun. After one week of this gradual adjustment, they are ready to transplant into the garden.

Direct-sowing seeds outdoors after your frost date

If you prefer not to start seeds indoors, you can sow seeds directly in the garden soil after your frost date has passed and the soil has warmed to at least 60 degrees Fahrenheit (ideally 70 degrees or warmer). Plant seeds about 1/4 inch deep in groups of 2 or 3, spaced 2 to 3 feet apart. Thin seedlings to one plant per spot once they have developed their first true leaves.

Direct-sown tomatoes will flower and fruit about 6 to 8 weeks later than indoor-started seedlings, since they start from seed in the ground rather than from a more mature transplant. In short-season climates, this delay can mean the difference between harvesting ripe fruit and having green tomatoes when frost arrives in fall.

Soil temperature and garden readiness

Tomato seeds and seedlings perform best when soil temperature is at least 60 degrees Fahrenheit, and they thrive when it reaches 70 degrees or higher. Planting into cold soil slows germination and growth and increases the risk of rot. If you are eager to plant but the soil is still cold, wait a week or two, or use black plastic mulch or row covers to warm the soil before planting.

Before planting, work compost or aged manure into the top 6 to 8 inches of soil. Tomatoes are heavy feeders and benefit from rich soil. If your soil is poor, amend it now rather than trying to catch up later with fertilizer alone.

Adjusting timing for your climate and season length

In short-season climates (northern regions with a frost date in late May or June), starting seeds indoors is nearly essential because the growing season is too short to wait for direct-sown seeds to mature. In warm climates with long seasons, you have the luxury of direct-sowing or starting indoors, whichever suits you.

If you live in a very hot climate, you may want to start seeds in late winter so plants are mature and producing before the peak heat of summer arrives. Some gardeners in hot regions also plant a second crop in mid-summer for a fall harvest when temperatures cool. Check with your local extension office for the best timing in your specific zone.

Keep a straightforward record each year of when you planted, when seedlings emerged, when you transplanted, and when you harvested. Over time, you will learn the rhythm that works best for your garden and your climate.

Common mistakes in timing

The most common mistake is planting seedlings outside too early, before the last frost date has passed. A single frost night can kill an entire crop of young plants. If you are unsure, wait a few extra days—the small delay in harvest is far better than losing the plants.

The second mistake is starting seeds indoors too early. Seeds started 10 or 12 weeks before the frost date will become overgrown and root-bound in their containers by the time they can go outside. Stick to the 6 to 8 week window for best results.

A third mistake is neglecting to harden off seedlings. Plants grown indoors in stable conditions are shocked by outdoor wind, direct sun, and temperature swings. Hardening off gradually acclimates them and prevents wilting or sunburn when they are transplanted.

Frequently Asked Questions

Can I plant tomatoes before my last frost date if I cover them?

Yes, row covers, frost cloth, or even old sheets can protect young plants from a light frost. However, if temperatures drop below 32 degrees Fahrenheit for more than a few hours, the cloth may not provide enough protection. It is safer to wait until after the frost date, or to have covers ready and monitor the forecast closely.

What if I missed my frost date and it is already summer?

You can still plant tomato seedlings in summer, but they will produce fruit later in the season, closer to fall. In warm climates, this can work well. In cool climates with an early frost, late-planted tomatoes may not have time to ripen before frost arrives. Check your first fall frost date to see if you have enough time.

Do I need to use a grow light, or will a windowsill work?

A sunny south-facing windowsill can work, especially if you rotate the seedlings daily so they grow evenly. A grow light produces sturdier, more compact plants and is worth the small investment if you start seeds every year. Either way, seedlings need 12 to 16 hours of light per day.

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

Seedlings are ready when they are 6 to 8 inches tall, have several sets of true leaves, and have been hardened off for one week. They should look sturdy and green, not pale or leggy. If they are still small and spindly, give them more light and a bit more time indoors.

Can I plant tomatoes in containers instead of the ground?

Yes, tomatoes grow well in containers 5 gallons or larger. The timing is the same—wait until after your last frost date to move containers outside. Container soil warms faster than garden soil, so you may be able to plant a few days earlier, but frost will still kill the plants if they are exposed.