Plant tomatoes in Washington after the last spring frost, which falls between mid-April in the south and late May in the north

Washington's frost dates vary dramatically by location. The Puget Sound region around Seattle stays cold until mid-May, while the Tri-Cities area near the Columbia River warms up by mid-April. Tomatoes planted before the soil reaches 60°F will sit dormant and rot; planted into soil below 50°F, they often die. The safest approach is to wait until nighttime temperatures stay above 50°F consistently—usually one to two weeks after your local last frost date.

You can find your exact last frost date by entering your zip code into the USDA Plant Hardiness Zone map or by calling your county's extension office. Washington State University Extension has offices in every county and can tell you the frost date for your specific area in a single call. Once you know that date, add 10 to 14 days and plant then, not before.

Key Takeaways

  • Washington's last frost date ranges from mid-April (southern valleys) to late May (northern mountains and Puget Sound), so your location determines your planting window.
  • Soil temperature matters more than air temperature—tomatoes need soil at least 60°F to grow, and they rot in soil below 50°F.
  • Start seeds indoors 6 to 8 weeks before your last frost date if you want mature transplants ready to go into the ground on planting day.
  • Early varieties like 'Early Girl' and 'Siberia' mature in 50 to 60 days and suit the shorter growing season in the Puget Sound; larger types like 'Brandywine' need 75 to 85 days and work better east of the Cascades.

Frost Dates by Washington Region

Western Washington—the Puget Sound area from Seattle to Olympia—has the latest frost dates in the state. Expect the last frost around May 15 to May 20. The marine influence keeps springs cool and wet. Plant transplants into the ground around May 25 to June 1 to be safe. This short window means you cannot afford to wait; once June arrives, the growing season is already halfway through.

Central Washington, including the Yakima Valley and Ellensburg, thaws earlier. The last frost typically falls around May 1 to May 10. You can plant transplants by mid-May. This region has a longer, warmer growing season than the coast, so larger, longer-season varieties have time to mature.

Eastern Washington—the Tri-Cities area (Richland, Kennewick, Pasco) and Spokane—has the earliest frost dates. The last frost in the Tri-Cities comes around April 15 to April 20; in Spokane, around May 10. You can plant transplants by late April or early May in the Tri-Cities, giving you a full three months of warm weather. This is ideal tomato country if you have the water to irrigate.

Starting Seeds Indoors vs. Buying Transplants

Starting seeds indoors 6 to 8 weeks before your last frost date gives you mature, hardened-off transplants ready to plant the moment soil warms up. In western Washington, that means starting seeds in mid-March. In the Tri-Cities, late February. Seeds need warmth (70 to 75°F), light (14 to 16 hours daily under grow lights), and consistent moisture. Most home gardeners find this easier than it sounds, but it requires space and attention.

Buying transplants from a nursery is simpler and works just as well. Nurseries in your region time their stock to your local frost date, so plants sold in late April in Seattle are ready for a mid-May planting. Plants sold in the Tri-Cities in early April are ready for mid-April planting. Buy transplants that are 6 to 8 inches tall with thick stems and dark green leaves—avoid tall, spindly plants or those already flowering. Harden them off by setting them outside in shade for a few hours each day for a week before planting.

Soil Temperature and Hardening Off

Soil temperature is the real gate. Tomatoes germinate and grow when soil is 60°F or warmer. Below 50°F, the roots cannot absorb water or nutrients, and the plant rots from the inside out. A soil thermometer costs $8 to $15 and takes the guesswork out. Stick it 4 inches into the soil where you plan to plant, measure in the morning, and check for three days in a row. When the average reads 60°F or higher, you are ready.

Hardening off—gradually exposing indoor-grown seedlings to outdoor conditions—prevents transplant shock. Start one week before planting day. Set plants outside in dappled shade for 2 to 3 hours, then bring them in. Each day, increase outdoor time and sun exposure by an hour or two. By planting day, they should have spent a full day and night outside. This toughens the stems and leaves so they do not wilt when planted.

Variety Selection for Washington's Growing Season

Early and mid-season varieties are safer in western Washington because the growing season is short and often cool. Early Girl matures in 50 to 60 days and produces medium fruit reliably. Siberia and Sub-Arctic Plenty are bred for short seasons and set fruit even in cool weather. Oregon Spring flowers in cool conditions and rarely fails in the Puget Sound. These varieties will produce ripe fruit by late August or early September, before fall rains and cool nights slow ripening.

Central and eastern Washington can grow longer-season varieties because the summer is warmer and longer. Brandywine, Cherokee Purple, and Mortgage Lifter need 75 to 85 days of warm weather to mature, but they are worth the wait if you have the time. In the Tri-Cities, planting by mid-April gives these varieties until late July or early August to ripen. In Spokane, plant by early May to finish before the first fall frost around September 15.

Protecting Young Plants from Late Frosts

Even after you plant, a surprise frost can kill tender seedlings. Keep frost cloth, row covers, or even old blankets on hand through mid-June in western Washington and through late May in central Washington. If frost is forecast after you have planted, drape the cover over the plants in the evening and remove it the next morning. A single frost cloth can save a $3 transplant and weeks of growth.

Another safeguard is to plant in a warm microclimate. South-facing walls, dark mulch, and raised beds all absorb and hold heat. A tomato planted against a south-facing fence warms up faster in spring and stays warmer at night. In western Washington, this small advantage can mean the difference between fruit ripening in September and October.

Frequently Asked Questions

Can I plant tomatoes earlier if I use a cold frame or row cover?

Yes, but only by two to three weeks. A cold frame or row cover traps heat and protects from frost, but it does not warm the soil much. If soil is 50°F, a cover might raise it to 55°F—still too cold for tomatoes to grow. Wait until soil reaches 55 to 60°F, then use a cover to protect against late frosts and speed up growth slightly.

What if I plant too early and the soil is cold?

The plant will sit still for weeks, looking pale and stunted. Roots cannot absorb water or nutrients in cold soil, so the plant is essentially drowning while looking dry. If this happens, do not add fertilizer or water more—that makes it worse. Wait for soil to warm, and the plant will usually recover and grow normally once temperatures rise.

Is it better to plant on a cloudy day or a sunny day?

Cloudy days are gentler on transplants because they reduce transplant shock. If you must plant on a sunny day, plant in the late afternoon so the plant has a cool night to recover before facing full sun. Either way, water deeply after planting and keep the soil consistently moist for the first two weeks.

Can I plant tomatoes in containers instead of in the ground?

Yes, and containers warm up faster than garden soil in spring. A 5-gallon pot of potting soil can reach 60°F a week or two before the ground does. This is especially useful in western Washington, where you can gain two to three weeks of growing time. Use quality potting soil, not garden soil, and water containers daily once temperatures rise.

What is the latest I can plant tomatoes in Washington?

In western Washington, plant by mid-June at the latest; in central Washington, by early June; in eastern Washington, by mid-May. Tomatoes planted later than this will not have enough warm days to mature before the first fall frost. Early varieties planted in late June in the Puget Sound might still produce a small harvest, but the odds drop sharply.