Plant tomatoes in Washington after the last frost date in your area, which falls between mid-April and mid-May depending on where you live in the state

Washington's growing season is shorter than most tomato-growing regions, and frost will kill tomato plants outright. The state divides into three main zones: the Puget Sound area around Seattle (last frost around May 10), central Washington around Spokane (last frost around May 15), and southwestern Washington (last frost around mid-April). Tomatoes need soil that has warmed to at least 60°F, ideally 65°F or higher. Planting too early into cold soil will stunt the plant and invite root rot.

Waiting until after your last frost date and giving the soil time to warm is the single most important timing decision you'll make. Air temperature and soil temperature are not the same thing—a warm day in April doesn't mean the ground has warmed enough. A soil thermometer, available at any garden center for under $15, takes the guesswork out of the decision.

Key Takeaways

  • The last frost date in western Washington is typically May 10, while eastern Washington and the southwest have earlier dates ranging from mid-April to mid-May.
  • Soil temperature matters as much as air temperature—wait until soil reaches at least 60°F before transplanting seedlings or direct seeding.
  • Starting seeds indoors 6 to 8 weeks before your last frost date gives you larger plants ready to go into the ground at the right time.
  • Choosing shorter-season varieties bred for cool climates will give you ripe fruit before fall frost arrives in September or early October.

Finding your exact last frost date

Washington State University Extension publishes frost date maps by county. Search "WSU frost date" plus your county name to find the specific date for your location. If you live in a city, your local Extension office can also tell you over the phone. The last frost date is the average date of the final spring frost in your area—some years it comes earlier, some later, but it's your best planning tool.

Don't rely on the calendar alone. Check your soil temperature with a soil thermometer before planting. Push it 4 inches into the ground in the morning. If it reads below 60°F, wait another week and check again. Cold soil is the reason many Washington gardeners plant too early and lose seedlings to rot.

Starting seeds indoors versus buying transplants

If you start seeds indoors, begin 6 to 8 weeks before your last frost date. For the Puget Sound area, that means starting seeds in late March or early April. For Spokane, start in early April. You'll need grow lights or a sunny south-facing window, seed-starting mix, and small pots. Keep the soil moist but not waterlogged, and maintain temperatures between 70°F and 75°F for germination.

Buying transplants from a nursery is simpler and works well if you start shopping in late April or early May. Look for stocky plants with green leaves and no flowers yet—plants already flowering have been stressed and will take longer to recover in the ground. Harden off transplants by setting them outside in a sheltered spot for a few hours each day for a week before planting, gradually increasing sun exposure. This toughens the plant and prevents transplant shock.

Soil preparation and timing

Prepare your planting area 2 to 3 weeks before you plan to plant. Work in 2 to 3 inches of compost or aged manure to improve drainage and add nutrients. Washington soils vary widely—western Washington tends toward clay and needs organic matter to drain properly, while eastern Washington is drier and benefits from compost to hold moisture. A soil test through your local Extension office costs $15 to $30 and tells you exactly what your soil needs.

Once you've amended the soil, let it settle for a couple of weeks. This gives the compost time to integrate and the soil to warm naturally. If you're planting in containers, use potting mix (not garden soil) and fill pots at least 5 gallons in size—tomatoes need room for roots to develop.

Choosing varieties suited to Washington's climate

Washington's short growing season and cool nights favor determinate (bush) and early-maturing varieties over large beefsteak types that need 80+ days to ripen. Look for varieties labeled "early" or "short season" with a days-to-maturity number of 65 to 75 days. 'Early Girl', 'Stupice', 'Glacier', and 'Siberia' are cold-hardy varieties that perform well in Washington. Cherry tomatoes like 'Sungold' and 'Black Cherry' also mature faster than slicing types.

Heirloom varieties often take 80+ days and may not ripen before fall frost arrives in late September or early October. If you want to grow them, start seeds very early (late February) and use row covers or cloches to extend the season in fall. For most gardeners, sticking to early varieties removes the guesswork.

Protecting young plants from late frost

Even after the last frost date, a surprise cold snap can damage or kill young tomato plants. Keep row covers, frost cloth, or old sheets on hand through mid-May. If frost is forecast after you've planted, drape the covers over plants in the evening and remove them the next morning once temperatures rise. A straightforward wire or PVC frame under the covers keeps the fabric from touching leaves.

Another strategy is to plant in a location that warms early—south-facing slopes, areas near heat-absorbing walls or pavement, or raised beds all warm faster than open ground. In western Washington, planting near a south-facing fence or wall can add 1 to 2 weeks to your effective growing season.

Spacing and planting depth

Space plants 24 to 36 inches apart to allow air circulation, which reduces disease pressure in Washington's humid western regions. Bury the stem deeper than it was growing in the pot—tomatoes will root along the buried stem, creating a stronger root system. You can bury the plant up to the first true leaves. This is especially helpful in cool soil because the deeper roots will eventually reach warmer soil.

Water thoroughly after planting and keep the soil consistently moist (not soggy) for the first 2 to 3 weeks while roots establish. Mulch around the base with 2 inches of compost or straw to retain moisture and keep soil temperature more stable.

Frequently Asked Questions

Can I plant tomatoes earlier if I use black plastic or row covers?

Black plastic warms the soil, and row covers protect from frost, so yes—you can plant 1 to 2 weeks earlier if you use both. However, remove row covers once daytime temperatures consistently reach 70°F to prevent overheating and allow pollinating insects access. Black plastic stays down all season to warm the soil and suppress weeds.

What if my area gets a late frost after I've planted?

Cover plants with frost cloth, blankets, or row covers the evening before the frost and remove them the next morning. Water the soil before covering—moist soil holds heat better than dry soil. If plants are already damaged by frost, prune off the blackened leaves and stems; the plant will usually recover and regrow.

Is it too late to plant if I miss the ideal window?

If you plant in early June instead of late May, you'll still get a harvest, but it will be smaller and later. Choose the earliest-maturing varieties available and plan to extend the season in fall with row covers or cloches to protect fruit from early frost. Planting in July is generally too late for most of Washington.

Do I need to start seeds indoors, or can I direct seed tomatoes outside?

Direct seeding works in theory but rarely in practice in Washington because the soil stays too cool for too long. Starting seeds indoors or buying transplants gives you a 6 to 8 week head start and a much better chance of ripe fruit before fall frost.

What's the difference between determinate and indeterminate varieties for Washington?

Determinate (bush) varieties mature all at once and stop growing, making them ideal for Washington's short season and canning. Indeterminate (vining) varieties grow all season and produce until frost, but they need more time to mature. For Washington, determinate or early indeterminate types work best.