Plant tomatoes in Seattle between mid-May and early June

Seattle's last frost date falls around April 21, but that does not mean you should plant tomatoes then. Tomato plants need soil that has warmed to at least 60°F, ideally 65°F or higher. In Seattle, the soil does not reliably reach that temperature until mid-May. Planting too early into cold ground causes seeds to rot and transplants to sit dormant, wasting weeks of the growing season.

The safest window is May 15 through June 5. If you plant by mid-May, you give the plant enough time to mature and produce fruit before the first fall frost, which typically arrives around October 10. Planting after early June cuts into the harvest window and risks a smaller yield.

Key Takeaways

  • Wait until mid-May to plant tomatoes outdoors in Seattle, even though the last frost date is April 21, because soil temperature matters more than air temperature.
  • Soil should reach 60°F to 65°F before planting; use a soil thermometer to check rather than guessing by date alone.
  • Starting seeds indoors 6 to 8 weeks before your outdoor planting date (late February or early March) gives you larger transplants ready to go in May.
  • Seattle's cool nights and short growing season favor determinate varieties and early-maturing types like 'Early Girl' and 'Siberia' over long-season beefsteaks.

How soil temperature affects planting success

A soil thermometer is the most reliable tool for deciding when to plant. Push it 4 inches into the ground in the spot where you plan to put tomatoes. If the reading is below 60°F, wait another week and check again. Soil warms gradually in Seattle because the region stays cloudy and cool through April and early May.

Planting into cold soil does real damage. Seeds germinate slowly or not at all. Transplants sit stunted for weeks, vulnerable to fungal diseases and pests. A tomato that goes into 50°F soil in late April may not catch up to one planted in 65°F soil in mid-May, even though the first one went in three weeks earlier.

Starting seeds indoors versus buying transplants

If you start seeds indoors, begin in late February or early March—about 6 to 8 weeks before your May 15 planting date. Seeds need warmth (70°F to 80°F) and light to germinate and grow stocky. A heat mat under seed trays and grow lights on a timer give you strong transplants ready to harden off in late April and plant out in mid-May.

Buying transplants from a nursery is simpler if you do not have indoor space or grow lights. Look for plants that are 6 to 8 inches tall with thick stems and true leaves (the second set of leaves that look like tomato leaves). Avoid tall, spindly plants that have been crowded in their pots. Harden them off by setting them outside in a sheltered spot for a few hours each day, gradually increasing exposure over 7 to 10 days before planting.

Varieties that thrive in Seattle's short season

Seattle's growing season is about 150 frost-free days, which rules out many large beefsteak tomatoes that need 80 to 90 days to mature. Choose determinate (bush) varieties and early-maturing types instead. 'Early Girl' produces ripe fruit in 50 to 60 days. 'Siberia' and 'Sub-Arctic' are bred for cool climates and mature in 55 to 65 days. 'Sungold' is a cherry type that ripens quickly and produces heavily even in cool weather.

Paste tomatoes like 'San Marzano' and 'Roma' mature in 70 to 80 days and can work if you plant by mid-May, but they are riskier. Avoid long-season varieties like 'Brandywine' and 'Mortgage Lifter' unless you have a greenhouse or can extend the season with row covers and walls of water.

Protecting young plants from late spring cold

Even in mid-May, Seattle can see a cold snap. Keep row covers, frost cloth, or cloches on hand. If temperatures drop below 50°F after planting, cover plants overnight. Remove covers during the day so plants get sun and air circulation. This protection is especially important in the first two weeks after planting, when transplants are still establishing roots.

Mulching around the base of plants (2 to 3 inches of straw or compost) helps retain soil warmth and moisture once the soil has warmed. Do not mulch until the soil is consistently warm, or you will trap cold in the ground.

Extending the season into fall

Seattle's fall frost typically arrives around October 10, giving you about five months of growing time from mid-May planting. To maximize ripening, stop fertilizing in late August and pinch off new flower clusters in early September. This redirects energy into ripening existing fruit rather than growing new flowers that will not mature before frost.

In late September, cover plants on cold nights (below 45°F) to protect developing fruit. You can also pick green tomatoes in late September and ripen them indoors on a windowsill or in a paper bag with a ripe banana, which releases ethylene gas and speeds ripening.

Common mistakes Seattle gardeners make

Planting too early is the most common error. The urge to get plants in the ground is strong in spring, but a tomato planted in April will not produce earlier than one planted in May—it will just sit there. Another mistake is choosing the wrong variety. A gardener new to Seattle often picks a tomato that worked in their previous home, not realizing that a 90-day beefsteak will not ripen here.

Overcrowding is also common. Tomatoes need air circulation to prevent fungal disease, especially in Seattle's humid climate. Space plants 24 to 36 inches apart. Finally, many gardeners skip the soil thermometer and plant by calendar date alone. A thermometer costs a few dollars and takes the guesswork out of timing.

Frequently Asked Questions

Can I plant tomatoes in April in Seattle?

You can, but it is not recommended. Soil is usually too cold, and plants will sit dormant for weeks. Mid-May is safer because soil has warmed enough for roots to grow and the plant to thrive.

What if I plant in early May and a frost comes?

Cover plants with frost cloth or row covers overnight. Remove covers during the day. If temperatures stay below 50°F for more than a few days, the plant may not recover well, so waiting until mid-May reduces this risk.

Do I need to use a soil thermometer?

No, but it removes guesswork. If you do not have one, mid-May is a safe default date for most Seattle gardens. A thermometer costs $10 to $20 and lasts for years.

Can I grow tomatoes in a greenhouse in Seattle?

Yes, and a greenhouse extends your season significantly. You can plant earlier (late April) and harvest later (November). Even an unheated greenhouse or cold frame helps, especially in fall.

What is the latest I can plant tomatoes in Seattle?

Early June is the practical limit. Planting after June 5 leaves little time for fruit to mature before the October frost. Early-maturing varieties planted by June 1 have the best chance of a full harvest.