Plant tomatoes in Seattle from mid-May through early June

Seattle's last frost date falls around April 20, but tomatoes need warmer soil and air to thrive. Wait until mid-May when nighttime temperatures stay above 50°F and soil has warmed to at least 60°F—ideally 65°F or higher. Planting too early is the most common mistake Seattle gardeners make; a tomato planted in cold soil will sit dormant, rot, or grow so slowly that you lose weeks of the short growing season.

If you start seeds indoors (which many Seattle gardeners do), begin them 6 to 8 weeks before your planting date. That means starting seeds in late March or early April so transplants are ready to go into the ground in mid-May. Buying nursery transplants in May is faster and sidesteps the indoor seed-starting step entirely.

Key Takeaways

  • Plant tomato transplants outdoors between mid-May and early June, when soil temperature reaches 60°F and nighttime lows stay above 50°F.
  • Starting seeds indoors in late March gives you transplants ready by mid-May; buying nursery plants in May skips this step.
  • Seattle's short growing season means choosing early and mid-season varieties (60 to 75 days to harvest) rather than long-season types.
  • Harden off indoor seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before transplanting.
  • Plant in full sun (6 to 8 hours daily minimum) and amend soil with compost to improve drainage and warmth.

Soil temperature matters more than air temperature

A soil thermometer is the single most useful tool for timing tomato planting in Seattle. Stick it 4 inches into the ground in the morning—that is where the roots will be. If the reading is below 60°F, wait another week or two. Soil warms slowly in Seattle's maritime climate, especially if your garden is shaded or sits on clay.

You can speed up soil warming by laying black landscape fabric or plastic mulch over the bed 2 to 3 weeks before planting. This traps heat and raises soil temperature by 5 to 10 degrees. Remove the plastic once you plant, or cut holes in it for the transplants and leave it in place to continue warming the root zone through the season.

Choose varieties bred for short seasons

Seattle summers are warm but brief. Select tomato varieties that mature in 60 to 75 days, not the 80+ day types sold in warmer regions. Early and mid-season varieties like 'Early Girl,' 'Siberia,' 'Glacier,' 'Stupice,' and 'Oregon Spring' are bred for cool climates and will produce fruit before fall rains and cooler nights slow ripening.

Cherry tomatoes like 'Sungold' and 'Black Cherry' also do well in Seattle because they set fruit quickly and keep producing even as temperatures drop in late summer. Avoid large beefsteak varieties unless you have a very sunny, sheltered spot and are willing to wait until late August or September for the first ripe fruit.

Harden off seedlings before moving them outside

If you started seeds indoors, your seedlings are soft and tender. Moving them straight from a warm, still indoor environment to outdoor wind and sun will shock them and stall growth for weeks. Instead, spend 7 to 10 days gradually exposing them to outdoor conditions.

Start by placing seedlings in a sheltered, shaded spot outdoors for 2 to 3 hours on the first day. Each day, increase the time outside and move them into brighter light. By day 7 or 8, they should be outside all day in dappled shade or morning sun. On the final days before planting, leave them out overnight if temperatures stay above 50°F. This process toughens the stems and leaves so transplants bounce back quickly once planted.

Plant in full sun and amend the soil

Tomatoes need at least 6 to 8 hours of direct sun daily—more is better in Seattle's cloudy climate. Choose the sunniest spot in your garden, ideally south-facing and sheltered from wind. If your garden is shaded, grow tomatoes in containers that you can move to the sunniest location.

Work 2 to 3 inches of compost into the top 8 to 10 inches of soil before planting. Seattle soil is often heavy clay or acidic; compost improves drainage, adds nutrients, and helps the soil warm faster. Avoid fresh manure, which can introduce disease. If you are planting in containers, use a quality potting mix (not garden soil) and choose pots at least 5 gallons for determinate varieties and 10 gallons for indeterminate types.

Space plants correctly and stake early

Plant transplants 24 to 36 inches apart, depending on the variety. Determinate (bush) types need less space; indeterminate (vining) types need more. Bury the stem deeper than it was growing in the pot—tomatoes will root along the buried stem and develop a stronger root system. You can bury up to the first true leaves.

Install stakes or cages at planting time, not later. Waiting to stake means you risk damaging roots when you drive a stake into the ground. Indeterminate varieties need sturdy stakes or cages at least 5 to 6 feet tall; determinate types can use shorter supports. Tying the stem loosely to the stake as it grows prevents wind damage and keeps fruit off the wet soil.

Watch for late frost and cool snaps

Even though mid-May is generally safe, Seattle can see frost as late as May 10 or even May 15 in some years. Check the extended forecast before planting. If frost is predicted within a week of your planting date, wait. If you have already planted and frost is forecast, cover plants with row cloth, old sheets, or cloches overnight and remove them in the morning.

Cool snaps in late May and early June are normal and will not kill established plants, but they will slow growth. Tomatoes straightforward pause when temperatures drop below 55°F at night. This is why starting early (mid-May) gives you the longest possible window before fall chill arrives in September.

Frequently Asked Questions

Can I plant tomatoes in April in Seattle?

Not safely. Soil is usually too cold, and nighttime temperatures often dip below 50°F. Tomatoes planted in April will sit dormant or rot before roots establish. Mid-May is the reliable window when soil and air have warmed enough for steady growth.

What if I start seeds indoors but they get leggy and tall?

Leggy seedlings mean insufficient light. Move them closer to a south-facing window or use a grow light positioned 2 to 3 inches above the leaves. Keep the light on for 14 to 16 hours daily. Leggy seedlings can still be planted deeper than normal to compensate, but prevention is easier than rescue.

Is it better to buy transplants from a nursery or start seeds indoors?

Both work. Nursery transplants are faster and require no indoor space or equipment. Starting seeds indoors costs less and gives you access to varieties not sold locally. For a first-time Seattle gardener, buying healthy transplants in mid-May is the simpler path.

Do I need to use fertilizer when I plant?

If you worked compost into the soil, you have enough nutrients to start. Once plants begin flowering (usually 4 to 6 weeks after planting), feed every 2 to 3 weeks with a balanced fertilizer or one slightly higher in phosphorus to encourage fruit set. Avoid high-nitrogen fertilizers, which promote leaves at the expense of fruit.

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

Cold soil slows root development and can cause the stem to rot at the soil line. The plant may sit for weeks without growing, wasting precious growing season. If you have already planted and realize the soil is too cold, mulch around the base with compost to help it warm faster, but waiting until mid-May prevents this problem entirely.