Plant tomatoes in Seattle after the last frost date, which falls around April 15th for most of the city

Seattle's last frost date is typically April 15th, though it can vary by a week or two depending on your exact neighborhood and elevation. Tomatoes cannot survive frost—even a light freeze will kill seedlings or young plants. If you plant too early, a late spring frost will set you back weeks or kill your plants entirely. Waiting until mid-April gives you a safe margin, though many Seattle gardeners wait until early May to be extra cautious.

The reason timing matters so much in Seattle is that the growing season is shorter than in warmer regions. Tomatoes need consistent warmth to produce fruit, and Seattle's cool springs mean every week counts. Planting at the right time ensures your plants have enough warm weather to mature fruit before fall rains and cooler temperatures arrive in September.

Key Takeaways

  • Seattle's last frost date is around April 15th, but many gardeners wait until early May to avoid late freezes that kill young plants.
  • Soil temperature should be at least 60°F before planting, ideally 65°F or warmer for faster growth.
  • Starting seeds indoors 6 to 8 weeks before your planting date gives you larger, stronger transplants ready to go into the ground.
  • Choose early and mid-season tomato varieties rather than long-season types, since Seattle's frost arrives by late September.
  • Planting in late April or early May gives tomatoes the full warm season to set fruit before fall weather arrives.

Soil temperature matters more than the calendar date

While April 15th is the official last frost date, soil temperature is the real trigger for planting. Tomatoes germinate and grow slowly in cold soil, and they are prone to disease and root rot if planted into soil below 60°F. A soil thermometer (available at any garden center) takes the guesswork out—wait until soil reaches at least 60°F, ideally 65°F or warmer.

In Seattle, soil typically reaches 60°F sometime in late April or early May, depending on how much sun your garden gets and whether you have amended the soil with compost. South-facing beds warm up faster than shaded areas. If you want to speed things up, lay black plastic or landscape fabric over the bed a week or two before planting—it traps heat and warms the soil faster.

Start seeds indoors in February or early March

If you want to grow tomatoes from seed rather than buying transplants, start them indoors 6 to 8 weeks before your planned planting date. For Seattle, that means starting seeds in late February or early March. Tomato seeds need warmth to germinate—around 70°F—and bright light once they sprout, so a sunny windowsill or grow light works well.

Seedlings started this early will be sturdy and ready to transplant into the garden by late April or early May. If you start too late, your plants will still be small and fragile when it is time to plant outside, and they will take longer to produce fruit. If you start too early, they become leggy and overgrown in their pots, which stresses them when you move them to the garden.

Choose varieties bred for short growing seasons

Seattle's frost arrives by late September, giving you roughly 140 to 150 frost-free days. Long-season varieties that need 80 to 90+ days to mature fruit often do not finish before the first frost. Instead, choose early and mid-season varieties that mature in 60 to 75 days. 'Early Girl', 'Siberia', 'Glacier', and 'Stupice' are all reliable choices for Seattle and will produce ripe fruit by late summer.

Cherry tomatoes also tend to mature faster than large slicing types, so they are a safer bet if you are new to Seattle gardening. Paste tomatoes like 'San Marzano' can work, but they need a long, warm season and often do not fully ripen outdoors in Seattle without extra help like row covers or a greenhouse.

Harden off transplants before planting outside

Whether you grew seedlings from seed or bought transplants from a nursery, they need to adjust to outdoor conditions before going into the ground. This process, called hardening off, takes 7 to 10 days. Start by placing plants in a sheltered, shaded spot outdoors for an hour or two, then gradually increase their time outside and their exposure to direct sun over the course of a week.

Hardening off prevents transplant shock—the wilting and stunted growth that happens when plants go straight from a warm, protected indoor environment into full sun and wind. By the time you plant them in the garden, they will be tough enough to handle outdoor conditions without setback.

Protect young plants from late frosts

Even though you are planting after the average last frost date, a surprise frost can still occur in late April or early May. Keep frost cloth, row covers, or even old blankets on hand. If frost is forecast after you have planted, drape the covers over your plants in the evening and remove them the next morning once temperatures rise above freezing.

A straightforward frost cloth or row cover (available at garden centers) is lightweight and lets light and water through while trapping warmth. It is much easier than trying to cover plants with blankets, and it will not damage tender new growth the way a heavy blanket might.

Plant spacing and depth for Seattle gardens

Space tomato plants 24 to 36 inches apart, depending on the variety—determinate (bush) types can be closer, while indeterminate (vining) types need more room. Plant them deeper than they were growing in their pots; bury the stem up to the first set of true leaves. Tomatoes will develop roots all along the buried stem, creating a stronger root system that handles Seattle's occasional dry spells better.

Water well after planting and keep the soil consistently moist (not waterlogged) for the first few weeks while roots establish. Once plants are growing vigorously, they are more drought-tolerant, though Seattle's summer rainfall usually means you will not need to water much after June.

Frequently Asked Questions

Can I plant tomatoes in Seattle in April, or is it too early?

Early April is usually too early—frost risk is still high. Mid-April is the earliest safe date for most of Seattle, but waiting until early May is safer and gives soil time to warm up. Check your soil temperature with a thermometer rather than relying on the calendar alone.

What if I plant too early and frost kills my plants?

If frost kills seedlings, you can replant. If it damages established plants, they often recover and put out new growth, though you will lose time. This is why many Seattle gardeners wait until May—the extra two weeks of safety is worth the delay in harvest.

Do I need to use a greenhouse or row cover to grow tomatoes in Seattle?

No, but they help. Row covers extend the season by a few weeks on both ends, and a straightforward cold frame or low tunnel can speed up ripening in late summer. Many Seattle gardeners grow tomatoes outdoors without any protection and get good harvests, especially with early varieties.

When will my tomatoes be ready to harvest if I plant in late April?

Early varieties planted in late April typically produce ripe fruit by mid-July to early August. You will continue harvesting through September, though production slows as temperatures drop and days shorten in fall.

Should I buy transplants from a nursery or start seeds indoors?

Both work. Nursery transplants are faster and require less equipment, but they cost more and may not include the varieties you want. Starting seeds indoors gives you more variety choices and costs less, but requires space, light, and attention for 6 to 8 weeks.