When to Plant Brussels Sprouts in Seattle, WA
Published: April 24, 2026
Brussels Sprout Planting Dates for Seattle, WA
| Start seeds indoors | April 2–9 |
| Last frost (average) | May 7 |
| Transplant outdoors | July 30–August 9 |
| Minimum soil temperature | 45°F |
| Expect first harvest | October 18 – November 7 |
| Fall crop planting | July 2–12 |
| Fall crop harvest | September 20 |
| First fall frost (average) | October 1 |
⚠ Brussels sprouts are planted in midsummer for a fall harvest. Count back 90 days from your first fall frost to find the transplant date. Start seeds indoors 4–6 weeks before that date. Flavor improves after frost.
Best Brussels Sprout Varieties for Seattle, WA
Consult a Seattle-area nursery or your state extension office for brussels sprout varieties proven in Zone 4b.
Growing Brussels Sprouts in Seattle
Seattle sits in Zone 4b, with an average last frost of May 7 and first fall frost around October 1 — giving a 147-day frost-free growing season. Cool-season crops like brussels sprout benefit from Seattle's cooler spring and fall windows, when temperatures stay in the 55–75°F sweet spot that produces the best flavor and least bolting.
Seattle's cooler summers are close to ideal for brussels sprout. The same conditions that limit tomato and pepper yields benefit cool-season crops — slower bolting, sweeter flavor, and longer harvest windows. The fall brussels sprout crop in Seattle is often more productive than the spring crop.
Seattle's well-draining loam soils are among the best for brussels sprout growing — focus on annual organic matter additions (2 to 3 inches of compost) and consistent moisture during drier months. Water brussels sprout at 1.5 inches per week; loam holds moisture evenly without the drought-crack cycle of heavy clay or the rapid-drain losses of pure sand.
Brussels Sprout Calendar for Seattle
| Month | Task |
|---|---|
| April | Start seeds indoors under grow lights |
| May | Last frost — harden off seedlings outdoors |
| July | Transplant outdoors into warm soil |
| October | Expect first harvest window to open |
| July | Start fall crop — transplants or direct sow |
Brussels Sprout Tips for Seattle Gardeners
- •Count back 90 days from first fall frost to find the transplant date — this is the most important Brussels sprout timing rule.
- •Pinch the top of each plant about a month before first frost — this channels energy into sprout formation.
- •Fall plantings in Seattle often out-produce spring plantings — cooler temperatures slow bolting and concentrate flavor. Light frost can actually improve the taste of brussels sprout.
Common Brussels Sprout Pests in Seattle
- •Cabbage Worm — peaks July in Zone 4b (active June–August); green caterpillars on brassicas; apply Bt spray or use row covers.
- •Aphid — peaks July in Zone 4b (active June–August); monitor closely during peak season and treat early.
- •Cabbage Looper — peaks July in Zone 4b (active June–August); loop-walking green caterpillars; Bt spray works well.
Check plants every 2–3 days during peak season — early intervention prevents most infestations from becoming serious.
What to Plant with Brussels Sprouts in Seattle
In Seattle's cold climate, Onion and Celery are the most beneficial plants to grow alongside brussels sprout. Onion deters carrot fly, aphids, and several root-zone pests through sulfur compounds. Keep brussels sprout away from Tomato — it competes for nutrients or shares pest pressure. In short-season gardens, interplanting companions at transplant time maximizes each bed's productive weeks.
Frequently Asked Questions
When should I plant brussels sprout in Seattle, WA?
In Seattle (Zone 4b), start brussels sprout seeds indoors around April 2–9 and transplant outdoors around July 30–August 9. The city's average last frost of May 7 is the anchor date — count 5 weeks back for seed starting and 12 weeks forward for transplanting.
What zone is Seattle, WA for brussels sprout growing?
Seattle is USDA Zone 4b. For brussels sprout, this means a frost-free growing season of roughly 21 weeks running from May 7 to October 1. Cool-season crops like brussels sprout thrive in this zone with both spring and fall planting windows available.
When is brussels sprout harvest season in Seattle?
Expect the first brussels sprout harvest in Seattle around October 18, with harvest continuing through November 7. This is based on 80–100 days from transplant or direct sow. A fall crop planted July 2–12 adds a second harvest around September 20.
How long does it take to grow brussels sprout in Seattle, WA?
From transplant or direct sow to first harvest, brussels sprout takes 80–100 days in Seattle's climate. Based on a typical planting date of July 30–August 9, expect your first harvest around October 18. Seattle's cooler Zone 4b climate often lands at the slower end of this range — cooler nights slow fruit development.
What soil does brussels sprout need in Seattle?
Seattle's loam soil is near-ideal for brussels sprout. Work in 2–3 inches of compost before planting to boost organic matter and nutrient content. Minimal amendment is needed beyond that — loam holds moisture evenly without the drought-crack cycle of clay or the nutrient-loss issues of sandy soil.