When to Plant Brussels Sprouts in Washington
Published: April 24, 2026
Washington gardeners in Zone 4b have a 21-week frost-free window for brussels sprout (May 7 to October 1). Cool-season crops like brussels sprout do well here because the short, cool summers match the plant's temperature preferences naturally. This guide gives exact dates for Washington based on your local frost calendar.
Brussels Sprout Planting Dates for Washington
| Start seeds indoors | April 2–9 |
| Last frost (average) | May 7 |
| Transplant outdoors | July 30–August 9 |
| Expect first harvest | October 18 – November 7 |
| Fall crop start | July 2–12 |
| Fall 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.
Growing Brussels Sprouts in Washington's Climate
Washington's cold Zone 4b climate is close to ideal for brussels sprout. The cool springs and moderate summers match the plant's natural temperature preferences, producing crisp, sweet, slow-bolting harvests that hot-climate gardeners cannot grow.
The fall brussels sprout crop in Washington is often better than the spring crop. Plants mature as temperatures cool in September and October, concentrating sugars and tightening heads or roots. Light frost can improve flavor; hard frost signals harvest time.
Common brussels sprout pests to watch for in Washington include Cabbage Worm and Aphid. The first line of defense is companion planting: Onion and Celery planted nearby discourage several of these pests by confusing host-plant identification or repelling adults before they can lay eggs. Weekly garden walks in the early morning catch problems when they're still manageable.
Brussels Sprout Garden Calendar for Washington
| Month | Task | Notes |
|---|---|---|
| January | — | Dormant season |
| February | — | Dormant season |
| March | — | Dormant season |
| April | Start seeds indoors | Use grow lights; warm soil to 70–75°F for best germination |
| May ← | Harden off seedlings | Set seedlings outdoors 1–2 hours daily, increasing gradually |
| June | — | Dormant season |
| July | Transplant outdoors | Soil temp must be 45°F+; nighttime lows above 50°F |
| August | — | Dormant season |
| September | — | Dormant season |
| October | First harvest window opens | Brussels Sprout begins producing 80–100 days after transplant |
| November | — | Dormant season |
| December | — | Dormant season |
Brussels Sprout Growing Tips for Washington 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.
- •Washington's short growing season means start seeds indoors under grow lights — don't rely on windowsill light which produces leggy, weak seedlings that struggle outdoors.
- •Cover young transplants with floating row covers for the first 2–3 weeks outside. The 3–5°F of protection reduces frost damage risk and accelerates early growth.
- •Harvest sprouts from the bottom up as they reach 1-inch diameter; top sprouts finish last.
Companion Planting for Brussels Sprouts in Washington
In Washington's cold climate, planting brussels sprout with Onion and Celery helps deter pests and improve soil. Avoid planting near Tomato and Strawberry, which inhibit brussels sprout growth through root chemistry or shared pest pressure. See the full brussels sprout companion planting guide for the complete list.
Brussels Sprouts Planting Dates by City in Washington
Top cities in Washington — select for city-level frost dates and planting calendars.
Frequently Asked Questions
When should I plant brussels sprout in Washington?
In Washington (Zone 4b), start brussels sprout seeds indoors around April 2–9 and transplant outdoors around July 30–August 9 — 12 weeks after the state's average last frost of May 7.
What is the last frost date in Washington?
Washington's average last frost date is May 7 in the state's most common Zone 4b. Frost dates vary by city — higher elevations and northern counties run 1 to 3 weeks later than the state average. See the city list below for city-level frost dates.
Can I grow brussels sprout year-round in Washington?
brussels sprout is a cool-season crop in Washington, with spring and fall windows. Summer heat bolts the plants before they can produce a harvestable crop. Succession planting every 2–3 weeks stretches the harvest window.
What brussels sprout varieties grow best in Washington?
For Washington's Zone 4b, consult a local nursery or extension office for brussels-sprouts variety recommendations.
When do I start brussels sprout seeds indoors in Washington?
Start brussels sprout seeds indoors in Washington around April 2–9, which is 5 weeks before the state's average last frost of May 7. Use grow lights and keep soil temperatures at 70–75°F for fastest germination.