When to Plant Onions in Washington
Published: April 21, 2026
Washington gardeners in Zone 4b have a 21-week frost-free window for onion (May 7 to October 1). Cool-season crops like onion 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.
Onion Planting Dates for Washington
| Start seeds indoors | February 26–March 5 |
| Last frost (average) | May 7 |
| Transplant outdoors | April 9–19 |
| Direct sow outdoors | April 9–19 |
| Expect first harvest | July 8 – August 7 |
| First fall frost (average) | October 1 |
⚠ CRITICAL: Choose variety based on your latitude. Short-day varieties for south of 35°N (Texas, Florida, California). Long-day varieties for north of 35°N (Ohio, Minnesota, New York). Intermediate varieties work in the middle band.
Growing Onions in Washington's Climate
Washington's cold Zone 4b climate is close to ideal for onion. 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 onion 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 onion pests to watch for in Washington include Onion Thrips and Onion Maggot. The first line of defense is companion planting: Carrot and Tomato 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.
Onion Garden Calendar for Washington
| Month | Task | Notes |
|---|---|---|
| January | — | Dormant season |
| February | Start seeds indoors | Use grow lights; warm soil to 70–75°F for best germination |
| March | — | Dormant season |
| April | Transplant outdoors | Soil temp must be 35°F+; nighttime lows above 50°F |
| May ← | Harden off seedlings | Set seedlings outdoors 1–2 hours daily, increasing gradually |
| June | — | Dormant season |
| July | First harvest window opens | Onion begins producing 90–120 days after transplant |
| August | — | Dormant season |
| September | — | Dormant season |
| October | First frost approaches | Harvest remaining onion; cover plants on frost nights |
| November | — | Dormant season |
| December | — | Dormant season |
Onion Growing Tips for Washington Gardeners
- •Match variety to latitude: short-day south of 35°N, long-day north of 35°N, intermediate in between.
- •Growing from sets (small bulbs) is fastest; growing from transplants gives the most variety options.
- •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.
- •Reduce watering as tops begin to fall over — dry conditions encourage tight, firm bulbs that store longer.
Companion Planting for Onions in Washington
In Washington's cold climate, planting onion with Carrot and Tomato helps deter pests and improve soil. Avoid planting near Bean and Pea, which inhibit onion growth through root chemistry or shared pest pressure. See the full onion companion planting guide for the complete list.
Onions 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 onion in Washington?
In Washington (Zone 4b), start onion seeds indoors around February 26–March 5 and transplant outdoors around April 9–19 — 4 weeks before 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 onion year-round in Washington?
onion 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 onion varieties grow best in Washington?
For Washington's Zone 4b, Long-day varieties like Walla Walla, Copra, and Yellow Sweet Spanish bulb under the 14+ hour days north of 35°N.
When do I start onion seeds indoors in Washington?
Start onion seeds indoors in Washington around February 26–March 5, which is 10 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.