Washington’s cooler coastal regions call for onion planting in early spring, around March or April, depending on your specific zone. Knowing the best time to plant onions in Washington is key to a successful harvest, as timing varies widely across the state’s diverse climates. Whether you’re in the rainy west or the drier east, getting the planting date right sets you up for big, flavorful bulbs.
Onions are a cool-season crop that thrive in Washington’s mild springs. But plant too early and frost can damage young seedlings. Plant too late and the long summer days might not give bulbs enough time to mature. This guide walks you through the exact timing for your region, plus tips for soil prep and variety selection.
Best Time To Plant Onions In Washington
The ideal window depends on where you live. In western Washington (zones 8a-8b), aim for mid-March to early April. Eastern Washington (zones 6a-7b) is colder, so wait until late April or early May. Coastal areas near the ocean can start as early as late February if the soil is workable.
Onions need 6-8 weeks of cool weather (below 75°F) for good leaf growth before bulbing begins. If you plant too late, the heat triggers bulbing early, giving you small onions. Use a soil thermometer—plant when the soil is at least 50°F at 2 inches deep.
Why Timing Matters For Washington Growers
Onions are photoperiodic: they form bulbs based on day length. Washington’s latitude means long summer days (15-16 hours), so you need “long-day” onion varieties. These need 14-16 hours of daylight to bulb. If you plant too late, the days are already long, and bulbs form before the plant is big enough.
Frost is another factor. Young onion seedlings can handle light frost (down to 28°F), but hard freezes (below 25°F) can kill them. Check your last frost date—for Seattle, it’s around March 15; for Spokane, it’s May 10. Plant 2-4 weeks before that date for best results.
Regional Breakdown For Washington
Western Washington (Puget Sound, Coast)
- Best planting window: March 15 to April 15
- Soil temperature: 50-55°F
- Last frost: Mid-March
- Recommended varieties: ‘Walla Walla Sweet’, ‘Redwing’, ‘Yellow Sweet Spanish’
In western Washington, the maritime climate keeps winters mild but springs wet. Onions need well-drained soil, so raised beds or mounds help. Start seeds indoors in late February if you want a head start. Transplants go out in March.
Eastern Washington (Inland, Higher Elevation)
- Best planting window: April 20 to May 10
- Soil temperature: 55-60°F
- Last frost: Late April to early May
- Recommended varieties: ‘Patterson’, ‘Copra’, ‘Red Bull’
Eastern Washington has colder winters and hotter summers. Onions need to be planted after the soil warms up. Use black plastic mulch to warm the soil faster. Direct seeding works well here if you plant in late April.
Coastal Areas (Olympic Peninsula, San Juan Islands)
- Best planting window: Late February to late March
- Soil temperature: 45-50°F (onions can tolerate cooler soil)
- Last frost: Early March
- Recommended varieties: ‘Ailsa Craig’, ‘White Sweet Spanish’, ‘Stuttgarter’
Coastal zones have the mildest winters. You can plant earlier, but watch for heavy rain that can rot bulbs. Use row covers to protect from slugs and cold snaps.
How To Plant Onions Step By Step
- Choose your method: Seeds, sets (small bulbs), or transplants. Seeds take 3-4 months to mature; sets are faster but less reliable for size.
- Prep the soil: Onions need loose, fertile soil with pH 6.0-7.0. Add compost or aged manure 2 weeks before planting. Avoid fresh manure—it can burn roots.
- Space correctly: Plant sets 1 inch deep, 4-6 inches apart. For transplants, bury them 1 inch deep, spacing 4-6 inches apart. Rows should be 12-18 inches apart.
- Water well: Give 1 inch of water per week. Onions have shallow roots, so consistent moisture is critical. Drip irrigation works best.
- Mulch: Use straw or grass clippings to retain moisture and suppress weeds. Onions hate competition.
- Fertilize: Apply a balanced fertilizer (10-10-10) at planting, then side-dress with nitrogen when bulbs start to swell (around June).
Common Mistakes To Avoid
- Planting too deep: Onions need shallow planting. If you bury them too deep, they won’t bulb well.
- Overwatering: Soggy soil causes rot. Let the top inch dry out between waterings.
- Ignoring day length: Only grow long-day varieties in Washington. Short-day onions won’t bulb properly here.
- Planting in heavy clay: Onions need good drainage. Amend clay soil with sand or grow in raised beds.
- Forgetting to harden off transplants: If you start indoors, acclimate them outside for a week before planting.
Best Onion Varieties For Washington
Long-day varieties are non-negotiable. Here are top picks for Washington’s climate:
- Walla Walla Sweet: Famous in Washington, this is a mild, sweet onion. It’s a long-day type that does well in western regions. Harvest in July-August.
- Copra: A storage onion with strong flavor. Great for eastern Washington’s hotter summers. Stores up to 10 months.
- Redwing: A red onion with good storage life. Adaptable to both sides of the state.
- Patterson: A yellow storage onion that handles heat well. Perfect for eastern Washington.
- Ailsa Craig: A giant, sweet onion for coastal areas. Needs a long growing season (120 days).
When To Harvest Onions In Washington
Onions are ready when the tops fall over and start to yellow. This happens 90-120 days after planting, depending on variety. In western Washington, harvest usually starts in late July. In eastern Washington, it’s mid-August to early September.
Stop watering when tops fall over. Let them sit in the ground for 2-3 days to cure. Then pull them, brush off soil, and cure in a warm, dry place for 2-3 weeks. Cut off tops and store in a cool, dark spot.
Frequently Asked Questions
Can I plant onions in the fall in Washington?
Yes, but only in mild coastal areas (zones 8b-9a). Plant sets in October for a spring harvest. In colder regions, fall-planted onions often freeze out. Stick to spring planting for most of the state.
What happens if I plant onions too late?
Late planting (after May in western WA, after June in eastern WA) results in smaller bulbs. The heat triggers early bulbing, and the plant doesn’t have enough leaf growth to support large bulbs. You might still get scallions, but not full-sized onions.
Should I start onions from seeds or sets?
Sets are easier for beginners and give faster results. Seeds offer more variety and better storage onions. In Washington, seeds started indoors in February work well for western regions. Sets are better for eastern Washington’s shorter spring.
How do I protect onions from frost?
Use row covers or cloches if a hard freeze is forecast. Onions can handle light frost, but cover them if temps drop below 28°F. Remove covers during the day to prevent overheating.
Can I grow onions in containers in Washington?
Yes, use deep containers (at least 10 inches) with drainage holes. Fill with loose potting soil. Water more frequently—containers dry out faster. Place in full sun (6+ hours daily).
Final Tips For Success
Test your soil pH before planting. Onions prefer slightly acidic soil. Add lime if pH is below 6.0. Also, rotate your onion bed each year to prevent soilborne diseases. Don’t plant onions where garlic, leeks, or other alliums grew the previous year.
Keep an eye on pests. Onion maggots are common in Washington. Use row covers to prevent flies from laying eggs. Slugs are also a problem in wet areas—use beer traps or diatomaceous earth around plants.
Weeding is critical. Onions have shallow roots and can’t compete with weeds. Hand-pull weeds carefully to avoid disturbing onion roots. Mulch heavily to smother weeds.
If you’re in a rainy part of Washington, consider planting on ridges or mounds. This improves drainage and prevents bulb rot. Raised beds are ideal for heavy clay soils.
Water consistency matters. Onions need steady moisture, especially during bulbing. If you let them dry out then water heavily, the bulbs can crack or split. Use a soaker hose for even watering.
Harvest on a dry day. Wet onions are prone to rot during curing. Cure them in a well-ventilated area out of direct sun. A garage or covered porch works well.
Store onions properly. Keep them in a cool (40-50°F), dry, dark place with good air circulation. Don’t store them with potatoes—potatoes release moisture that can spoil onions.
For sweet onions like Walla Walla, eat them within a few months. They don’t store as long as pungent varieties. Use storage onions like Copra for winter meals.
Experiment with different varieties each year. Washington’s microclimates vary widely, so what works in Seattle might not work in Yakima. Keep notes on planting dates and harvest results.
If you’re new to gardening, start with onion sets. They’re more forgiving than seeds. Plant them in early spring and you’ll have onions by late summer. It’s a rewarding crop for beginners.
Remember that the best time to plant onions in Washington is a moving target based on your exact location. Check your local frost dates and soil temperature. A little planning goes a long way toward a bountiful harvest.
Onions are one of the easiest vegetables to grow in Washington if you get the timing right. They require minimal fuss once planted. Just give them sun, water, and space, and they’ll reward you with fresh flavor all year.
Don’t be afraid to try direct seeding in late April if you missed the transplant window. Onions are resilient. Even late-planted ones can produce decent scallions or small bulbs for cooking.
Join a local gardening group or check with your county extension office for region-specific advice. Washington State University Extension has excellent resources for onion growers. They can tell you the exact best planting dates for your zip code.
With the right timing and a little care, you’ll be pulling up beautiful onions from your Washington garden. Start planning now for a successful season. Your taste buds will thank you.