Planting Guides

When to Plant Onions in Seattle: Complete Guide + Best Varieties for Zone 8b

Seattle, Washington

USDA Zone 8b

Last Frost: Mar 15

Last updated: October 30, 2025

Plant onions in Seattle March 1-April 15 for summer harvest or September 15-October 15 for early summer harvest in Zone 8b's mild climate.

MMike Thompson

October 30, 2025

When to plant onions in Seattle Zone 8b garden with proper spacing and mulch — — Image © PlantReference.org 2026

Quick Answer

Plant onions in Seattle March 1-April 15 for summer harvest. Fall planting September 15-October 15 produces earlier, larger bulbs.

TL;DR

Plant onions in Seattle March 1-April 15 for summer harvest or September 15-October 15 for early summer harvest. Zone 8b's mild winters allow both spring and fall planting with long-day varieties like Allium cepa 'Walla Walla' thriving in the Pacific Northwest climate. Fall-planted onions overwinter successfully and produce larger bulbs by June.

Frequently Asked Questions

When is the best time to plant onions in Seattle?

Plant onions in Seattle March 1-April 15 for summer harvest or September 15-October 15 for early summer harvest. Seattle's Zone 8b climate allows both traditional spring planting and strategic fall overwintering. Fall-planted onions often produce larger bulbs due to extended growing time through mild winters.

What onion varieties grow best in Seattle's climate?

Long-day varieties perform best in Seattle's latitude (47.6°N). Allium cepa 'Walla Walla' is the Pacific Northwest classic, while Allium cepa 'Red Wing' provides excellent storage for wet winters. Allium cepa 'Copra' and Allium cepa 'Yellow Sweet Spanish' also thrive in Seattle's 245-day growing season with proper care.

Can I plant onions in fall in Seattle?

Yes, fall planting works excellently in Seattle's Zone 8b climate. Plant transplants September 15-October 15 for harvest the following May-June. Fall-planted onions develop extensive root systems through mild winters and often produce 25-50% larger bulbs than spring-planted varieties. Apply heavy mulch protection during winter months.

How do I prevent onion storage problems in Seattle's humid climate?

Cure onions indoors rather than outdoors due to frequent rain. Use fans for air circulation during the 2-4 week curing process and maintain humidity below 70%. Store completely cured onions in unheated garages or basements at 32-40°F (0-4°C). Only storage varieties like Allium cepa 'Red Wing' keep well through Seattle's wet winters.

Should I plant onion sets or transplants in Seattle?

Transplants offer the best results in Seattle, providing superior variety selection and handling the region's cool, wet springs better than sets. Transplants work for both spring and fall planting windows. Choose onion sets smaller than a dime if using them for spring planting, but avoid sets for fall planting due to bolting risks.

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Tagged with

#Seattle gardening #Zone 8b planting #Pacific Northwest onions #when to plant onions #Walla Walla onions #fall planting onions #Seattle vegetables #Washington state gardening #long-day onions #onion varieties

Written By

Mike Thompson

Mike's been designing residential landscapes in the Seattle area, with a particular focus on Pacific Northwest natives and sustainable practices. He's a Washington State Master Gardener and worked in construction before landscaping, which gives him a practical edge when it comes to building raised beds and hardscaping projects. He runs a small side business propagating and selling unusual perennials at farmers markets on weekends. His property serves as a testing ground for low-maintenance, wildlife-friendly garden designs. Mike's philosophy: if it requires weekly maintenance, it's not sustainable.