March garden tasks: Pacific Northwest

March in the Pacific Northwest is the beginning of active gardening season despite weather that reads as winter by other regions' standards. Portland (zone 8b), Seattle (zone 8b/9a), and the Willamette Valley are climatically mild in winter but persistently wet -- and that wetness defines every.

March in the Pacific Northwest is the beginning of active gardening season despite weather that reads as winter by other regions' standards. Portland (zone 8b), Seattle (zone 8b/9a), and the Willamette Valley are climatically mild in winter but persistently wet — and that wetness defines every March gardening decision. Working soil that's too wet is the most common and most damaging mistake.

Per Oregon State Extension, average last frost dates in the PNW: Portland March 21, Salem March 19, Seattle March 24, Olympia March 10. Many lowland PNW gardens are frost-free by mid-March, but late cold snaps are possible through April in cooler inland valleys.

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Cool-season planting

The mild PNW winter means cool-season crops can be planted substantially earlier than other regions. Per WSU Extension, by March the following can be planted outdoors:

Direct sow (March 1—31):

• Spinach, arugula, lettuce: all can go in when soil is workable; these germinate in soil as cold as 40°F
• Peas: plant by March 17 (St. Patrick's Day is the traditional marker in Western Oregon and Washington); earlier in the warmest sites
• Beets, chard, carrots, parsnips: sow March 15+ when soil begins to dry slightly
• Broccoli and cauliflower transplants: set out under floating row cover in March for early June harvest

Cold frames and floating row cover: Per Oregon State Extension, a single layer of floating row cover fabric (1.25 oz/yd) raises the temperature under it by 4—6°F and provides frost protection to about 26°F. Row covers make March planting practical even in inland areas that can still dip below freezing.

Onion sets and transplants: Plant onion sets and transplants in March — onions are among the most cold-tolerant crops and can withstand temperatures to 20°F after hardening.