May garden tasks: Pacific Northwest
May in the Pacific Northwest is the shoulder season -- early May still looks like April (cool, wet, uncertain) and late May starts to hint at what June will bring (drying out, longer days, faster growth). The key transition is soil temperature, which climbs slowly in the PNW due to persistent cloud.
May in the Pacific Northwest is the shoulder season — early May still looks like April (cool, wet, uncertain) and late May starts to hint at what June will bring (drying out, longer days, faster growth). The key transition is soil temperature, which climbs slowly in the PNW due to persistent cloud cover and soil moisture.
Per Oregon State Extension, average soil temperature at 4 inches in Portland area reaches 60°F around May 20 in a typical year. This is the reliable threshold for tomato and pepper transplanting. Setting them out earlier is possible with soil warming practices but not without them.
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Warm-season crop transplanting
Per WSU Extension, May planting dates for Western Oregon and Washington:
- Tomatoes: transplant May 15—June 1 when soil is 60°F+ at 4-inch depth; use black plastic mulch or IRT (infrared-transmitting) mulch to pre-warm soil; per Oregon State, cold soil below 55°F causes tomato roots to shut down and can trigger early blight from the soil
- Peppers: transplant May 20—June 1; even more cold-sensitive than tomatoes; cold soil (below 60°F) stunts pepper growth for weeks and the plants never fully recover their early-season potential
- Cucumbers: direct sow or transplant May 15—25; cucumbers germinate rapidly in warm soil but can rot if soil is still cold
- Squash and zucchini: direct sow May 15—25; germinates rapidly and doesn't benefit from early starting
- Basil: transplant May 20—June 1 when night temperatures are reliably above 50°F; basil chilled below 50°F shows black spotting on leaves and remains stunted
Per Oregon State Extension, choose tomato varieties adapted to cool PNW summers: 'Early Girl' (57 days), 'Siletz' (70 days), 'Legend' (68 days), 'Willamette' (65 days). Varieties requiring 80+ days often don't ripen before fall rains arrive in September.