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.
Strawberry care
Per WSU Extension, May strawberry management:
- Remove row covers once plants are actively blooming; pollinators need access during flowering — per WSU, covers left on during bloom reduce fruit set significantly
- Fertilize June-bearing strawberries with 1 lb ammonium nitrate per 100 sq ft when growth is vigorous (early May); apply before bloom for best effect
- Day-neutral and everbearing: apply monthly light fertilization through August to sustain continuous production
- Runner management: remove all runners from the first-year planting to keep energy directed to the crown and root system; in the second year, allow runners to fill in the row
- Bird netting: install before first fruit colors; birds begin taking berries well before they're fully ripe
Strawberry root weevil (Otiorhynchus ovatus): Per Oregon State Extension, this is the primary strawberry pest in the PNW. Adults emerge from soil in May—June, feed on leaf margins (creating notched pattern), and lay eggs near crowns. The larvae damage roots over summer. Adult feeding is cosmetic; larval damage is more serious in established beds. beneficial nematodes (Heterorhabditis bacteriophora) applied in June when soil is warm can reduce larval populations.
Preparing for the dry season
May is the month to prepare irrigation infrastructure before the dry season that begins in June in the PNW. Per Oregon State Extension:
- Inspect drip and soaker systems; repair or replace damaged heads, drip emitters, and connectors
- Set timer for early morning runs (4—6 AM) to allow foliage to dry before evening
- Calculate water needs: most vegetables need 1—1.5 inches per week; established trees and shrubs need deep infrequent watering (2—3 hours drip every 1—2 weeks once the dry season begins in June)
Starting irrigation before plants visibly stress is better practice than waiting for wilt. Per WSU Extension, water stress at critical growth stages (flower set, fruit fill) reduces yield even after recovery — the damage is not fully reversible.
Slug transition
May marks the beginning of the slug pressure decline as soils dry. Per Oregon State Extension, slug activity drops significantly when the soil surface dries between rain events. By late May, the daily bait applications of March—April can be reduced to weekly or as-needed. Monitor new transplants for the first 2 weeks after setting out — young transplants in newly warmed soil attract slugs even as overall populations decline.
Cool-season crops wrapping up
Per WSU Extension, most cool-season crops planted in March—April are reaching the end of their productive life in May:
- Harvest peas aggressively: overmature pods become mealy within days; harvest every 2—3 days during peak production; once pods are past peak, remove plants and replant with warm-season crops
- Cut lettuce frequently: once a bolt stalk begins, the leaves turn bitter within a week; harvest immediately or pull the plant
- Start transition: remove finished cool-season crops promptly and amend beds with 2—3 inches of compost before replanting with warm-season crops
Berry and fruit management
- Blueberries: thin flower clusters if the plant was transplanted last year — allowing the plant to fruit heavily in its first productive year reduces long-term vigor; per Oregon State, allow only 30—50% of full fruit load in the second year after planting; after that, full production is appropriate
- Raspberry primocanes: this is the month when new primocanes (first-year canes) emerge and grow rapidly; do not remove them — they will fruit next year; tie or train to trellis system as they reach 12—18 inches; per WSU, crowded untied primocanes develop weaker stems that are more prone to breaking under fruit load
Common mistakes
| Mistake | Consequence | Correct approach |
|---|---|---|
| Transplanting tomatoes before May 15 without soil warming | Cold stall; early blight risk; poor establishment | Pre-warm soil with black plastic; transplant after May 15 |
| Removing strawberry runners on established beds | Thins the matted row too aggressively | In established matted rows, allow runners to fill gaps; thin to target density |
| Not preparing irrigation before June | Plants stress when dry season arrives suddenly | Test and set up all irrigation in May |
| Leaving row covers on strawberries through bloom | Reduced fruit set from poor pollination | Remove covers when plants begin blooming |
Frequently asked questions
Should I use black plastic mulch for tomatoes in the PNW? Per Oregon State Extension, black plastic mulch is one of the most effective tools for tomato production in the PNW. It raises soil temperature 8—12°F over bare soil, which dramatically improves early-season growth. It also controls weeds and maintains consistent soil moisture — both important in PNW's variable early summer. The downside is plastic disposal; biodegradable plastic mulch is available as an alternative.
When should I plant garlic for fall harvest in the PNW? Per WSU Extension, garlic is planted in fall (October—November) for the following July harvest. Any May-planted garlic will produce undersized bulbs because it misses the required cool-season root development period. If you want garlic this season, plant immediately — May garlic produces small but usable bulbs by August.
Sources
- Oregon State Extension — Home Vegetable Gardening
- WSU Extension — Vegetable Planting Calendar
- Oregon State Extension — Strawberry Production
- Oregon State Extension — Tomato Production in the Pacific Northwest
- Oregon State Extension — Strawberry Root Weevil