What to Plant in Portland in May: Zone 8b Warm Season Kickoff

Warm soil at last, rhodies at peak, drip lines under audition before June flips the tap off.

The question of what to plant in Portland in May finally has a warm answer this week: nighttime lows are holding above 50°F, soil at four inches is pushing past 60°F, and the last frost is safely three weeks behind us. That means tomatoes, peppers, basil, beans, squash, and cucumbers can all go in the ground now, while rhododendrons at peak bloom and dahlias breaking dormancy need attention on a completely different schedule. Portland's rainfall typically drops off sharply after May 25, so this week is also your last realistic window to walk drip lines and mulch beds before the dry season lands.

This Week's Action List

Frequently Asked Questions

When can I plant tomatoes outside in Portland Oregon?

The safe outdoor window in Portland opens around May 15, once nighttime lows hold above 50°F and soil at four inches reaches 60°F. Planting earlier stalls the plants in cold soil and rarely produces earlier fruit. If a cold snap threatens after transplant, drape row cover or an old sheet over cages for the night.

Should I prune my rhododendron after it blooms in May?

Only deadhead and lightly shape, nothing more. Snap spent flower trusses off at the base with your fingers, being careful not to damage the new green shoots pushing out just below. Portland rhododendrons set next year's flower buds in June and July, so any real pruning done after early June sacrifices the following spring's display.

How much should I water new vegetable transplants in Portland in May?

Deep soak new transplants every two to three days for the first two weeks, delivering roughly a gallon per plant slowly at the base rather than a quick daily sprinkle. Portland typically still gets one to two inches of rain in May, so check soil moisture two inches down before watering. Once June arrives and rainfall drops off, shift to drip on a timer.

Is it too late to start a vegetable garden in Portland in mid May?

Not at all. Mid May is actually prime time to plant warm season crops from starts, including tomatoes, peppers, eggplant, basil, and squash, and to direct sow beans, cucumbers, and corn. You still have a full 180 day growing window before the mid November first frost, which is more than enough for nearly every summer vegetable variety sold locally.