✨ Rhody trusses snap off, dahlias get pinched, tomato foliage stays bone dry. Portland's dry season starts Monday.
Late blight prevention for Portland tomatoes moves from theory to daily practice this week, because nighttime lows are finally holding above 50°F and the last May rains are on their way out. That combination, warm wet nights followed by muggy mornings, is exactly what the Phytophthora infestans spores wait for. Set up your beds correctly in the next seven days and you buy yourself a much easier July.
This Week's Action List
- 1
Switch tomatoes to drip or soaker irrigation this week if you haven't already. Overhead watering wets the foliage at night, and wet leaves after dark are the direct trigger for late blight infection. Run drip lines 30 to 45 minutes at dawn, two or three times a week, not daily.
- 2
Snap spent rhododendron trusses off at the base with your fingers, not pruners, right after the last flower browns. Break the truss just above the two new leaf whorls pushing out below it. Cut too low and you lose next year's flower buds, which are setting now for a 2027 bloom.
- 3
Pinch dahlia shoots back to the fourth set of true leaves once plants hit 12 inches tall. This forces two side shoots at each node and roughly doubles your bloom count from August through October. Do it once, on a dry afternoon, and let the plant recover before you fertilize.
- 4
Lay 3 inches of arborist chip or fine bark mulch over every vegetable bed before June 1. Mulch cuts irrigation demand 40 to 50 percent through July and August, buffers soil temperature, and, critically for tomatoes, blocks soil borne blight spores from splashing up onto lower leaves during any late spring shower.
- 5
Walk every drip zone with the system running for 15 minutes and mark broken emitters, kinked lines, and dry spots with landscape flags. Fix problems now while nurseries have parts in stock. Finding a failed emitter in a 96°F July week is how tomatoes die overnight.
- 6
Strip the bottom 8 to 10 inches of leaves off every tomato plant once it clears 18 inches tall. Bare stem near the soil dramatically slows both late blight and early blight, and improves airflow through the canopy. Cage or stake at the same time, before plants get floppy.
- 7
Start weekly rose fertilizing with a balanced granular formula, roughly a half cup per established shrub, worked into the top inch of soil and watered in. Aphids are at peak right now on new growth. Blast colonies off with a hard water spray every three or four days before reaching for anything stronger.
- 8
Check under dahlia leaves for silvery streaking and tiny black specks, the signature of thrips damage. Rub affected leaves off and drop them in a sealed bag, not the compost. Encourage lacewings and hoverflies by leaving alyssum and calendula flowering nearby instead of spraying broad spectrum insecticides.
Frequently Asked Questions
When does late blight usually show up on Portland tomatoes?
Late blight typically appears in Portland gardens between late June and mid July, once nighttime temperatures hold above 50°F and morning humidity stays above 90 percent. The infection window opens earlier in wet years and in shaded gardens with poor airflow. Prevention set up in May, drip irrigation and heavy mulching, is far more effective than any spray applied after symptoms show.
Can I still plant tomatoes in Portland during the last week of May?
Yes, late May is actually prime planting time in Zone 8b once soil temperature reaches 60°F at 4 inches deep. Choose 4 to 6 inch transplants, bury them up to the first true leaves to build root mass, and water in with a diluted fish emulsion. Varieties like Legend, Stupice, and Oregon Spring are bred for Portland and outrun blight better than most heirlooms.
Should I prune my rhododendron now that it's finished blooming?
Only very lightly. Snap off the spent flower trusses at their base to redirect energy into next year's buds, and cut back any obviously damaged or crossing branches. Heavy structural pruning on rhododendrons should wait until late June at the latest, because flower buds for 2027 begin setting in July and any cuts after that remove next year's show.