The First August Heat Wave Is The Story Of The Week.
This Week's Action List
- 1
Direct sow spinach, arugula, and lettuce Monday or early Tuesday before the heat lands, then cover the seedbed with shade cloth or a light board until germination. Soil above 75°F triggers dormancy in lettuce and bolts spinach fast, so keep the top inch damp with a light morning misting through Thursday.
- 2
Hold fall broccoli, cabbage, and kale transplants in their pots through the Wednesday and Thursday peak if you can. Move them into the ground Friday evening or Saturday morning once highs drop back to the mid 80s, and water in with a diluted kelp solution to reduce transplant shock. The August 15 target still gives you a week of cooler planting weather.
- 3
Two consecutive days near 97°F will stress established tomatoes into blossom drop and can scald exposed fruit. Deep soak the root zone Tuesday morning with 2 to 3 gallons per plant.
- 4
Order tulip, daffodil, and allium bulbs this week for October planting.
- 5
Watch dahlias for botrytis gray fuzz on petals and stem joints, especially on tightly packed varieties like ball and pompon types. Cut affected blooms into a bag, not the compost, and thin interior foliage to improve airflow before humidity climbs behind the heat wave.
- 6
Take 4 inch dahlia cuttings from strong side shoots if you want extra plants for 2027. Strip the lower leaves, dip in rooting hormone, and root in moist perlite in bright shade, they should callus within two weeks and root by Labor Day.
- 7
Begin tapering nitrogen on woody shrubs and trees now so new growth hardens before fall. Switch dahlias to a low nitrogen, higher potassium feed like 3 to 8 to 8 to firm up tubers for lifting in October.
- 8
Rake and bag fallen rose leaves and spent blooms weekly through August. Every black spot lesion you remove now is one fewer spore load overwintering in the mulch for next spring's flush.
Frequently Asked Questions
Can I plant fall vegetables in Portland during a 97 degree heat wave?
Yes for direct sown seeds if you shade the bed and keep the top inch of soil moist, spinach, arugula, and lettuce germinate fine when the seedbed itself stays under 75°F. For transplants like broccoli and cabbage, wait until highs drop back into the mid 80s later in the week to avoid transplant shock. The fall planting window in Zone 8b runs through August 15 for brassica starts.
Why can't I prune my rhododendrons or hydrangeas in August in Portland?
Any pruning cut in August triggers a flush of soft new growth that cannot lignify before Portland's first frost around October 15. That tender growth dies back over winter and creates entry points for disease. The next safe structural pruning window for woody shrubs opens in November once plants are fully dormant, or after bloom next spring for spring flowering species.
When should I order spring bulbs for Portland gardens?
Order in August for October planting. Portland nurseries and mail order suppliers stock their best selection in mid August, and popular tulip cultivars, species alliums, and specialty daffodils sell out by early September. Store bulbs in a cool, dry, dark spot like a garage or basement until soil temperatures drop below 55°F, usually mid October in Zone 8b.
How much should I water tomatoes during a Portland heat wave?
Established tomatoes need 2 to 3 gallons per plant every two to three days during a stretch of 95°F plus days, delivered by drip or a slow soak at the base. Water in the early morning so foliage dries before evening, and mulch with two inches of straw or shredded leaves to buffer soil temperature. Skip overhead watering entirely, wet leaves in humid mornings are the main trigger for late blight.