Dahlia care in August: dial down nitrogen, keep the cut flower rotation aggressive, and use this dry week to place your spring bulb order.
This Week's Action List
- 1
Switch dahlias to a low nitrogen, higher potassium feed this week (something like a 3-8-8 or a tomato formula at half strength). Excess nitrogen now delays tuber ripening and gives you soft stems that flop in the first September rain.
- 2
Cut dahlias deep and often. Reach 12 to 18 inches down into the plant and take the stem back to a set of leaves, even if the side buds look small. Shallow snips leave stubby branches; deep cuts trigger long new stems and keep the plant blooming into October.
- 3
Order tulip, daffodil, and allium bulbs by Friday. Portland Nursery, Cornell Farm, and mail order specialists like Old House Gardens sell out of premium varieties (think 'Menton', 'Queen of Night', Allium 'Globemaster') by early September. Bulbs go into the ground in October, but the picking happens now.
- 4
Watch for botrytis gray mold on dahlia petals as Thursday's heat gives way to Saturday's cooler 80°F. Brown, water soaked petal edges spreading into the flower center are the tell. Cut affected blooms into the trash, not the compost, and thin crowded foliage to move air through the plant.
- 5
Water dahlias deeply Wednesday evening or Thursday at dawn ahead of the 92°F peak. Two to three gallons per mature plant, delivered slowly at the base, will carry them through the heat. Overhead watering in this window invites botrytis and wastes water to evaporation.
- 6
- 7
Direct sow spinach, arugula, and mache into a shaded, pre moistened bed this week. Soil temperatures above 75°F stall germination, so sow in the late afternoon, water in cool, and lay shade cloth or a board over the row until you see cotyledons in 5 to 8 days.
- 8
Take 4 inch dahlia cuttings from strong side shoots if you want free plants next year. Strip the lower leaves, dip in rooting hormone, and stick in moist perlite in a bright but shaded spot. Roots form in 10 to 14 days at this temperature.
Frequently Asked Questions
When should I stop fertilizing dahlias in Portland?
Cut off nitrogen feeding by mid August and switch to a low nitrogen, potassium heavy formula through early September. Portland's first frost typically lands around October 15, and tubers need six to eight weeks of ripening to store well. Late nitrogen keeps plants pushing soft green growth instead of building tuber reserves.
When is the best time to order spring bulbs in Portland Oregon?
Order in mid to late August for the best variety selection. Bulbs ship in September and October for planting once soil temperatures drop below 60°F, usually mid October through early November in Portland. Waiting until September often means the tulip and allium cultivars you actually want are already sold out.
Can I still prune my hydrangeas in August in Portland?
No. August pruning on hydrangeas, rhododendrons, camellias, or any woody shrub pushes tender new growth that cannot harden off before winter and will die back at the first hard freeze. The next safe pruning window opens in November for dormant cuts, or immediately after bloom for spring flowering shrubs next year.
Why are my dahlia flowers turning brown on the edges?
Brown, mushy petal edges spreading toward the flower center are classic botrytis gray mold, driven by warm days and cool humid nights. Cut affected blooms off and discard them in the trash, not the compost. Improve airflow by thinning crowded foliage and stop any overhead watering, which keeps petals wet long enough for spores to take hold.