Dahlias are the showpiece of the autumn garden — their colours deepen as cool nights arrive, hitting their richest just before the first frost ends the season in a single night. Plant tubers after last frost when soil is consistently above 60°F. They are slow to start: resist the urge to water until you see the first shoots, then water deeply and consistently.
Pinch the growing tip at 12 inches to encourage branching and more stems. In Zone 6b and colder, lift tubers after the first killing frost, cure them for a week in a dry, airy space, and store packed in barely-damp vermiculite at 40–50°F over winter.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Mosaic virus
Yellowing, mottled, or distorted leaves. No cure — spread by aphids.
Remove and destroy infected plants. Control aphid populations to prevent spread. Do not propagate from infected tubers.