Blue mistflower is a native perennial that earns its place as one of the most valuable late-season pollinator plants in the garden — the soft purple-blue floss-like flowers open in August and carry through to frost, providing critical nectar for migrating monarch butterflies and other pollinators at a time when almost nothing else is blooming. Native to 26 US states and Ontario, it thrives in average soil with minimal intervention.
The plant spreads in two ways — by wind-dispersed seed and by rhizome — and will double in size each season if left unchecked. Cut flower tops before seed sets to limit spread, and divide clumps in spring to keep it in bounds. A spring haircut to the ground encourages fresh compact growth. Worth growing deliberately despite the vigour; nothing else fills the late-season pollinator gap so reliably.
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.
Aster Yellows
Deformed, greenish flowers, stunted growth, and yellowed foliage. Flower centers may produce tufts of leafy growth instead of normal petals.
No cure. Remove and destroy infected plants immediately to stop spread. Control leafhoppers, which transmit the disease, and remove nearby weed hosts.
Rust
Orange to brown raised pustules on the undersides of leaves, with yellow spotting on the upper surface. Heavy infections cause leaves to yellow and drop.
Remove and destroy infected leaves. Avoid overhead watering and improve air circulation. Apply a sulfur or copper-based fungicide if it spreads. Clear plant debris in fall.