Yarrow is a hardy perennial grown for its flat clusters of small flowers and its fine, fern-like foliage. It tolerates poor soil, drought, and heat, making it reliable in low-maintenance and pollinator beds. Flowers attract bees, predatory wasps, and beneficial insects, and the blooms hold up well as cut and dried flowers.
It spreads by rhizomes and can become aggressive in good soil, so give it room or contain it. Plants flop in rich or shaded conditions; lean soil and full sun keep growth compact. Cut back after the first bloom to encourage a second flush, and divide every two to three years to maintain vigor.
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.
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.
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.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.