Achillea millefolium · Perennial
Powdery mildew: Improve air circulation by thinning crowded plants and water at the base to keep foliage dry.
Aphids: Squash infestations by hand or spray with a strong jet of water; introduce ladybirds for biological control.
Slugs and snails: Use beer traps, copper tape barriers, or iron phosphate-based pellets to deter feeding.
Root rot: Ensure well-drained soil and avoid overwatering; lift and divide congested clumps in spring.
Rust: Remove affected leaves immediately and avoid overhead watering to reduce humidity around the plant.