Giant ironweed is one of the most striking native perennials you can grow for cutting — tall, bold stems topped with flattened clusters of vivid purple-magenta flowers that hold for weeks in late summer and fall. It is native to the eastern US and extraordinarily easy: it tolerates wet soils, dry soils, clay, and part shade with equal indifference. Monarch butterflies and bumblebees work the flowers constantly.
The scale is dramatic — plants reach five to eight feet in good conditions — and they work beautifully cut with dahlias and late cosmos. The seed heads are also attractive left standing through winter. Start from transplants or divisions; germination from seed is slow and erratic.
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.
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.
Fungal leaf spot
Brown or purplish spots on the leaves, often ringed with a yellow halo, that spread and merge in warm, humid, or wet weather; heavily spotted leaves yellow and drop early.
Pick off spotted leaves and clear fallen debris to remove overwintering spores. Improve air circulation by thinning, and water at the base so foliage dries quickly. If it persists, apply a copper- or sulfur-based fungicide at the first sign.
Aphids
Aphididae →Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.