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.
Ironweed Rust
Orange to reddish-brown raised pustules on the undersides of leaves, with yellow spotting on the upper surface. Heavy infection causes leaves to yellow and drop early.
Remove and destroy infected leaves. Space plants for airflow and avoid overhead watering. Apply a sulfur or copper fungicide if infection is widespread, and 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.
Vernonia Leaf Spot
Small brown or purple-bordered spots on leaves that may enlarge and merge, causing yellowing and early leaf drop in humid conditions.
Improve air circulation by thinning stems and spacing plants. Avoid overhead watering. Remove and discard infected leaves; clear debris at season's end.
Aphids
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.