Eupatorium capillifolium aka Dog Fennel

Eupatorium capillifolium is also called dog fennel. It belongs to the daisy family. It is native to the central and eastern United States. Its native range also includes Bermuda and the Bahamas. Garden sources call it a herbaceous perennial: a plant with soft growth that lives for years. Kew's data instead lists it as an annual. It grows at woodland edges, along roads, and in fields. It also grows on ground that people have disturbed. It can be a problem weed in the southeastern United States.

Garden plants reach about 3–8 feet [0.9–2.5 metres] tall. They spread 2–5 feet [0.6–1.5 metres] wide. Shoots in Florida can exceed 8 feet [2.4 metres]. Stiff stems stand upright, with woody bases. They may look reddish, and fine hairs cover them. Bright green leaves split into thin, threadlike parts. These leaves give the plant a soft, feathery look. Wind stirs the leaves and adds motion to a border. Crushed leaves have a strong smell. Broken stems may smell bad.

Small flowers appear from September to November. Sources call their color greenish-white or pale pinkish-white. Many small flower heads form broad groups atop the stems. Frost may stop the blooms before their buds open. The flowers give way to smooth fruits called achenes. Hairs help these fruits travel on the wind. Stems with fruit add interest in fall and winter. Bees and wasps visit, and the plant draws butterflies. Its leaves also feed the larvae of two moth species. Male scarlet-bodied wasp moths take toxins from the plant. These toxins help keep their predators away.

Grow dog fennel in sun or part shade. Shade in the afternoon helps where summers are hot. It prefers moist soil that drains well. It accepts clay, loam, sand, and a wide pH range. Once settled, plants cope with drought and poor soil. Missouri Botanical Garden lists USDA zones 6–9. NC State gives a wider range of zones 3–10. Its text calls for winter mulch and shelter north of zone 6. The RHS rates it H3 for UK conditions. Plant it in borders, cottage gardens, or at woodland edges. Cut stems back when the growing season ends.

Seeds and spreading underground stems can form dense groups. Broken stems may regrow if removal leaves parts behind. This spread can make garden care hard work. Deer and rabbits tend to leave it alone. Serious pests and diseases rarely cause problems. Powdery mildew, aphids, slugs, and leaf miners may occur. Do not eat this plant. NC State reports alkaloids that can harm the liver. Florida research also reports leaf toxins that harm cattle. Sources note past uses on the skin for bites. They also record past uses to control insects. These records do not show safe treatment or food uses.