Eupatorium perfoliatum aka Chapman's Thoroughwort

Eupatorium perfoliatum is known as boneset or thoroughwort. It belongs to the aster family, Asteraceae. This perennial grows in eastern and central North America. Its native range includes parts of Canada and the United States. Wild plants grow in marshes, bogs, fens, and damp woods. They also grow beside rivers and in wet ditches.

Boneset forms upright clumps and spreads through rhizomes into colonies. Garden height varies among sources. RHS gives about 3–5 feet [1–1.5 metres]. NC State gives 4–6 feet [1.2–1.8 metres]. Chicago Botanic Garden reports shorter plants, at 2–3 feet [0.6–0.9 metres]. Spread reaches about 3–5 feet [1–1.5 metres]. Long white hairs cover the main stem and side stems. The narrow, pointed leaves grow in opposite pairs. Their joined bases wrap around the stem. This makes the stem seem to pass through each pair. Leaves have fine teeth, clear veins, and a rough surface. The lower leaves are larger, with hairy undersides. The plant loses its foliage for winter.

Small white flowers form broad clusters atop the stems. More clusters grow where upper leaves meet the stems. Each cluster contains many tiny, fragrant, tubular disc florets. Flowers open from July to September in most accounts. Chicago Botanic Garden also records bloom into October. Bees and butterflies visit the flowers for food. Some moths use boneset as a host plant. Small dry fruits, called achenes, appear from September to November. Hair tufts help the wind carry them away. Some birds eat the seeds.

Grow boneset in sun or partial shade with steady moisture. It accepts sand, loam, clay, and chalk soils. It also tolerates a wide range of soil pH. Brief flooding does not harm it, but dry soil can scorch leaves. Sources place it in USDA zones 3–8, with NC State extending to zone 9. RHS rates its cold hardiness H7. Use it in damp borders, rain gardens, or beside ponds. Its late flowers suit native, cottage, and wildlife gardens. The blooms also work as cut or dried flowers.

Boneset needs little routine care and has few serious problems. Aphids and slugs may attack it. Deer and rabbits tend to leave it alone. Cut stems back when the growing season ends. Propagate by seed or divide clumps in spring. Boneset has a history of use for colds and fevers. It is bitter and toxic, not a food plant. Eating it can cause vomiting and diarrhoea. Contact may cause dermatitis, and long-term use risks liver damage.