Eupatorium altissimum aka Tall Thoroughwort

Eupatorium altissimum is a perennial in the daisy family. Its common names include tall boneset and tall thoroughwort. It grows wild across eastern and central North America, including Ontario. Its range extends south to Texas and Florida. It grows in prairies, fields, thickets, and open woods. Some wild sites have limestone soils. It is not native to Britain or Ireland.

Plants grow 2–6 feet [0.61–1.83 metres] tall. Chicago Botanic Garden gives a narrower range of 4–5 feet [1.22–1.52 metres]. They spread 2–3 feet [0.61–0.91 metres] wide. Each upright stem branches near its top. White hairs cover the green stems. Branching rhizomes help the plant spread below ground. Paired green leaves have little or no stalk. Their narrow shape tapers toward both ends. Three clear veins run along each leaf. Small teeth line the upper part of each blade. Fine hairs cover the undersides, and smaller leaves may fill the axils.

Flowers open from late summer into fall, with records spanning August through October. Each broad, flat cluster holds many small, star-shaped flowers. The flowers range from white to cream. They have five lobes but lack ray flowers. Brown central columns bear stamens with white tips. Bees, wasps, flies, beetles, and small butterflies visit the blooms. The plant also hosts larvae of the three-lined flower moth. Small, dry fruits called achenes follow the flowers. White to tan hair tufts help wind carry the seeds. Dead, hollow stems give native bees places to nest.

Grow tall boneset in sun or light shade. Afternoon shade suits places with hot summers. It grows in clay, loam, or sand with good drainage. It accepts dry to moist soil and tolerates drought. NC State lists neutral soil as suitable. Plants grow at a medium rate. Missouri Botanical Garden and NC State list USDA zones 4–8. Chicago Botanic Garden gives the wider range of 3–9. Gardeners can raise it from seed or divide established plants.

Its height suits the back of mixed beds and borders. It also fits meadows, native gardens, and open woodland plantings. Flowers support pollinator gardens, while deer resistance adds landscape value. It can become weedy through seeds and rhizomes. Removing spent flowers can limit unwanted seedlings. Serious pest and disease problems are not usual. Powdery mildew, aphids, and leaf miners may still occur. NC State records past medicinal use but gives no details. The supplied sources do not establish edible parts or toxicity.