Ageratina altissima aka White Snakeroot

Ageratina altissima, or white snakeroot, belongs to the aster family. This leafy perennial comes from eastern Canada and the United States. It grows in woods, thickets, woodland edges, and rocky sites. Older plant books may call it Eupatorium rugosum. Its leaves fall away at the end of the season.

In gardens, it forms upright clumps with a bushy shape. Plants often reach 2–5 feet [0.6–1.5 metres] tall. RHS describes plants around 5.6 feet [1.7 metres], with an upper range of 8.2 feet [2.5 metres]. Clumps spread 2–4 feet [0.6–1.2 metres] across. Smooth, round stems range from pale green to tan or brown. The simple leaves grow in opposite pairs and lack hairs. Lower leaves have broad, heart-shaped bases and clear veins. Upper leaves are smaller and shaped like a lance. All have toothed edges.

Small white flowers form dense, flat-topped corymbs at stem tips. Each head has tubular disk flowers but no ray florets. Bloom spans late summer and fall, with some sources reporting earlier summer flowers. Butterflies, bees, moths, and wasps visit the blooms. The small black fruit is a cypsela, rather than a true achene. Each has ribs and a tuft of white hairs. Wind carries these fruits away, and songbirds feed on the seeds.

Grow this plant in moist soil with good drainage. It suits loam or clay with neutral to alkaline pH. It accepts full sun, though partial shade also suits it. Afternoon shade helps in the southeastern United States. It can cope with brief dry spells once growing well. Sources list USDA zones 3–8 or 3–9; RHS rates it H5. Frost may harm fresh shoots. Use it in woodland gardens, cottage borders, or naturalized ground. It spreads through rhizomes and seed, sometimes with speed. Remove spent flower heads to limit unwanted seedlings. Spring seed sowing and division offer ways to raise more plants.

Serious disease problems are uncommon, but several pests can damage leaves. These include leaf miners, flea beetles, aphids, and slugs. White snakeroot is poisonous and has no known food use. Eating it can cause grave illness or death in people and animals. Its toxins can also reach people through milk from affected cattle. Roots have a history of folk use for several ailments. That history does not make this toxic plant safe to take.