white snakeroot Identification (Ageratina altissima)

Check the leaves, stems, and flower clusters for a quick match.

• Leaves are opposite, broadly triangular to ovate, with a pointed tip, coarse teeth, and three main veins from the base.
• Leaf blades are often 2 to 6 inches long (5 to 15 cm) on noticeable stalks.
• Stems are upright, branching near the top, and usually smooth rather than hairy.
• Flower heads form flat-topped clusters and each small head has only white disk florets, with no ray petals.

A combination of opposite, triangular leaves and fuzzy-looking white clusters is especially useful.

Ageratina altissima shows clear structural changes from early leaf growth through seed production.

• Seedling: Seedlings show paired first leaves on a thin upright stem; early true leaves are small, triangular to ovate, with coarse teeth and long stalks.
• Young plant: Young plants form several opposite leaves and a simple or lightly branched stem, staying leafy and compact before the open upper branching seen later.
• Flowering stage: At flowering, stems branch near the top and carry broad, flat clusters of small fuzzy white flower heads; plants often stand 2-4 feet (0.6-1.2 m) tall.
• Fruiting stage: After flowering, the white heads turn into looser clusters of slim dark achenes, each topped with a small tuft that gives the cluster a rougher look.

In summer, white snakeroot is easiest to spot when flat-topped clusters of small white flower heads appear above the foliage, usually from late summer into fall. After frost, the plant dies back. Winter leaves bare, upright stems topped with persistent seed heads that can remain visible for a time.

Several late-season white wildflowers can be mistaken for white snakeroot at a glance.

Ageratina altissima is native to eastern and central North America, from southern Canada south into the eastern United States and west into the Great Plains in parts of its range. It is most often found in deciduous woods, shaded woodland edges, streambanks, thickets, and disturbed forest ground. Seeing it in partial shade in moist to fairly dry woodland settings can help separate it from similar white-flowered composites of open prairie or wet marsh habitats.

• Toxic if eaten: Ageratina altissima contains tremetol, mainly in the leaves and stems. Ingestion is poisonous to humans and pets, and it is especially dangerous to grazing animals.
• Indirect food-chain risk: When livestock eat this plant, tremetol can pass into milk or meat. That exposure has poisoned humans, a condition known as milk sickness.