Ageratina altissima (L.) R.M. King & H. Rob.

White Snakeroot

© SRTurner

Family - Asteraceae/Eupatorieae

Habit - Fibrous-rooted perennial forb.

Stems - Ascending to erect, usually solitary, to 1.5 m, glabrous or short-hairy, usually branched.

	Stem and node.
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Leaves - Opposite, petiolate, simple. Blades to 18 cm, triangular or triangular-ovate, the uppermost often narrowed, sharply pointed, the margins broadly toothed, the surfaces glabrous or sparsely hairy, with 3 main veins. Main venation is adaxially impressed, abaxially expressed.

	Leaf adaxial.
© SRTurner

	Leaf abaxial.
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Inflorescence - Panicles at branch tips, flat-topped to convex.

	Inflorescence.
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Heads - Discoid, with 9-25 florets. Involucre 3-5 mm long, cup-shaped, the bracts 8-14 in usually 2 unequal, overlapping series, linear to narrowly oblong-elliptic, bluntly or sharply pointed, glabrous or finely short-hairy, sometimes only along the margins. Receptacle flat or nearly so.

	Heads.
© SRTurner

	Involucres.
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Florets - Corollas 5-lobed, white. Pappus of numerous capillary bristles.

	Florets.
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Fruits - Achenes 1.7-3.0 mm long, 5-angled, somewhat wedge-shaped in profile to nearly linear, glabrous, brown to black.

	Developing achenes.
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	Fruiting heads.
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Flowering - July - October.

Habitat - Forest, streambanks, roadsides, open disturbed areas.

Origin - Native to the U.S.

Lookalikes - Eupatorium serotinum.

Other info. - This plant is very common throughout the state, and throughout most of the eastern half of the U.S. When fresh, the flowering heads are pure white in color, and this along with the triangular leaves make this plant fairly easy to recognize. It is most easily confused with Eupatorium serotinum, which is typically a taller, less branched plant with dirtier-looking flower heads and shorter leaf petioles. In fact, Ageratina altissima was once classified as Eupatorium rugosum. The species has been subdivided into infraspecific forms, with all Missouri plants currently assignable to var. altissima.

The plant is toxic, causing a potentially fatal condition known variously as "trembles" or "milk sickness." The toxic constituents, known collectively as "tremetol," are fat soluble and pass into the milk of livestock which ingest the plant. Thousands of human deaths resulted from consumption of this tainted milk in the early 19th century, before the condition was recognized. The most famous victim, in 1818, was Nancy Hanks Lincoln, the mother of Abraham Lincoln. The toxicity of the plant was finally properly documented by Anna Pierce Hobbs Bixby, a frontier doctor who learned of the condition from an elderly Shawnee woman.

Photographs taken near Valmeyer, Monroe County, IL, 10-3-2011; near Glencoe, St. Louis County, MO, 10-31-2011; Shaw Nature Reserve, Franklin County, MO, 9-8-2012; and LaBarque Creek Conservation Area, Jefferson County, MO, 9-13-2012 (SRTurner).