Foothills, montane. Woodlands, meadows, ridges, washes. Summer, fall.
Vallecito Creek Trail, September 13, 2010.

Ageratina herbacea is a woody-based perennial that can grow to 80 centimeters tall and to over a meter wide (see the last photograph below).  Leaves are mainly opposite, notched, from two-to-seven centimeters long, and a pale yellow/green.  Flowers are white, numerous, and showy. (The yellow flowers are those of Heterotheca pumila which always is in quiet, yet spectacular bloom.)

Ageratina herbacea might be mistaken for a Brickellia, especially in its typical rocky habitat and its triangular-shaped leaves.  But a careful look, especially at the flowers, shows distinct differences.

In 1853 Asa Gray named this plant Eupatorium herbaceum from specimens gathered in Mexico by Charles Wright when he participated in the Mexican Boundary Survey of 1851-1852.  Edward Greene renamed the plant Ageratina herbacea in 1901.  "Ageratina", a genus name given by Edouard Spach in 1841, is a diminutive of "Ageratum", a genus many of us know from our garden.

Ageratina herbacea.   Synonym: Eupatorium herbaceum.   (Fragrant Snakeweed)
Asteraceae (Sunflower Family)

Foothills, montane. Woodlands, meadows, ridges, washes. Summer, fall.
Vallecito Creek Trail, September 13, 2010.

The flowers of Ageratina herbacea present a densely packed, swirling confusion of white.  The filamentary projections are the stamens which bear their pollen on the underside of the curved tips.  About eight-to-twenty flowers are crowded into each head.  There are no ray flowers and the individual disk flowers are tubular, white (often with tinges of light red just above the phyllaries), and only about 1 1/2 millimeters wide.  Phyllaries are nearly equal in length and usually in one inner and one outer row.

Foothills, montane. Woodlands, meadows, ridges, washes. Summer, fall.
Vallecito Creek Trail, September 13, 2010.

Range map © John Kartesz,
Floristic Synthesis of North America

State Color Key

Species present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Eradicated
Questionable presence

Range map for Ageratina herbacea