Foothills, montane. Woodlands, meadows, ridges, washes. Summer, fall.
Above and left: Perimeter Trail, Ouray, October 8, 2017 and 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.  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.)

Ageratina herbacea might be mistaken for a Brickellia, especially because of their shared preference for rocky habitats and their triangular-shaped leaves and relatively similar-looking flower heads.  Most floras show the two genera immediately next to each other in their keys.

a) Seeds have five angles and the involucral bracts are nearly equal in length or sometimes in two ranks..... Ageratina
b) Seeds have 10 angles and the involucral bracts are overlapping in different lengths.... Brickellia

In addition there are several more easily noted characteristics that separate the two genera, especially separating the two fairly common species, Ageratina herbacea and Brickellia grandiflora::
Ageratina herbacea flower heads are upright and white; Brickellia grandiflora's are almost always nodding and pale yellow-green.
Ageratina herbacea typically grows to about 14 inches tall in a wide, shrub-like manner; Brickellia grandiflora grows to about 28 inches in a more narrow and open form.
Ageratina stems are fairly brittle: Brickellia's are more supple.
Ageratina involucres are 4-5 mm long; Brickellia's are 7-12 mm long.
Ageratina has minute hairs; Brickellia has longer hairs.

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 home flower gardens.

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

Foothills, montane. Woodlands, meadows, ridges, washes. Summer, fall.
Left: Vallecito Creek Trail, September 13, 2010.
Below: Vallecito Creek Trail, September 9, 2015.

The flower heads of Ageratina herbacea present a densely packed, swirling confusion of white.  The filamentary projections are the styles which have pushed through the fused anther sacks picking up pollen. In the photograph below, the pollen gives the styles a bumpy, granular appearance. 

Eight-to-twenty disk 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.

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

Foothills, montane. Woodlands, meadows, ridges, washes. Summer, fall.
Perimeter Trail, Ouray, October 8, 2017.

Leaves are almost always opposite with petioles to 25 mm long. Leaf blades are triangular, ovate, or lance-ovate. Leaf margins are wavy-edged, varying from dentate  to serrate-dentate, and leaf surfaces range from minutely hairy to glabrous (hairless).

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