Ageratina

Spach

Hist. Nat. Vég. 10: 286. 1841

Common names: Snakeroot
Etymology: Generic name Ageratum and Latin - ina, diminutive
Treatment appears in FNA Volume 21. Treatment on page 547. Mentioned on page 461, 462, 541, 552.
Perennials, subshrubs, or shrubs [trees], mostly 20–220 cm. Stems usually erect, rarely scandent, sparsely to densely branched. Leaves cauline; mostly opposite (distal sometimes alternate); petiolate; blades 3(–5)-nerved from bases, usually deltate, lanceolate, ovate, rhombic, or triangular, sometimes orbiculate, margins entire, crenate, dentate, or serrate, faces glabrous or hispidulous, pilose, or puberulent, sometimes gland-dotted (A. occidentalis, A. adenophora). Heads discoid, usually in compact, (terminal and axillary) corymbiform arrays, sometimes borne singly. Involucres campanulate, 3–6 mm diam. Phyllaries persistent, 8–30 in 2(–3) series, 0- or 2-nerved, lanceolate to linear, ± equal (herbaceous). Florets 10–60; corollas white or lavender, throats obconic to campanulate (lengths 1.5–2 times diams.); styles: bases sometimes enlarged, glabrous, branches linear, seldom distally dilated. Receptacles convex (glabrous or hairy), epaleate. Cypselae prismatic or ± fusiform, usually 5-ribbed, scabrellous and/or gland-dotted; pappi usually persistent, sometimes fragile, rarely falling, of 5–40, barbellulate bristles in 1 series. x = 17.

Distribution

North America, Mexico, Central America, Andean South America.

Discussion

Species ca. 250 (14 in the flora).

Key

1 Shrubs > 2
1 Perennials or subshrubs > 3
2 Petioles (2–)3–5 mm; leaf blades 1–2 cm, margins entire or shallowly crenate, facesgland-dotted Ageratina wrightii
2 Petioles 3–10(–15) mm; leaf blades (2–)3–5(–7) cm, margins coarsely crenate, facesnot gland-dotted Ageratina havanensis
3 Leaves alternate on at least distal 1/4–1/2 of stems > 4
3 Leaves opposite > 6
4 Heads usually borne singly (rarely 2s or 3s); involucres 11–12 mm Ageratina shastensis
4 Heads usually 5–10 (axillary clusters usually forming elongate or broad aggregates); involucres 2.5–3.5(–4) mm > 5
5 Leaves alternate on distal 1/4–1/2 of stems; involucres 3–3.5(–4) mm; corollas pink, bluish, or white tinged with purple (not orange-veined); cypselae sessile-glandular Ageratina occidentalis
5 Leaves alternate (from bases to apices of stems); involucres 2.5–3 mm; corollaswhite (prominently orange-veined); cypselae eglandular Ageratina thyrsiflora
6 Peduncles densely stipitate-glandular Ageratina adenophora
6 Peduncles puberulent, glabrous, or glabrescent (not glandular) > 7
7 Flowering in spring; leaves usually narrowly lanceolate, 0.5–1.5 cm wide,apically long-acuminate; involucres 2.5–3 mm; cypselae glabrous Ageratina paupercula
7 Flowering in fall; leaves ovate to lanceolate-ovate, triangular, or lanceolate, (0.5–)1.5–9 cm wide, apically acute to acuminate; involucres (in A. jucunda 2.5–)3.5–7 mm; cypselae usually hairy (glabrous in A. altissima) > 8
8 Leaves mostly sessile; heads in open, loose arrays, peduncles 10–60 mm Ageratina lemmonii
8 Leaves distinctly petiolate; heads in compact clusters, ultimate peduncles 1–15(–20) mm > 9
9 Petioles 1–22 mm (distal leaves greatly reduced in size well proximal to heads); leaf blades 2–7(–9) × 1.5–4 cm (relatively thick) > 10
9 Petioles (5–)10–70 or 2–20 (in A. rothrockii) mm; leaf blades 2–11(–13) × 1.5–9 cm (relatively thin) > 11
10 Petioles 1–8(–12) mm; leaf margins crenate or less commonlycrenate-serrate to dentate or subentire Ageratina aromatica
10 Petioles 7–15(–22) mm; leaf margins coarsely serrate or incised orless commonly crenate to subentire Ageratina jucunda
11 Leaves (yellow-green or grayish yellow-green): blades triangular to lanceolate-ovate or ovate, 2–5(–7) × 1.5–3.5(–4.5) cm; phyllariesusually granular-puberulent Ageratina herbacea
11 Leaves (green, rarely yellowish): blades lanceolate to lanceolate-ovate, 4–11(–13) × 2.5–9 cm; phyllaries glabrous, villous, or villous-puberulent > 12
12 Stems glabrous; leaf blades broadly deltate-ovate (thin, delicate), apices obtuse; corolla lobes glabrous or sparselypuberulent Ageratina luciae-brauniae
12 Stems puberulent; leaf blades deltate-ovate to ovate or broadly lanceolate, apices acute to acuminate; corolla lobes short-villous > 13
13 Peduncles 1–5 mm; involucres 4–5 mm; cypselae glabrous;e United States and Canada Ageratina altissima
13 Peduncles 5–12(–20) mm; involucres 5–7 mm; cypselae sparsely and finely strigose-hirsute; Arizona, New Mexico, sw Texas Ageratina rothrockii