Ageratum houstonianum
Ageratum
bluemink, floss flower, Houston's whiteweed
whiteweed
erect to decumbent, sparsely to densely pilose.
often decumbent (rooting at proximal nodes), sparsely to densely branched.
blades deltate to ovate, mostly 3–8 × 2.5–4 cm, margins toothed, abaxial faces sparsely to densely pilose, not evidently gland-dotted.
cauline; all or mostly opposite; petiolate;
blades mostly 1-nerved, deltate to ovate, or elliptic to lanceolate, margins entire or toothed, faces glabrous or ± pilose, puberulent, or strigoso-hispid, sometimes gland-dotted.
viscid-puberulent, pilose, and stipitate-glandular.
ca. 4 × 5–6 mm.
campanulate, 3–6 mm.
conic, epaleate [paleate].
20–125;
corollas white or bluish to lavender, throats ± campanulate (lengths 2 times diams.);
styles: bases not enlarged, glabrous, branches ± linear to clavate (usually papillose and dilated distally).
usually lavender, rarely white.
narrowly lanceolate (0.6–1 mm wide), stipitate-glandular, sparsely to densely pilose, eciliate or inconspicuously ciliate, tips gradually tapering, indurate-subulate, 0.8–2 mm.
persistent, 30–40 in 2–3 series, usually 2-nerved, lanceolate, ± equal (often indurate, margins scarious).
discoid, in dense to open, cymiform to corymbiform arrays.
sparsely strigoso-hispidulous;
pappi of 5 distinct, oblong scales 2–3 mm.
prismatic, 4–5-ribbed, glabrous or sparsely strigoso-hispidulous;
pappi persistent, of 5–6 aristate scales, or coroniform, or 0.
= 10.
= 20.
Ageratum houstonianum
Ageratum
Ageratum houstonianum is apparently native to southeastern Mexico and Central America; the North American plants are escapes and naturalized from cultivars. M. F. Johnson (1971) observed that forma isochroum (B. L. Robinson) M. F. Johnson (type from the state of Veracruz, Mexico) sometimes may be nearly eglandular.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Species ca. 40 (4 in the flora).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Plants colonial; stems and leaves glabrous or glabrate | A. maritimum |
1. Plants not colonial; stems and leaves hairy | → 2 |
2. Stems puberulent to minutely strigoso-hispid; cypselae glabrous | A. corymbosum |
2. Stems sparsely to densely pilose (usually in combination with other forms of vestiture); cypselae sparsely strigoso-hispidulous. | → 3 |
3. Peduncles minutely puberulent and sparsely to densely pilose, eglandular; phyllaries oblong-lanceolate, abruptly tapering to subulate tips 0.5–1 mm, glabrous or sparsely pilose, margins often ciliate, abaxial faces eglandular | A. conyzoides |
3. Peduncles mixed pilose, stipitate-glandular, and viscid-puberulent; phyllaries narrowly lanceolate, gradually tapering to indurate-subulate tips 0.8–2 mm, margins not ciliate or inconspicuously ciliate, abaxial faces stipitate-glandular and sparsely to densely pilose | A. houstonianum |
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