33. Vernonia Schreber, Gen. Pl. 2: 541. 1791.
Ironweed [For William Vernon, d. 1711, English botanist]
John L. Strother
Perennials, 2–20(–30+) dm (rhizomatous or not). Leaves usually mostly cauline (rarely mostly basal or basal and cauline); sessile or petiolate; blades ovate, elliptic, lanceolate, oblanceolate, spatulate, linear, or filiform, bases usually ± cuneate (rounded-truncate in V. pulchella), margins usually toothed (rarely entire), apices acute to attenuate, abaxial faces usually ± scabrellous to strigillose or tomentose to pannose, sometimes glabrate or glabrous, usually resin-gland-dotted (sometimes ± pitted), adaxial faces ± scabrellous or glabrate, sometimes resin-gland-dotted (rarely pitted). Heads discoid, ± pedunculate, not subtended by foliaceous bracts, (6–)40–100+ in ± corymbiform to paniculiform arrays (6–)10–25+ cm diam. Involucres ± campanulate to obconic or hemispheric, 3–8(–11+) mm diam. Phyllaries 18–70+ in 4–7+ series, the outer ovate to lanceolate or subulate, inner ± lanceolate to oblong, all ± chartaceous, margins entire, often ciliolate, tips rounded (then sometimes apiculate), or acuminate, subulate, or filiform, faces glabrous or sparsely strigillose to tomentose, sometimes ± gland-dotted. Florets 9–30(–65+); corollas usually purplish or pink (rarely white), tubes longer than funnelform throats, lobes 5, lance-linear, ± equal. Cypselae ± columnar, sometimes arcuate, 8–10-ribbed, glabrous or ± strigillose to hirtellous, often resin-gland-dotted; pappi persistent, of 20–30+ outer, erose to subulate scales or bristles plus 20–40+ inner, longer, subulate to setiform scales or bristles. x = 17.
Species 20 or so (17 in the flora): mainly c, e North America, n Mexico, 2–3 species in South America.
The circumscription of Vernonia adopted here follows that of H. Robinson (1999).
Vernonias hybridize; almost every one of the species recognized here has been noted as sometimes hybridizing with one or more others. Putative hybrid plants are usually intermediate between parentals in some traits; such plants may not "key" satisfactorily to any of the species treated here. Some putative hybrids have been named. Vernonia guadalupensis is "without much doubt a hybrid of V. baldwinii Torrey and V. lindheimeri Engelmann & Gray" (L. H. Shinners 1950); V. vulturina Shinners (known only from the type collection) may be a product of V. baldwinii × V. marginata; V. ×georgiana Bartlett may refer to V. acaulis × V. angustifolia. Additional putative hybrids (S. B. Jones 1964) are V. ×concinna Gleason (V. ovalifolia × V. angustifolia), V. ×dissimilis Gleason (V. altissima × V. angustifolia), and V. ×recurva Gleason (V. pulchella × V. angustifolia).
In the key and descriptions, "l/w = " refers to lengths divided by widths for blades of leaves; lengths of phyllaries include subulate to filiform tips (if any).
Gleason, H. A. 1922b. Vernonia. In: N. L. Britton et al., eds. 1905+. North American Flora…. 47+ vols. New York. Vol. 33, pp. 52–95. Jones, S. B. 1964. Taxonomy of the narrow-leaved Vernonia of the southeastern United States. Rhodora 66: 382–401. Jones, S. B. and W. Z. Faust. 1978. Vernonia. In: N. L. Britton et al., eds. 1905+. North American Flora…. 47+ volumes. New York. Ser. 2, part 10, pp. 180–195. Shinners, L. H. 1950. Notes on Texas Compositae. IV. Field & Lab. 18: 25–32.