Annuals, biennials, perennials, subshrubs, or shrubs [treelets], 1–120[–400] cm. Stems ± erect, usually branched. Leaves usually cauline, sometimes in rosettes; alternate; petiolate or sessile; blades elliptic, lanceolate, linear, lyrate, oblanceolate, obovate, ovate, rounded-deltate, spatulate, sometimes (1–)2-pinnately lobed, ultimate margins entire or toothed, faces usually hairy and gland-dotted (at least the abaxial). Heads usually radiate, sometimes ± disciform (P. alpinum), usually borne in corymbiform or paniculiform arrays (in glomerules in P. argentatum, borne singly in P. alpinum and P. ligulatum). Involucres ± hemispheric [rotate], 3–8(–12+) mm diam. Phyllaries falling, 10(–16) in 2 series, usually distinct (partially connate in P. alpinum), outer 5(–8) herbaceous to scarious, inner 5–8 ± scarious to membranous. Receptacles flat to conic; paleae cuneate to flabelliform, scarious or membranous, distally papillate and/or fimbrillate, all or the peripheral each ± enfolding a disc floret. Ray (pistillate) florets 5(–8), fertile; corollas ochroleucous, tubes stout, glandular, laminae oblong to reniform or orbiculate, or ± coroniform (none in P. alpinum). Disc florets 12–60+, functionally staminate; corollas ochroleucous, funnelform, lobes 5 (filaments distinct, anthers connate). Cypselae (black) oblanceoloid, obovoid, or pyriform, often ± obcompressed (shed together with subtending phyllary and 2 contiguous disc florets and their investing paleae); pappi 0 (shoulders of cypselae may bear 1–3 pappus-like, triangular to ovate, or ± subulate enations). x = 9.

Distribution

New World, mostly warm-temperate and tropical, some temperate, introduced in Old World.

Discussion

Species ca. 16 (7 in the flora).

Selected References

Mears, J. A. 1975. The taxonomy of Parthenium section Partheniastrum DC. (Asteraceae–Ambrosiinae). Phytologia 31: 463–482.

Rollins, R. C. 1950. The guayule rubber plant and its relatives. Contr. Gray Herb. 172: 1–73.

Lower Taxa

Parthenium alpinum, Parthenium argentatum, Parthenium confertum, Parthenium hysterophorus, Parthenium incanum, Parthenium integrifolium, Parthenium ligulatum

Key

1	Plants 1–2 cm (plants cespitose or forming mats); heads borne singly	> 2
1	Plants (5–)15–120 cm (plants not cespitose or forming mats); heads in glomerules, or in simple or compound, corymbiform or paniculiform arrays	> 3
2	Corollas of pistillate florets bearing laminae 1–2 mm	Parthenium ligulatum
2	Corollas of pistillate florets lacking laminae	Parthenium alpinum
3	Shrubs	> 4
3	Annuals, biennials, or perennials	> 5
4	Leaf blades oval-elliptic to obovate (sometimes pinnately 3–7-lobed or round-toothed, ultimate margins entire)	Parthenium incanum
4	Leaf blades lanceolate to oblanceolate (margins mostly entire, some with 1–5 sharp teeth)	Parthenium argentatum
5	Perennials; leaf blades usually crenate to serrate, sometimes coarsely toothed or somewhat lobed (then mostly toward bases); pappus-like enations from shoulders of cypselae 0 or 2(–4), erect to spreading, ± subulate or threadlike	Parthenium integrifolium
5	Annuals or biennials (sometimes persisting); leaf blades mostly 1–2-pinnately lobed; pappus-like enations from shoulders of cypselae 2, erect, ± deltate to ovate.	> 6
6	Biennials (sometimes flowering first year or persisting); leaf blades ± pinnately lobed, abaxial faces strigillose (usually with erect hairs 1–2 mm as well)	Parthenium confertum
6	Annuals (rarely persisting); leaf blades (1–)2-pinnately lobed, abaxial faces scabrellous (seldom with erect hairs 1–2 mm)	Parthenium hysterophorus