64. Ambrosia L. (ragweed)

(Payne, 1964)

Plants annual or perennial herbs (shrubs elsewhere), sometimes with taproots, rhizomes, or a branched, woody rootstock. Stems few- to many-branched, mostly erect or ascending, finely ridged or grooved, sparsely to densely pubescent with often pustular-based hairs. Leaves alternate and/or opposite, variously sessile to long-petiolate. Leaf blades variously shaped, simple and entire or more commonly 1–3 times pinnately or palmately lobed, variously sparsely to densely hairy. Inflorescences of separate staminate and pistillate heads, the staminate heads in spikes or spikelike racemes terminal on the branch tips, these sometimes appearing paniculate, the pistillate heads sessile, solitary or in clusters toward the base of the staminate spikes or in the adjacent upper leaf axils. Heads discoid (but this not evident in pistillate heads), the staminate heads pendant. Involucre of the staminate heads cup-shaped to saucer-shaped, often somewhat asymmetrical, the 5–12 involucral bracts in 1 series, fused irregularly well above the base, green. Involucre of the pistillate heads with the main body globose to ovoid or somewhat pear-shaped, the involucral bracts closely enclosing the florets and fused into a bur, the outer surface with straight spines or tubercles, more or less beaked at the tip (where an opening allows exsertion of the stigmas), green or sometimes purplish-tinged. Receptacle flat (difficult to observe in pistillate heads), not elongating as the fruits mature, that of the staminate heads with chaffy bracts subtending the florets, these narrowly linear to narrowly lanceolate, usually hairy and sometimes also glandular, not wrapped around the florets. Staminate heads with 10–150 disc florets, these with a minute, nonfunctional ovary and undivided style, the stamens with the filaments more or less fused into a tube and the anthers free but positioned closely adjacent to one another in a ring, the corolla 2–4 mm long, narrowly bell-shaped, white to pale yellow, sometimes purplish-tinged toward the tip, usually minutely hairy and often also glandular. Pistillate heads with 1 or 2 florets, the corolla absent. Pappus of the staminate and pistillate florets absent. Fruits 3.0–4.5 mm long, more or less globose to ovoid, not flattened (or less commonly somewhat flattened), not angled, grayish tan to nearly black, glabrous, completely enclosed in the persistent pistillate involucre and dispersed as an intact bur. About 46 species, nearly worldwide, but most diverse in North America.

Ambrosia and its relatives form a specialized group within the Heliantheae that at one time was treated as a separate tribe, Ambrosieae Cass., by some botanists. Payne (1964) justified the inclusion of species formerly separated as Franseria Cav. (Steyermark, 1963) within Ambrosia. More recent molecular studies (Miao et al. 1995c) generally supported this conclusion and resulted in the additional lumping of the four southwestern species of the genus Hymenoclea Torr. & A. Gray ex A. Gray into Ambrosia. The ragweeds are wind-pollinated, and especially the species that form large stands in disturbed areas are a leading cause of hay fever during the late summer.