1. Cyclachaena xanthiifolia (Nutt.) Fresen. (big marsh elder, carelessweed)

Iva xanthiifolia Nutt.

Pl. 284 a, b; Map 1169

Plants annual, with taproots. Stems 25–150(–200) cm long, unbranched or more commonly several-branched, erect or ascending, finely ridged or grooved, sparsely to moderately pubescent with fine, curved or curled, slender hairs, often nearly glabrous toward the base, sometimes also with scattered, longer, stiff, spreading, pustular-based hairs toward the tip. Leaves opposite or the uppermost few alternate, mostly long-petiolate (the uppermost leaves sometimes short-petiolate). Leaf blades 3–25 cm long, simple or shallowly 3-lobed (usually with 3 main veins), lanceolate to more commonly ovate or broadly ovate, angled or short-tapered at the base, mostly tapered at the sharply pointed tip, the margins irregularly and often coarsely toothed and minutely hairy, the upper surface nearly glabrous to sparsely or moderately roughened with short, more or less appressed hairs, sometimes only along the main veins, the undersurface moderately to densely pubescent with appressed hairs, often appearing pale or whitened, both surfaces also with minute, sessile, yellow glands. Inflorescences panicles at the branch tips and from the upper leaf axils, the heads sessile or very short-stalked, solitary or in small clusters of 2–4 at the nodes, the inflorescence branches mostly with short, leaflike bracts, but the individual heads or clusters not subtended by bracts. Heads discoid, spreading in several directions. Involucre 1.5–3.0 mm long, cup-shaped to broadly cup-shaped, not or only slightly asymmetrical, the 5 involucral bracts in 1 series, free to the base and overlapping, green, sparsely pubescent with somewhat tangled hairs and also sessile glands. Receptacle flat, not elongating as the fruits mature, with chaffy bracts subtending the florets, those subtending the pistillate florets obovate, hairy along the margins (also glandular) and wrapped around the florets, those subtending the staminate florets linear to narrowly oblanceolate, glandular, not wrapped around the florets. Central staminate florets 8–20, 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 2.0–2.5 mm long, funnel-shaped to narrowly bell-shaped, 5-lobed, white to pale yellow, sometimes purplish-tinged toward the tip, glandular. Marginal pistillate florets usually 5, the corolla absent (a persistent, minute, raised collar sometimes present at the tip of the ovary and fruit). Pappus of the staminate and pistillate florets absent. Fruits 2–3 mm long, obovoid, sometimes somewhat flattened, not angled or very bluntly angled on 1 face, otherwise appearing minutely pebbled, dark brown to nearly black, glabrous. 2n=36. June–October.

Uncommon, known only from historical collections from northwestern Missouri; possibly both native and introduced in Jackson County, and introduced in St. Louis City and County (Wisconsin to Washington south to North Dakota and Texas; Canada; introduced eastward to the East Coast and in Europe). Floodplains of rivers and margins of lakes; also gardens, railroads, and open, disturbed areas.

This species is an important cause of hay fever in regions where it is abundant, but it is too uncommon in Missouri to present a severe problem. Pruski (2005) has argued that the correct author citation for this taxon should be C. xanthiifolia Fresen. Additionally, many authors have followed older misspellings of the name as C. xanthifolia, but compound epithets intended to convey the meaning “leaves like Xanthium” should have ii as the combining vowels.