1. Iva annua L. (marsh elder, sump weed)

I. ciliata Willd.

I. annua var. caudata (Small) R.C. Jacks.

Pl. 284 c, d; Map 1206

Plants annual, with taproots. Stems 30–120(–200) cm long, unbranched or more commonly several-branched, erect or ascending, finely ridged or grooved, sparsely to moderately roughened with short, ascending, pustular-based hairs, often also with longer, spreading hairs toward the tip, usually glabrous or nearly so toward the base. Leaves opposite or the uppermost few alternate, variously sessile to long-petiolate. Leaf blades 2–15 cm long, simple, lanceolate to broadly ovate, angled or short-tapered at the base, tapered at the sharply pointed tip, the margins entire or more or less toothed and hairy, the surfaces nearly glabrous to sparsely or moderately roughened with short, more or less appressed hairs, often also with sparse, longer, spreading hairs along the main veins and/or near the base, usually also sparsely to moderately gland-dotted. Inflorescences of spikelike racemes terminal on the branch tips, these sometimes appearing paniculate, the heads very short-stalked, solitary at the nodes, subtended by ovate to narrowly lanceolate, leaflike bracts. Heads discoid, pendant. Involucre 2–4 mm long, more or less cup-shaped, somewhat asymmetrical, the 3–5 involucral bracts in 1 series, fused irregularly at or just above the base, green, sparsely long-hairy, especially along the margins. Receptacle flat, not elongating as the fruits mature, with chaffy bracts subtending the florets, these narrowly linear to narrowly oblanceolate, usually glandular along the margins, not wrapped around the florets. Central florets 9–16, staminate, 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, usually glabrous. Marginal florets 3–5, pistillate, the corolla 1.0–1.5 mm long, narrowly tubular, often slightly oblique at the tip (but unlobed), white to pale yellow, often persistent at fruiting. Pappus of the staminate and pistillate florets absent. Fruits 2.5–4.0 mm long, broadly obovoid to somewhat pear-shaped, somewhat flattened, not angled or at most very bluntly angled on 1 face, otherwise appearing smooth or with numerous faint, fine longitudinal lines, brown to dark brown, glabrous but with minute, sessile glands. 2n=34. July–October.

Scattered nearly throughout the state, but most commonly in the Unglaciated Plains Division and counties adjacent to the Missouri and Mississippi Rivers (Indiana to North Dakota south to Mississippi and New Mexico; Mexico; introduced eastward to Maine and Florida). Banks of streams and rivers, margins of ponds and lakes, swamps, sloughs, bottomland prairies, bottomland forests, and rarely moist depressions of upland prairies; also fallow fields, crop fields, ditches, pastures, railroads, roadsides, and open, disturbed areas.

Jackson (1960) recognized plants with the bracts of the inflorescence narrowly lanceolate, with slender, long-tapered tips (vs. ovate to lanceolate and more gradually tapered) as var. caudata. In the Midwest, these differences appear to form a more or less continuous spectrum of variation, and segregation of varieties to document the extreme cases seems ill-advised. Blake (1939) described a form of the species based on relatively large fruits found by archaeologists at rock shelter sites in Arkansas, Kentucky, and Missouri. Jackson (1960) included this taxon in his monograph of the genus as I. annua var. macrocarpa (S.F . Blake) R.C. Jacks. (I. ciliata Willd. var. macrocarpa S.F. Blake). This fossil marsh elder appears to be extinct in modern times and to have existed only as a food or medicinal plant cultivated by prehistoric Native Americans.