1. Tripleurospermum inodorum (L.) Sch. Bip. (scentless mayweed, scentless false chamomile)

Matricaria inodora L.

M. inodora var. agrestis (Weiss) Wilmott

M. maritima L. var. inodora (L.) Soó

T. maritimum ssp. inodorum (L.) Applequist

M. perforata Mérat

Pl. 227 d, e; Map 958

Plants annual, with taproots. Stems 8–40(–70) cm long, erect or ascending, usually branched, finely ridged, glabrous or very sparsely hairy. Leaves alternate and basal (basal leaves usually withered by flowering time), sessile or short-petiolate with winged petioles and somewhat broadened, more or less clasping bases. Leaf blades 1.5–6.0(–10.0) cm long, elliptic to oblong-obovate, deeply 1–3 times pinnately lobed, glabrous, the ultimate segments 3–20 mm long, mostly linear to threadlike, sharply pointed at the tip, mostly 1-veined. Inflorescences of solitary heads or more commonly loose clusters at the branch tips, the stalks glabrous or sparsely hairy, bractless. Heads radiate. Involucre 3–4 mm long, cup-shaped to hemispheric, the bracts more or less in 3 or 4 loosely overlapping series, subequal, elliptic-lanceolate to narrowly oblong or narrowly ovate-triangular, mostly bluntly pointed at the tip, glabrous, tan to brown, often with a narrow, green midvein, the midrib not keeled, the margins thin and papery. Receptacle convex to hemispheric, tending to elongate but remain relatively rounded as the fruits mature, solid, naked. Ray florets 12–25, pistillate, the corolla 6–12(–16) mm long, white, rarely pinkish-tinged. Disc florets perfect, numerous, the corolla 1–2 mm long, yellow or sometimes greenish yellow, minutely glandular, the 5 lobes with resin canals (appearing as minute, brown lines or streaks), persistent, the tube often somewhat flattened toward the tip, becoming somewhat swollen at fruiting. Pappus a short collar or crown. Fruits 1.5–2.0 mm long, nearly rectangular to slightly wedge-shaped in profile, unevenly triangular in cross-section, slightly flattened, truncate at the base, the tip often slightly obliquely truncate, strongly 3-ribbed, the ribs on the margins and 1 of the faces almost winglike, the surface otherwise finely wrinkled or finely tuberculate, glabrous, brown to dark brown with lighter ribs. 2n=18, 36. May–October.

Introduced, known thus far only from the city of St. Louis (native of Europe; introduced widely in the U.S. [except some southern states], Canada). Railroads and open, disturbed areas.

Some authors prefer to treat this species as a variety or subspecies of the closely related T. maritimum (Applequist, 2002), which differs in its biennial to perennial habit, more numerous and more spreading stems, leaves with shorter, thicker lobes, and fruits with thicker ribs. True T. maritimum occurs natively mostly in coastal regions in northern North America, Europe, and Asia. Steyermark (1963) used the name Matricaria maritima var. agrestis (Knaf) Wilmott for this taxon, but this appears to represent an unpublished combination based on the illegitimate earlier name Dibothrospermum agreste Knaf (Applequist, 2002).