Annuals (sometimes biennials or perennials), (5-)30-60(-80) cm. Stems 1, ascending to erect, usually branched distally, sometimes proximally, glabrous or glabrate (sparsely hairy when young). Leaf blades 2-8 cm, ultimate lobes filiform, 4-20 mm, not fleshy, apices apiculate. Heads (1-)10-200+, 3-4.5 cm diam., in corymbiform arrays of solitary heads at ends of branches. Phyllaries centrally dark greenish or brownish, oblong, subequal, scarious margins colorless to light brown, 0.1-0.2 mm wide. Ray florets 10-25; corollas (4-)10-13(-20) mm. Disc corollas 1-2.5 mm. Cypselae pale brown, ribs separated by 1/3+ their widths, abaxial-apical resin glands ± circular, faces minutely roughened between ribs. 2n = 18, 36. Flowering May-Sep. Fields, dry shorelines, waste places; 0-1500+ m; introduced; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Calif., Colo., Conn., Idaho, Ill., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Y., N.Dak., Ohio, Oreg., Pa., S.Dak., Utah, Wash., Wis., Wyo.; also introduced in Europe; Pacific Islands (New Zealand). Tripleurospermum inodorum has been classified as a noxious weed (class C) in the state of Washington and is considered invasive in other states (it is resistant to some herbicides); it is a weed of cereals in western Canada. W. L. Applequist (2002) has shown that the name Matricaria inodora is not a superfluous new name for M. chamomilla as earlier stated by S. Rauschert (1974). Therefore, the appropriate name under Tripleuro-spermum is T. inodorum. She also considered its type to belong in T. maritimum and formally recognized it there as subsp. inodorum, on the basis of hybridization with other T. maritimum subspecies (A. Vaarama 1953); on the same basis, however, Hämet-Ahti maintained the species distinction between T. inodorum and T. maritimum, while making T. phaeocephalum a subspecies of the latter. Q. O. N. Kay (1994), in a more extensive review of the literature and of hybridization data, also maintained T. inodorum and T. maritimum as distinct species, a conclusion followed here. From the standpoint of weed science, taxonomic merging of T. inodorum and T. maritimum has the inconvenience of grouping under a single specific name taxa that have different physiologies, ecologies, weed potentials, and, possibly, reactions to weed control measures.
The name Matricaria inodora var. agrestis Weiss was not validly published.
Short-lived, often annual, nearly inodorous herb 1-7 dm, glabrous or nearly so; lvs 2-8 cm, bipinnatifid, the ultimate segments mostly elongate, linear or linear-filiform, sometimes shorter and broader; heads several or numerous, the disk 8-15 mm wide; outer invol bracts narrow and scarcely or barely scarious-margined; rays 12-25, 6-13 mm; disk-cors 5-toothed; receptacle hemispheric, rounded; achenes as described in the key; 2n=18, 36. Native of Europe, established along roadsides and in waste places in our range. July-Sept. (M. perforata; M. inodora; Chamomilla i.; C. maritima)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.