scentless mayweed

The species may grow to be 20–80 cm (7.9–31.5 in) in height. It is usually 1-stemmed, with the stem erect–ascending, branching, glabrous, green. Stems are single, erect, branched in the upper plant, weakly ridged or lined, hairless though sparsely hairy when young. Leaves are alternate, short-stalked–stalkless. The blade is 2–3 times pinnately lobed (–with leaflets), glabrous, lobes (or leaflets) long, thread-like narrow, sharp-pointed. Leaves are ¾ to 3 inches long, feathery with a few to numerous thread-like branching lobes. The cotyledons are oribicular to oblong, very small, 3 to 5 mm long, and stalkless. Flowers are single flower-like, usually with a 3–5 cm (1.2–2.0 in) capitula, surrounded by involucral bracts. The capitula's ray-florets are white, tongue-like, tip shallowly 3-toothed; disc florets are yellow, tubular, small. Stamens 5. Pistil of 2 fused carpels. Involucral bracts are of different lengths, 1–1.5 mm (0.039–0.059 in) broad, light brown–white margins. Disc is stacked, full. Capitula is 1–20 borne in a corymbose cluster. It flowers from June–October. Pollen is collected by solitary bees. The fruit is a flattish, ridged achene, with 2 round–angular oil spots, tip sometimes with small, membranous ring.

Similar species. Tripleurospermum maritimum (false mayweed) is morphologically similar to T. inodorum. …

T. inodorum grows in fields, fallow land, lawns, wasteland, roadsides, yards, gardens. It is an annual or short-lived perennial. It is native to Eurasia. 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. According to Canadian regulations, it is classified as Secondary Noxious, Class 3 and Noxious, Class 5 in the Canadian Weed Seeds Order, 2016 under the Seeds Act.

In Sweden and Norway, it is called Baldr's brow, but in Iceland, it is the close relative sea mayweed (Matricaria maritima) that carries this name. In Gylfaginning, Snorri Sturluson explains that the name Balder's brow comes from the plants' whiteness: