- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Biennial or perennial herb.
- Height: 10–60 cm (4–25 in.). Many-branched, stem erect–ascending, glabrous, often reddish at base.
- Flower: Single flower-like, usually 4–6 cm (1.6–2.4 in.) capitula, surrounded by involucral bracts. Capitula’s ray-florets white, tongue-like; disc florets yellow, tubular, small. Stamens 5. Gynoecium composed of 2 fused carpels. Involucral bracts different or same lengths, 1.5–5 mm (0.06–0.2 in.) broad, brown–black-edged. Disc stacked, full. Capitula several–20 borne in a corymbose cluster.
- Leaves: Alternate, basal leaves short-stalked, stem leaves almost stalkless. Blade 2–3 times pinnately lobed, lobes thread-like, narrow, short–long, round–with tapered tips, sometimes fleshy.
- Fruit: Flattish, ridged, dark brown achene, of which 2 long, reddish–yellow oil spots, tip sometimes with small, membranous ring.
- Habitat: Sea-shores, rocky shores, fissures in coastal rocks, seaweed piles, fields, roadsides, ditches, wasteland, culturally-influenced land.
- Flowering time: June–September.
Sea mayweed and scentless mayweed (T. inodorum) are a difficult pair for botanists – they were formerly regarded as separate species, but more recent research views them as the same species or as subspecies. The type species sea mayweed is a biennial or perennial while scentless mayweed is a biennial at most. Other differences between the two are sea mayweed’s straggly branches and the way its basal leaves lie partly on the ground. Sea mayweed’s involucral bracts are 1.5–5 mm (0.06–0.2 in.) broad with dark margins while scentless mayweed’s are 1–1.5 mm (0.04–0.06 in.) broad with light-coloured margins. The surest difference is quite difficult to detect: the oil-spots on the achenes are almost round on scentless mayweed, while on sea mayweed they are elliptic. The achenes are quite large, however, so such details are quite visible to the naked eye or with a small magnifying device.
Of sea mayweed’s many subspecies, ssp. maritimum grows on rocky outcrops by the sea, on seaweed piles, on bird rocks and rocks as far as around Vaasa. It loves a bit of nitrogen in the soil and the best bird rocks can be identified by its abundance. Ssp. maritimum is not picky or choosy, however, and is often among the first flowers to colonise cracks in bird rocks and rocks that are regularly submerged beneath the waves. It shuns human company but might grow on rocky coastal ballast. Sea mayweed can be differentiated from other subspecies by its involucral bracts which are different lengths, 1.5–2 mm (0.06–0.08 in.) broad with narrow brown margins, and its leaf lobes are narrow, roundish and quite fleshy.
Despite its name, sea mayweed is not exclusively a coastal plant: the subspecies ssp. subpolare grows inland in damp fields and ditch banks. Its northern and eastern habitat partly overlaps with scentless mayweed and the two can be easily confused. Compared to other sea mayweed subspecies ssp. subpolare has involucral bracts which are different lengths, 2–3 mm (0.08–0.12 in.) broad with wide brown margins, and its leaf lobes are quire long and tapered.
The northern subspecies ssp. phaeocephalum grows on waste ground in Norwegian Finnmark and Inari Lapland. It is originally from the coast of the Arctic Ocean and is quite a recent arrival in Finland. Its involucral bracts are the same length, 3–5 mm (0.12–0.2 in.) broad with very wide dark brown–black edges. The leaf lobes are longish, tapered and quite thick. Identifying the subspecies is not made any easier by hybrids, which can be the most common type in some places.