Annual. Solitary herb with a single root and ascending stems to 10–20 cm. Stems 3–4 mm thick, often tinged purple, often branched at base and farther up, with numerous leaves. Stems and leaves glabrous or with very sparse, soft, white hairs.

Leaves alternate, 3–7 × 1–2.5 cm, oblong in outline, irregularly 2- to 3-pinnatisect with numerous, linear segments, green or sometimes tinged purplish. Leaf segments succulent, subacute, without or with a minute yellowish mucro.

The primary inflorescence of Asteraceae is a head (capitulum) surrounded by an involucrum of one or more rows of phyllaries (involucral bracts). The flowers sit on a flat, concave or convex receptacle, sometimes with scales (the bracts of the single flowers). The flowers are epigynous with perianth at top of the gynoecium. The sepals are always transformed into a pappus, mostly by hairs or sometimes by scales. The 5 stamens are inserted in the corolla tube and the anthers form a ring through which the style grows and pushes the pollen outwards. Gynoecium of two fused carpels, 2 stigmas. The fruit is an achene with one seed.

Inflorescence of 1–several heads on long peduncles, up to 5–10 cm. Heads 3–5 cm in diameter. Involucrum of phyllaries imbricate in several rows, up to ca. 5 x 3 mm, oblong, the outer and inner ones equally long (a diagnostic character for this subspecies), with a green centre and a 0.5–0.8 mm broad, medium brown hyaline margin. Receptacle hemisphaerical, without scales. Outer flowers monosymmetric, female, ligulate with all petals fused into a ligula with ca. 3 irregular teeth apically. Ligulas 18–25 × 5–7 mm, white. Inner flowers bisexual (hermaphrodite), wit a cylindrical, 5-lobed and yellow corolla. Pappus reduced to a small corona (i.e., no pappus hairs).

Fruit an achene. Achenes compressed laterally with broad, more or less confluent ribs, and 2 elongated resin-glands on the abaxial (ventral) face, glands 1.5–2 times as long as wide.

Sexual reproduction by seeds; no vegetative reproduction. Insect pollinated (and obligately out-crossing). It is not clear whether this plant ever has reproduced in Svalbard (see Comments).

Fruits may be dispersed mainly by man.

Tripleurospermum occurs with two species in northern regions: the weedy T. inodorum (L.) Sch.Bip. and the seashore but also weedy T. maritimum. The significant diagnostic feature (except for the chromosome number, 2n = 36 in T. inodorum, 2n = 18 in T. maritimum) is found in the achenes where T. inodorum has round resin-glands and narrow, not confluent ribs whereas T. maritimum has elongated resin-glands and broad, confluent ribs. As few Svalbard plants ever have approached the fruiting stage, these characters are not very helpful. However, there is also a small difference in the leaves: T. inodorum has non-succulent leaf segments with a long acuminate (and pale) mucro, whereas T. maritimum has broader and succulent leaf segments with at most a very short mucro. Applying this character, the plants of the Russian settlements of Barentsburg and Pyramiden belong to T. inodorum and never have reached a flower stage, whereas the others belong to T. maritimum and fairly often have reached a flower stage.

There are three acknowledged subspecies in T. maritimum. Two of these have been recorded in Svalbard: ssp. phaeocephalum and ssp. subpolare. They are distinguished by their phyllaries and fruits. Subspecies subpolare has oblong phyllaries, the outer as long as the inner, and the hyaline margin is less than 0.8 mm broad and medium brown; ssp. phaeocephalum has triangular phyllaries, the outer shorter than the inner, and the hyaline margin is more than 0.8 mm broad and blackish brown. In addition, if fruits are developed, the resin glands of ssp. subpolare are 1.5–2 times as long as broad, those of ssp. phaeocephalum more than 2 times as long as broad.

Ruderal ground in settlements.

Introduced. Tripleurospermum maritimum ssp. subpolare has been recorded from three settlements: Longyearbyen in 1928 (flowering) and 1960 (Sverdrupbyen, flowering), Nordenskiöld Land, Ny-Ålesund in 1928 (flowering) and 1958 (flowering), Oscar II Land, and Colesbukta in 1936 (Russian settlement, in bud), Nordenskiöld Land. There is no evidence that any of these plants produced any mature fruits.

Tripleurospermum maritimum ssp. subpolare is a widespread, mainly ruderal plant in N Europe and NW Siberia. It is the common ruderal Tripleurospermum in Greenland, Iceland, and N Fennoscandia.

The main work on the N European Tripleurospermum is still Hämet-Ahi (1967). Her conclusions, based on cytology and morphology, have not been contested. She found T. inodorum and T. maritimum to differ in ploidy levels and only with sterile intermediates, i.e., two species with hybrids. Within T. maritimum, she identified three main trends in morphology and ecology, which she treated as subspecies because there were some transitions. Subspecies maritimum is restricted to W Europe, possibly with some transgression across the Atlantic to southernmost Greenland and E North America. Its northern limit in Europe is probably in Nordland. Subspecies subpolare is a northern, mainly ruderal race, and it is not surprising that it appears among the Svalbard ruderals as it is common in the port areas and the fodder regions in N Norway, where supplies to Svalbard came from. Subspecies phaeocephalum is nearly circumpolar in the arctic and northern boreal zones.

Hämet-Ahti, L. 1967. Tripleurospermum (Compositae) in the northern parts of Scandinavia, Finland and Russia. – Acta Botanica Fennica 77. 19 pp.