Annual to short-lived perennial. Solitary herb with a single root, a more or less branched caudex (in perennial plants) 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 × 3 mm, broadly triangular, the outer ones distinctly shorter than the inner ones (a diagnostic character for this subspecies), with a green centre and an up to 1–1.5 mm broad, blackish 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 15–22 × 4–7 mm, white. Inner flowers radially symmetric, bisexual (hermaphrodite), tubular, with a cylindrical, 5-lobed, yellow corolla. Pappus reduced to a small corona (i.e., no pappus hairs).

Fruit an achene with floating tissue in the ribs. Achenes compressed laterally with broad, more or less confluent ribs, and 2 elongated resin-glands on the abaxial (ventral) face, glands more than 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, but it is possible (see Comments).

Fruits are mainly dispersed by sea currents.

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: Tripleurospermum 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 Tripleurospermum 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 by cabins and in settlements.

Only recorded from Spitsbergen at Hollendergruva on Kapp Boheman at the north side of Isfjorden (Oscar II Land) in 1924 and from Ny-Ålesund (Oscar II Land) in 1958 and 1965. These occurrences have all been assumed based on introduced plants; see, however, Comments.

Tripleurospermum maritimum ssp. phaeocephalum is the geographically major race of the species and the only race present well outside Europe. It is one of the more regular seashore plants throughout arctic Eurasia from Finnmark in NE Norway to the Bering Straits, throughout arctic North America (Elven et al. 2011), and in Greenland in the west and with an isolated area in E Greenland across from Svalbard (Bay 1992). It is rarely weedy except for inside arctic settlements within its native range.

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., they are 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 southermost 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 mainly came from. The third race is ssp. phaeocephalum.

The occurrence in Svalbard of ssp. phaeocephalum is more problematic. This race occurs in Norway only in the northeast, in Finnmark (see Elven et al. 2013). This has not been an important region for supplies to Svalbard, and at least one of the finds in Svalbard is in a place where no 'ruderal' should be expected: Hollendergruva at Kapp Boheman. We are not convinced that T. maritimum ssp. phaeocephalum is an introduced plant in Svalbard, even if it has been found only in a settlement (Ny-Ålesund) and near a cabin (Hollendergruva). Note that this is a species native to E Greenland (and in Iceland and northernmost Norway) and easily dispersed by sea currents.

Bay, C. 1992. A phytogeographical study of the vascular plants of northern Greenland – north of 74 northern latitude. – Meddelelser om Grønland, Bioscience 36. 102 pp.

Elven, R., Fremstad, E. & Pedersen, O. (eds.) 2013. Distribution maps of Norwegian vascular plants. IV The eastern and northeastern elements. – Akademika Publishing, Trondheim.

Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/

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