Mat-forming graminoid herb with an extensive horizontal system of long, slender, branched rhizomes with reddish brown, scaly leaves, typically with branch lengths of 3–7 cm between aerial shoots, forming very open stands. Culms 7–18 cm.

Basal leaves 5–15 cm long, 0.8–1.2 mm broad, involute, smooth. Culms with 1–2 leaves above the basal ones, the uppermost culm leaf with an inflated sheath 1.5–2.5 cm long with hyaline margins (no red cells, see E. scheuchzeri) and with a reduced blade 1–2 cm long with 5–7 veins.

Inflorescence a dense cluster (both early and late in season) of 1–3 oblong to globular spikes, lateral spikes smaller than the central one. Central spike 1.3–1.5 × 0.9–1.2 cm before the fruit stage and up to 2–2.3 cm long in the fruit stage, subtended by 1–3 bracts with (5)7(9) distinct veins, with dark olive grey mid parts and a narrow, pale grey hyaline margin without red cells. The lowermost bract without or with a very short (< 5 mm) blade. Spikes sessile or subsessile with very short peduncles with short, stiff hairs. Each flower subtended by a triangular, obtuse to subacute scale with a dull grey mid part (without red cells). Lower scales usually with a shiny, silvery white hyaline margin, upper scales with grey margin. Flowers are bisexual, having 3 stamens with short anthers, 1.0–1.5 mm, and a one-seeded gynoecium with 3 stigmas. Stamens and gynoecium are surrounded by a ring of hairs (transformed tepals) attached to the base of the gynoecium and elongating strongly as wool in the fruiting stage. Wool white.

Each flower in the spike produces a nut.

Probably sexual reproduction by seeds; efficient local vegetative reproduction by rhizomes, resulting in large stands that may become fragmented. Wind pollination. In spite of being an assumed hybrid, anther and pollen development is good, at the same level as in the two other species of Eriophorum in Svalbard, and nuts seem to mature some years. Flowering and seed-set is, however, probably intermittent.

Nuts are efficiently spread by wind due to the attached wool. Dispersal of rhizome fragments by birds or water is possible.

The three taxa of Eriophorum in Svalbard are distinguished by several features. Eriophorum scheuchzeri has a single spike (head) with only scaly hyaline bracts, whereas both E. sorensenii and E. triste have two or more spikes with one or more extended, firm, leaf-like bracts. Eriophorum scheuchzeri and E. sorensenii both have filiform involute leaves, whereas E. triste has flat leaves with a V-shaped depression on the upper surface that ends well below the apex. In E. triste, the 2–4 spikes are all of the same size and sit on short, but distinct, peduncles, whereas E. sorensenii usually has one large spike and 1–2 smaller, subsidiary ones, all subsessile. The anthers of E. triste are distinctly longer (1.8–2.2 mm) than those of E. sorensenii (1.0–1.5 mm), and E. sorensenii has a very reduced uppermost culm leaf and some lower bracts in the spikes with silvery hyaline margins, whereas E. triste has a well-developed blade on the uppermost culm leaf and no bracts with silvery hyaline margins.

Eriophorum sorensenii occurs in similar environments as E. triste, i.e., shallow mires and marshes on calcareous ground, but also in less calcareous mires and sometimes in moss tundra, always on permanently moist to wet, fine-grained substrates.

Thermophilous. Recorded from the middle arctic tundra zone and the weakly and clearly continental sections. Restricted to Spitsbergen and to the middle and inner parts of Isfjorden: Sassen in Sabine Land, Gipsdalen in Bünsow Land and the Kapp Thordsen and Kapp Wijk areas in Dickson Land; Kongsfjorden: Blomstrandøya in Haakon VII Land; Liefdefjorden: Ringertzfjellet in Haakon VII Land; and Wijdefjorden: the east side in Austfjorden and Flatøyrdalen in Ny-Friesland and the west side on Krosspynten in Andrée Land). This range is generally similar to that of E. triste, however, the two are not often found together in the same mires or marshes. Eriophorum sorensenii is thereby largely independent of one of its assumed parents, E. triste, in Svalbard.

This is mainly an American arctic species, reaching from Alaska eastwards across Canada and N Greenland to Svalbard. Eriophorum sorensenii is reported to be rare within its entire range but may have been extensively overlooked (easily mistaken for E. triste).

There are several assumed hybrid species in Eriophorum, E. medium being the one most widely distributed and well known in N Europe (see Elven et al. 2013). In a study of Eriophorum in NW North America, Elven & Murray (in prep.) found it easy to distinguish between fertile species and nearly or fully sterile hybrids (in the latter anthers shrivelled and were retained in the wool, fruits undeveloped, wool not extending fully). Only one collection of an assumed fertile hybridogenous species was found in all the material from Alaska and Yukon (E. sorensenii from Peard Bay, Alaska). Cayouette (2004) found the same pattern and hybrids in E Canada. Novoselova (1984, 1988) accepted only E. medium (assumed from E. russeolum × scheuchzeri ssp. scheuchzeri) as such a hybrid species from Russia. Ball & Wujek (2002) mentioned neither hybrids nor hybridogeneous species in their treatment. Retaining or revival of fertility in Eriophorum hybrids is probably a rare phenomenon. For instance, no convincing specimens have been seen supporting presence of the hybrid species E. rousseauianum Raymond (Raymond 1950: 67) as different from primary hybrids E. angustifolium × scheuchzeri ssp. scheuchzeri. Incidentally, The Plant List (http://www.theplantlist.org/) synonymizes E. rousseauianum and E. sorensenii with the former as the priority name. This is only true if E. angustifolium and E. triste is considered one species, which we do not.

Ball, P.W. & Wujek, D.E. 2002. Eriophorum Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 23. Magnoliophyta: Commelinidae (in part): Cyperaceae: 21–27.

Cayouette, J. 2004. A taxonomic review of the Eriophorum russeolum–scheuchzeri complex (Cyperaceae) in North America. – Sida 21: 791–814.

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/

Elven, R. & Murray, D.F. in prep. The cottongrasses (Eriophorum L., Cyperaceae) of Alaska and Yukon Territory revisited.

Novoselova, M.S. 1994. Sistema roda Eriophorum (Cyperaceae). I. Podrody Erioscirpus, Eriophoropsis, Phyllanthela. – Botanicheskii Zhurnal 79(11): 77–89.

Novoselova, M.S. 1998. Nomenklaturnie kombinatsii v rode Eriophorum L. (Cyperaceae). – Novosti Sistematiki Vysshykh Rastenii 31: 7–9.

Raymond, M. 1950. Quelques entites mineures de la flore du Quebec. – Le Naturaliste Canadien 77: 55–71.