Moderately mat-forming herb with stout, horizontal and branched rhizome and more or less vertical caudex branches ending in leaf rosettes without or with reproductive stems. Reproductive stems erect with a few basal leaves and ca. 3 stem leaves beneath the inflorescence. The entire plant glabrous or petioles, leaves and upper parts of stems slightly puberulent with very short, stumpy hairs.

Leaves alternate. Basal leaves with petioles 2–10 cm and blades 3–10 × 1.5–3 cm, 2–3 times as long as broad, oblong or lanceolate, sagittate, with two often acute basal lobes turned downwards (in parallel with the petiole). Stem leaves similar but more short-petiolate, smaller, often much more narrow (3–5 times as long as broad), the uppermost sessile and with bases clasping. Sheaths (ochreas, see Bistorta vivipara) 1–1.5 mm, brown, with petiole attached about midway, upper margin with long, slender fringes (fimbriate). Leaves green or often tinged red.

In Svalbard, this species has only been observed with immature inflorescences with buds, and the characters for inflorescence and flowers are therefore given in general without much measures.

Inflorescence a panicle, branched one time (very rarely with secondary branching of the lowermost main branches), with minute, brown bracts at the base of branches and flower clusters. Flowers in small clusters (whorls) along the branches, usually 3–5 together. Inflorescence usually red or at least strongly tinged reddish.

Plants dioecious (separate male and female plants). Flowers radially symmetric, with perianth of 3 + 3 free tepals. Female flowers with outer tepals flimsy and strongly reflexed, inner tepals more or less orbicular, pink or with pink margins, clasping the gynoecium and enlargening to ca. 3–4 × 3–4 mm and becoming firm in the fruit stage, protecting the fruit. Gynoecium of 3 carpels, with a dense bunch of long, intensely red and conspicuous stigmatic branches with long papillae. Male flowers with the two whorls of tepals nearly equal, and 3 + 3 stamens. Stamens with very short filaments and anthers as long as tepals.

Fruit a trigonous nut clasped inside and dispersed with the 3 inner tepals (valves). Fruits have never been seen in Svalbard.

In principle, sexual reproduction by seeds; very local vegetative reproduction by branching of rhizome. Obligate out-crossing due to being dioecious. Wind pollinated. However, we assume that there is no seed-set in Svalbard plants. The plants on Bjørnøya, where the only stable stands are located, have been observed with stems with young inflorescences at four sites in two seasons in the second half of August, too late for any seed-set. The stands persist by vegetative growth (rhizome).

The sagittate leaves of Rumex acetosa have no parallel in the Svalbard flora.

Near huts and settlements, probably on more or less disturbed ground.

Introduced. Rumex acetosa is reported from five localities on Bjørnøya (Engelskjøn & Schweitzer 1970), and here it seems to persist, at least at lake Krillvatn, as it has been observed on nearly every visit (even if visits have been rare) since first found in 1957. It is also recorded six times from five localities in settlements on Spitsbergen, but only as seedlings with a few leaves: Barentsburg in 2008, Hotellneset in 1897, Longyearbyen in 1939, all in Nordenskiöld Land, Ny-Ålesund in 1928 and 1964 (Oscar II Land), and Virgohamna in 1898 (on Danskeøya, Albert I Land, a settlement long abandoned).

The general range is Eurasian.

Nordic authors (see Nilsson 2000, Elven et al. 2013) have usually treated Rumex acetosa s. lat. as a polymorphic species with at least 5 races in the their area, 3 of these major geographic ones: ssp. acetosa widespread in Fennoscandia, mainly in agricultural landscapes or in areas much influenced by human activity and husbandry; ssp. lapponicus in forests and mountains in more upland and northern parts of Fennoscandia (and obviously native); and ssp. islandicus in Iceland and the Faeroes. Many other European authors, and also the genus author in Flora of North America (Mosyakin 2005), have rather accepted two species, R. acetosa and R. alpestris Jacq. and assigned ssp. lapponicus to the latter species. This solution was suggested as an alternative by Elven et al. (2013) and supported by part abortion of fruits in plants intermediate between ‘acetosa’ s. str. and ‘lapponicus’ (Nilsson 2000). Rumex alpestris Jacq. is, however, a synonym for the rather different species R. scutatus L., and the relevant species name is R. arifolius All. Rumex acetosa s. str. and R. arifolius  (incl. ssp. lapponicus) differ in several characters in leaf shape and panicle but most distinctly in the ochreas (sheaths) which have long fringes in the margin in R. acetosa but are nearly entire in R. arifolius.

Rumex arifolius ssp. lapponicus is the most hardy of the two and the one reaching high up in the mountains and far north, both in Fennoscandia and elsewhere in Eurasia. It is perhaps surprising that the plants of Svalbard belong to R. acetosa, with distinctly fringed ochreas, including plants from four of the five known sites on Bjørnøya. The reason is probably that the plants have been introduced from lowland parts of Norway, where R. acetosa s. str. is the only species present.

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/

Mosyakin, S.L. 2005. Rumex Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 5. Magnoliophyta: Caryophyllidae, part 2: 489–533.

Nilsson, Ö. 2000. Rumex L. – In: Jonsell, B. (ed.), Flora Nordica 1 Lycopodiaceae – Polygonaceae: 290–298.