Mat-forming or solitary herb with a horizontal (ssp. friesianus) or more or less erect (ssp. acris) rhizome. Cauxed unbranched or sometimes branched, covered with black marcescent remains of petioles and sheaths, often disintegrating into fibres. Leaves in a basal rosette and on stems. Stems erect, up to 7–8 mm thick at base and up to 85 cm (!) tall, branched in the upper part, slightly ribbed, glabrous or with appressed white hairs.
Leaves alternate. Rosette leaves with petioles up to 15(20) cm with broad sheaths half clasping the stem, glabrous or with appressed white hairs; blades up to 10 × 10 cm, deeply palmately divided nearly to base (palmatifid to palmatisect) into (3)5 main segments, each segment divided again for 3/4–4/5 of its width into (2)3(4) cuneate lobes with secondary lobes and teeth, ultimate lobes and teeth often narrow and acute, sparsely pubescent with appressed or spreading hairs, green. Stem leaves with shorter petioles and simpler, often divided to base into 3(5) nearly linear segments on sometimes up to 2 cm stalks (petiolules).
Inflorescence an irregular cyme with up to 20 flowers (or sometimes more), pedicels up to 4–5 cm, densely pubescent with appressed or subappressed hairs, bracteates.
Pedicels up to 4–5 cm, densely pubescent with appressed or subappressed hairs, with small bracteoles. Flowers radially symmetric, up to 2.5 cm in diameter, with 5 sepals and petals. Sepals 4–6 × 2–3 mm, more or less appressed to petals, elliptic or oblong, obtuse, more or less densely pubescent with white, subappressed hairs, yellowish green, sometimes purplish at apex. Petals 10–12 × 7–9 mm, broadly obovate, rounded at apex, contiguous (margins touching the neighbour petals but not overlapping), shiny yellow. Stamens numerous, ca. 5 mm; filaments ca. 4 mm; anthers ca. 1 mm, yellow. Receptacle tap-formed, 1–2 times as tall as broad. Gynoecium of numerous small, free carpels with a hooked style.
Fruit nutlets (one seed) in a nearly globular head of 15–30 fruits.
Sexual reproduction by seeds; local vegetative reproduction by rhizome, especially in ssp. friesianus. Insect pollinated but some self pollination may occur (stamens and stigmas sit very close together). Fruits probably ripen only intermittently in Svalbard; plants observed in August rarely have ripe fruits but there are no observations from September. The large population in Barentsburg suggests that some reproduction by seeds takes place.
The nutlets are dispersed by animals (and people) as they easily attach to fur and footwear due to the hooked style.
Ranunculus acris and the other introduced species of Ranunculus, R. repens and R. subborealis, differ from the native, yellow-flowered species of Ranunculus by being much taller and usually more or less pubescent on stems, petioles and blades (the others are glabrous). Ranunculus repens differs from all others in its coarse stolons and tripartite leaves with stalked (petiolulate) leaflets, from R. acris and R. subborealis in addition in its usually single flowers with overlapping petals (vs. usually several flowers and contiguous petals). Ranunculus acris and R. subborealis are best separated by the pubescence of stems and petioles, appressed and white in R. acris, patent, coarse and yellowish in R. subborealis. The basal leaves of R. acris usually have 5 primary segments and acute terminal segments, those of R. subborealis ssp. villosus usually 3 primary segments and subacute to obtuse terminal segments.
Ruderal ground within settlements, usually on fairly coarse, well-drained substrates.
Introduced. Ranunculus acris has been recorded from three settlements but is stable in one of these only: the Russian settlement of Barentsburg (Nordenskiöld Land). Here it was first recorded in 1988 (Liška & Soldán 2004, as ssp. friesianus, see Comments) and has since been recorded during every visit up to 2011, in fairly large and vital stands. The other records are from Longyearbyen in 1939 and 2011 (both ephemeral) and Hiorthhamn in 1939, a long abandoned settlement close to Longyearbyen, both in Nordenskiöld Land. Other records, in the literature and herbaria, refer to R. subborealis.
The species (s. lat.) is European and W Asian in several races. It occurs as an introduced species in Greenland, North America, and probably in E Asia.
Even if we keep Ranunculus subborealis apart, R. acris is polymorphic. The plants found in Longyearbyen and Hiorthhamn correspond to R. acris s. str. (ssp. acris), and they have probably been introduced from areas where this species and race prevails (SE or C Norway). The plants found and persisting in Barentsburg are rather different. The morphological description above is based on these plants as they are the most complete and the ones who are established and probably reproduce in Svalbard today. We understand why Liška & Soldán (2004) assigned them to ssp. friesianus, and they may be right. The main diagnostic character reported for the two subspecies in European floras is found in the rhizome, more or less horizontal and branching in ssp. friesianus, erect and little branched in ssp. acris. There is no material with rhizome system available for study. However, also the dissection of the blade is rather different between the plants from Barentsburg and those from Longyearbyen (and mainland Norwegian plants). The source area of the Barentsburg plant is probably in E Europe (Russia) where ssp. friesianus may be the predominant race.
Liška, J. & Soldán, Z. 2004. Alien vascular plants recorded from the Barentsburg and Pyramiden settlements, Svalbard. – Preslia 76: 279–290.