Solitary graminoid herb growing in dense, often large tussocks with all branching inside leaf sheaths (intravaginal, i.e., no runners). Culms to 40(50) cm, erect, smooth or very slightly scabrous. Base of shoots densely surrounded by pale, straw-coloured sheaths of marcescent leaves. Prophylls (scaly leaves without developed blade at base of shoots) 1–2, hyaline.

Leaves flat or slightly involute (margins rolled inwards), tapering towards apex, with 5–10 veins, distinct on the lower surface, raised as pale ribs on the upper surface, glabrous, smooth on the lower surface, strongly scabrous in margin and on ribs on the upper surface. Basal leaves of aerial shoots 10–30(35) cm long, shorter than culms, 2–4 mm broad. Culm leaves 2–3, 5–15 cm long, much decreasing in length upwards on culm, 1–3 mm broad, flat, blade of flag leaf attached at middle of culm or above. Ligula long, 2–8 mm, obtuse to acute, entire.

The units of the inflorescence of Poaceae are the spikelets, nearly always numerous in a panicle or spike-like inflorescence. Spikelets are composed of 2 glumes (bracts for the spikelet) and one or more flowers (the term used below) or rather floral units often named ‘florets’ because we do not know what is the exact flower. A flower or floret is composed of a lemma with 1 mid vein (probably the floral bract), a palea with 2 mid veins (either 2 fused bracteoles or perhaps 2 fused perianth leaves), 3 small organs called ‘lodiculae’ and essential in the opening of the flower at anthesis (possibly transformed perianth leaves or transformed stamens), 3 stamens (mostly), and a gynoecium of 2 fused carpels with 2 feathery stigmas and one seed.

Inflorescence an open panicle (8)12–20(25) cm long, occupying 1/3–1/4 of culm length, with ascending to spreading branches. Panicle with 8–12 well separated nodes and with 3–6 branches at each of the lower nodes. Branches 30–70 mm, moderately to strongly scabrous, each with 5–15 spikelets. Spikelets 4–6 × 1–1.5 mm, with 2(3) flowers, the third, if present, usually reduced and non-functional. Bracts (glumes and lemmas) with rounded backs. Glumes lanceolate, acute, lower glume 2.5–3.5 mm, upper glume 3.5–4.5 mm, as long as spikelet, with mid vein and 2 indistinct lateral veins (more distinct on upper glume), mid vein smooth or slightly scabrous apically, more scabrous on upper glume, glume surface shiny, with green or violet mid part and broad, violet and yellow, hyaline margin and apex. Lemmas 2–4.5 mm, lanceolate with lacerate apex, with (3)5–7 indistinct veins, shiny, smooth and glabrous, with a straight or geniculate awn 1–6 mm attached at the lower part of the mid vein, mostly not exerted from the spikelet. Paleas smaller than lemmas or reduced. Stamens 3; anthers 1.5–2 mm (but Svalbard plants rarely reach full anthesis).

Fruit an achene (with one seed).

Sexual reproduction by seeds but probably not in Svalbard; no vegetative reproduction. Adapted to pollination by wind but no Svalbard plants observed to have fully reached a pollination stage (even if they have developed young spikelets) and probably no seed production.

Deschampsia cespitosa resembles D. alpina before the latter develop its bulbils. There are, however, three differences. Whereas D. alpina has almost smooth ribs on the upper leaf surface, smooth branches in the panicle, and narrow and involute leaves, D. cespitosa has strongly scabrous ribs with forwards-pointing spines (easy to feel with fingers when you try to strike downwards along the upper surface), strongly scabrous branches in the panicle, and usually flat leaves. Late in season, D. alpina turn into autumn colours in Svalbard, whereas D. cespitosa stays green.

The character most clearly distinguishing young plants of D. alpina and D. cespitosa from those of D. sukatschewii is the number of veins in the leaves, 5–10 in D. alpina and D. cespitosa, 3–5 in D. sukatschewii which also has significantly narrower, filiform leaves.

On disturbed ground in settlements and close to cabins, mostly in dry meadows. The species is probably indifferent as to soil reaction (pH).

Introduced. Deschampsia cespitosa is recorded from several places on Bjørnøya (Engelskelva, Hedvighamn and Krillvasshytta 1957, Tunheim 1967 and 1983, at Krillvatn 1984) and from several places on Spitsbergen: Calypsobyen at Recherchefjorden 1939 (Wedel Jarlsberg Land), Barentsburg at Grønfjorden 1993 (Nordenskiöld Land), several places in and near Longyearbyen (Longyearbyen 1939, Hotellneset 1928, Advent City 1939; Nordenskiöld Land), at Pyramiden 2011 and 2014 (Dickson Land), and at Kongsfjorden in Ny Ålesund 1939, 1958, 1964 (Oscar II Land) and at Ny-London on Blomstrandøya 1923 (Haakon VII Land). Deschampsia cespitosa seems to persist for many years, e.g., in Pyramiden where it was found several places as late as in 2014, 16 years after the settlement was abandoned. Due to the lack of seed set, it is expected to die off if no new seeds are introduced.

For global distribution, see Comments.

Deschampsia cespitosa s. lat. is a very complicated group, by some authors treated as one species with more than 20 subspecies (and including both the other Svalbard species of Deschampsia as subspecies). This matter is discussed, with many references, by Elven et al. (2011). It does not concern us very much as the introduced plants in Svalbard belong to D. cespitosa s. str. (ssp. cespitosa), which those accepting several species consider to be a species of Europe and W Siberia. It is predominantly diploid with 2n = 26 and sexual.

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