For plants with bulbil reproduction (vivipary: entire spikelet except the glumes transformed into a single bulbil), see Poa arctica / pratensis viviparous plants.

Mat-forming graminoid herb growing in often extensive, loose stands due to extravaginal, horizontal and branched rhizomes, typically with rhizome branches of 2–5 cm between aerial shoots. Aerial shoots ascending from rhizome. Culms usually 7–15(20) cm, slender, erect, smooth and glabrous. Base of aerial shoots with a few withered leaves forming a loose sheath. Lower parts of leaf sheaths, blades and culms green or purplish, rarely deep purple.

Leaves with keel, flat to moderately folded (convolute), smooth and glabrous. Basal leaves 5–10 cm long, very narrow, 0.7–1.0 mm wide, gradually tapering towards apex. Culm leaves usually 2, narrow, blade 1.5–3 cm, the flag leaf blade usually attached near the middle of the culm. Ligula 1–3 mm, subacute or obtuse, often lacerate.

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, pyramidal panicle 2.5–5.5 × 2–4.5 cm, occupying 1/4–1/6 of culm length, with spreading to pendulous or retrorse, slender branches. Panicle with 4–6 nodes with 1–2(3) branches at each of the lower nodes. Branches long (the longest ca. 7–15 mm), usually with 2–3 spikelets in their distal parts, smooth. Spikelets 4.5–6.7 × 1.5–4.0 mm, with 2–4 flowers. Bracts (glumes and lemmas) with keels, often fairly sharp. Glumes acute to acuminate, smooth and glabrous, purplish throughout or green at base, with bronze yellow or whitish hyaline apex. Lower glume 3–3.7 mm, usually more than 1/2 the length of spikelet, 0.5–0.7 mm wide, narrowly lanceolate, with 1(3) veins; upper glume 3.8–4.2 mm, more than 1/2 the length of spikelet, 0.75–0.9 mm wide, lanceolate, with 3 veins, margin hyaline for usually more than 1/2 the width of glume. Lemmas acute, 3.3–4.0 × 0.9–1.0 mm, with 3–5 veins, with long, more or less straight hairs on the keel and veins, short hairs between the veins, purplish or green at base, with a broad hyaline margin, often variegated in bronze yellow and white. Paleas shorter than lemmas, with pubescent veins. Anthers developed, 1.6–2.2 mm, or more rarely shrivelled (non-functional).

Fruit an achene (with one seed).

Reproduction by seeds, probably mainly asexual; efficient local vegetative reproduction by growth by the rhizomes, often forms very large stands that may be fragmented by soil movement (cryoturbation, solifluction), other disturbance, or simply through age. Flowering and seed-set is more or less regular (see below). Seed germination of 2.3 %, 15 % (Eurola 1972), and 59 % have been observed (Alsos et al. 2013), suggesting that germination conditions may be important.

In some grass genera, Poa among them, there is a fairly good correlation between ploidy levels and modes of reproduction. Diploids (mostly 2n = 14) and tetraploids (mostly 2n = 28) usually have sexual seed-set, whereas higher ploidy levels are often (but not always) asexual with either seed-set without fertilization (agamospermy) or vegetative propagation by bulbils replacing the flowers in the spikelet (vivipary). Poa arctica ssp. arctica is a highly polyploid (6×–ca. 11×), seed-reproducing plant, partly with asexual seed-set (agamospermy). Both seed and bulbil reproduction are reported in ssp. arctica but the assignment of viviparous plants to ssp. arctica is debatable. The amount of seed production may be small; very often the culms stand with panicles with intact spikelets (where no grains have been shed) into the winter and until the next growth season. However, recruitment on fresh ground (e.g., river sediment plains, road verges, screes) by seedlings of P. arctica (both ssp. arctica and ssp. caespitans) is commonly observed and vouches for a significant level of seed-set. Haugen (2000) investigated seed-set in a laboratory situation without and with removal of anthers (emasculation) in a pilot study of a few plants of Svalbard P. arctica ssp. arctica. She found 18 % seed-set (55 fruits from 309 florets) in control plants (not manipulated) and much fewer, 7 % (5 fruits from 70 florets), in emasculated plants, suggesting some agamospermy but predominantly sexual seed-set.

Fruits (inside florets) are probably dispersed mainly by wind but possibly also by birds.

Poa arctica s. lat. and the P. pratensis group differ from all other Poa species in Svalbard in their extensive, extravaginal rhizomes, evident even in apparently tussocky individuals when dug up. The rhizomes may be short or long but always run some distance horizontally before bending up and producing aerial shoots. These two species or species complexes also differ from the other Svalbard species of Poa (except for P. alpina) in usually larger and more open panicles with longer branches.

The differences between P. arctica s. lat. and the P. pratensis group are more subtle (see Haugen 2000). The key difference lies in the lemmas: in P. arctica the lemmas have veins with more or less straight hairs and there are short hairs or spinules between the veins; in the P. pratensis group the lemmas have veins with crisped hairs and the lemmas are glabrous and smooth between the veins. Another feature is that the hyaline margin of the lemmas in P. arctica is broad, ca. 1/2 the width of the lemma in the upper part and narrows to less than half the width towards base, in the P. pratensis group it is more narrow, less that 1/2 the width of the lemma the upper part but narrows less towards the base. The panicle branches are smooth in P. arctica but at least slightly scabrous in the P. pratensis group. Less reliable but more easily observable differences are found in the shape of the panicle (triangular in outline with spreading to even retrorse branches in P. arctica, more elongated ovate in outline with more ascending branches in the P. pratensis group), number of branches at the lower nodes in the panicle, 1–2(3) in P. arctica and usually more than 3 in the P. pratensis group, and the number and position of spikelets on branches (one of very few and only distally in P. arctica, several and along the branches in the P. pratensis group)..

The differences between Poa arctica ssp. arctica and ssp. caespitans are less pronounced. "Typical" plants of the two are rather different: ssp. arctica with the long rhizome branches and loose mat growth, very narrow leaves, slender culms, mostly green or only purplish culm leaf sheaths and the graceful panicle with smaller spikelets; ssp. caespitans with the very short rhizome branches and dense mat or tussocky growth, broader leaves, stiffer culms, deep purplish culm leaf sheaths and a stouter panicle with stiffer branches and slightly larger spikelets. Haugen (2000) emphasized the following additional characters for separation: In ssp. arctica panicle branches with usually 2–more spikelets, lowest panicle branch 7–15 mm, lower glume 0.5–0.7 mm wide (keel to margin), upper glume 0.75–0.9 mm wide with hyaline margin 1/2 or more of width, lemmas 0.85–1.0 mm wide, and anthers usually well developed; in ssp. caespitans panicle branches with usually 1 spikelet, lowest panicle branch 5–10 mm, lower glume 0.6–0.8 mm wide, upper glume 0.8–1.0 mm wide with hyaline margin less that 1/2 of width, lemmas 0.95–1.2 mm wide, and anthers always shrivelled. These characters are quantitative and overlapping rather than qualitative, and some plants not conforming to this list of differential characters are found (especially in the herbaria where abnormal plants are over-represented). The genetic support for ssp. caespitans, as different from ssp. arctica, is perhaps better than the morphological one (see Haugen 2000).

Poa arctica s. lat. occurs in both stable and unstable vegetation covers and on open ground, sometimes as the sole or one of very few vascular plants (e.g., in boulder screes). It is also largely indifferent as to soil moisture, ranging from the tussock level in mires and moderately soaked sediment plains to dry ridges and screes, but with its main occurrences in moist to moderately dry mossy sites. The species is probably not a strong competitor but may sustain (sometimes vegetatively) in very adverse conditions as on coastal cliffs, in over-manured patches in bird colonies, or in deep shade. It is largely indifferent as to soil reaction (pH), present on both acidic and basic substrates.

Of the two subspecies, ssp. arctica is the plant of the moister site types and is the one found in meadows and moss mats, ssp. caespitans is the plant of drier and more often unstable sites types and predominates in screes and on dry gravel slopes. The subspecies do not seem to differ much in soil reaction preferences; both are found on both acidic and basic soils.

Poa arctica s. lat. is largely independent of climate and found in a broad range of site types and probably throughout the islands, i.e., along the entire gradients from the middle arctic to the polar desert zones and from the weakly oceanic to the clearly continental sections. Both subspecies are found more or less throughout the investigated parts of Svalbard (including the outlying islands of Kong Karls Land and Hopen). There are no records from Edgeøya and only one from Barentsøya, however, we suspect the lack of records to be due to under-collecting. Outside Svalbard Poa arctica s. lat. is one of the most widespread of all arctic plants, present in nearly all areas, and also with extensive ranges in temperate mountains south to S Scandinavia, C Asia, and the southern Rockies. Whereas ssp. arctica may be circumpolar or nearly so (doubts about its range in Beringia), ssp. caespitans is amphi-Atlantic. Confirmed records of ssp. caespitans are from the eastern parts of the Canadian Arctic, Greenland, Svalbard, N Scandinavia, and Novaya Zemlya. No reports from other regions have been confirmed.

Poa arctica s. lat. is a large and much disputed group within northern Poa. The variation in N Canada, Greenland, and arctic Europe has usually been treated as two subspecies (Böcher et al. 1978; Elven et al. 2011, 2022): ssp. arctica and ssp. caespitans. Edmondson (1980) did not accept any races for Europe. Soreng et al. (2003) provisionally accepted ssp. caespitans, with their weaker "acceptance level 2", and Soreng (2007) accepted ssp. caespitans as one of five North American subspecies. Our view (Elven, Haugen) concerning the plants of Greenland and N Europe is that there are two disjunctly different groups (‘arctica’ and ‘caespitans’) without documented signs of transition (even if some incomplete plants collected may be difficult to assign). Additional races and species of a P. arctica group are reported from the Beringian areas, both on the American and on the Asian side.

Whereas ssp. caespitans is quite monomorphic throughout its range, ssp. arctica is very polymorphic. In Scandinavia, 6 regional races (subspecies or varieties) have been named and 5 of these formally described (see summary by Elven et al. 2022): ssp. elongata (Blytt) Nannf. and ssp. depauperata (Fr.) Nannf. in S Norway, ssp. caespitans Simmons ex Nannf. and ssp. microglumis Nannf. in N Norway and N Sweden, ssp. tromsensis Nannf. in Tromsø in N Norway, and ssp. wahlenbergii Nannf. ined. in E Finnmark in NE Norway. In addition, the vast majority of the plants found in the mountains of N Scandinavia have not been assigned to any race. The differences found between the races in Scandinavia are probably due to isolated populations and population groups and agamospermy. In the Arctic, where Poa arctica occurs everywhere, similar variation patterns are not geographically separated and therefore impossible to identify and give names.

Alsos, I.G., Müller, E. & Eidesen, P.B. 2013. Germinating seeds or bulbils in 87 of 113 tested Arctic species indicate potential for ex situ seed bank storage. – Polar Biology 36: 819–830. Doi 10.1007/s00300-013-1307-7.

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

Eurola, S. 1972. Germination of seeds collected in Spitsbergen. – Annales Botanici Fennici 9: 149–159.

Haugen, J. 2000. Possible hybrid origins of Poa hartzii and Poa arctica ssp. caespitans (Poaceae) investigated by morphology and isoenzymes. – Cand. scient. Thesis, Univ. Oslo, Oslo.

Lid, J. & Lid, D.T. 1994. Norsk Flora. Ed. 6 by R. Elven. – Det Norske Samlaget, Oslo.

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Soreng, R.J., Peterson, P.M., Davidse, G., Judziewicz, E.J., Zuloaga, F.O., Filgueiras, T.S. & Morrone, O. 2003. Catalogue of New World grasses (Poaceae): IV. Subfamily Pooideae. – Contributions from the U.S. National Herbarium 48. 730 pp.