Solitary graminoid herb growing in dense tussocks with all branching inside leaf sheaths (intravaginal, i.e., no runners or stolons). Culms to 20 cm, erect, smooth and glabrous. Base of aerial shoots surrounded by a cylinder of thin, withered, shiny, pale yellowish grey leaf sheaths.

Leaves with keel, narrowly folded (convolute), smooth and glabrous. Basal leaves 5–15 cm long, nearly always more than half the length of the culms, narrow, 0.5–1.5 mm broad, abruptly tapering at the apex. Culm leaves 2–3, similar to basal leaves but with slightly inflated sheaths and much reduced blades, 1–2 cm long, the flag leaf blade often attached near the middle of the culm. Ligula 1.5–3 mm, acute. Culms and leaves glabrous.

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 a dense, narrow panicle (2.0)2.2–4.5(5.5) × (0.3)0.5–1.0(1.3) cm, occupying 1/3 to 1/5 of culm length. Panicle with 4–7 nodes, with 2 branches (rarely 1) at each of the lower nodes. Branches 10–30 mm, erect to ascending, smooth or slightly scabrous, each with 3–5 spikelets. Spikelets slender, 5–7 × 1.5–2.5 mm, with 3–4 flowers. Bracts (glumes and lemmas) with less distinct keels than in many other species of Poa. Glumes acute, glabrous, purplish (rarely whitish) with a narrow, bronze yellow hyaline margin. Lower glume 2.5–3.5 mm, about 1/2 the length of the spikelet or more, narrowly lanceolate, with 1 vein, rarely also two short, lateral veins; upper glume 3.5–4.5 mm, distinctly more than 1/2 the length of the spikelet, narrowly lanceolate, with 3 veins. Lemmas 3.8–5.0 mm, acute, often with a slightly lacerate apex (a character shared with P. abbreviata and P. glauca among Svalbard species of Poa), with 3–5 veins, densely pubescent with long, fairly straight hairs on and between veins in the proximal half, glabrous in the distal and hyaline part, pale purplish (rarely whitish) with a broad bronze yellow and white hyaline margin. Paleas shorter than lemmas, with pubescent veins. Anthers shrivelled (non-functional).

Fruit an achene (with one seed).

Asexual reproduction by seeds (agamospermy); no vegetative reproduction. Seed-set is regular.

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 reproduction, whereas higher ploidy levels are often (but not always) asexual with either seed production without fertilization (agamospermy) or vegetative propagation by bulbils replacing the flowers in the spikelet (vivipary). Poa hartzii is a mainly decaploid species (2n = 70) and no pollen production has been observed in Svalbard (Haugen 2000). 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. hartzii. She found 71 % seed-set (174 fruits from 246 florets) in control plants (not manipulated) and exactly the same, 71 % (165 fruits from 233 florets), in emasculated plants, suggesting entirely asexual seed-set.

There is no special adaptation to seed dispersal but the hairy lemmas may facilitate wind dispersal of fruits within florets, perhaps also dispersal with animals (furs and feathers). Populations are usually rich in individuals and the species is found in most sites and microsites where it could be expected based on habitat preferences, suggesting an efficient seed-set, dispersal and establishment.

Poa hartzii differs from P. arctica s. lat. and the P. pratensis group in forming tussocks due to its intravaginal branching and total lack of horizontal rhizomes. It differs from the tussock forming P. alpina in, e.g., its much narrower and more convolute leaves, lemmas densely hairy on and between the veins and with an often lacerate lemma apex, and by much smaller and more narrow spikelets; P. alpina has much broader and less convolute leaves that narrow abruptly in the apex, lemmas with long hairs on the veins only and with an evenly tapering apex, and presence of solid ‘socks’. It differs from P. glauca all branching being intravaginal, by smooth or nearly smooth panicle branches, and by lemmas densely hairy; P. glauca has a mixed intravaginal and extravaginal branching even if the plant usually is densely tussocky, the panicle branches are scabrous, often coarsely so, and the lemmas sparsely hairy on the veins only.

The species most similar to P. hartzii, and probably its closest relative in Svalbard, is P. abbreviata. They can be distinguished by the following characters: P. hartzii is more tall-grown, culms ca. 20 cm, has much less firm basal sheaths, longer ligulas (1.5–3 mm), erect culms, leaves nearly always more than half the length of the culms, the flag leaf blade attached at the middle of the culm, 2–5 spikelets per branch, and narrowly lanceolate glumes and lemmas; P. abbreviata is short-grown, culms rarely more than 10 cm, has very firm basal sheaths, shorter ligulas (0.5–1 mm), prostrate or ascending culms, leaves much shorter than half the length of culms, flag leaf blade attached below the middle of the culm, 1–3 spikelets per branch, and more broadly lanceolate glumes and lemmas.

Mistaking this Poa for a Puccinellia, especially P. angustata, is possible because they share the lacerate lemmas. However, no species of Puccinellia have keeled glumes and lemmas.

Poa hartzii may be among the most frost tolerant of Svalbard plants, restricted to valley silt plains and low ridges with nearly no snow protection in winter and exposed to the lowest temperatures and to abrasion by ice and snow crystals blown along the ground in strong winds. The buds are protected by the sheaths of leaf remains, and the tussocks catch snow and function as miniature snow banks, giving some protection. Also in the summer, its sites are harsh, often drying out and exposed to strong winds. The vegetation is always very open; P. hartzii has never been observed in a closed vegetation cover. The substrate is fine-grained (usually loam), a difference from P. abbreviata which has a preference for coarser substrates, and P. hartzii may be the most frequent or the only vascular plant species on loam plains or low loam ridges with strong winds. All observed sites of P. hartzii have substrates with a basic soil reaction, often with a very high pH (8 or more).

Perhaps moderately thermophilous (in spite of its frost tolerance). It occurs in the middle and northern arctic tundra zones and in the weakly and clearly continental sections. The range is confined to Spitsbergen: to the inner parts of Isfjorden, Bockfjorden, and Wijdefjorden, and to Agardhbukta on the east coast. It is especially frequent in the arctic steppe areas of Wijdefjorden. Its localities are usually far apart and are almost always plains and low ridges dominated by basic silt, site types appearing very locally within more closed vegetation. The species seems to have been able to reach and establish in the majority of such sites, where extensive enough and located in the inner fjords.

Poa hartzii is predominantly an American species reaching across the North Atlantic to Svalbard and across the Bering Straits to Wrangel Island in NE Asia. Reports from other Russian areas need confirmation. The connections of the Svalbard population are obviously to N Greenland or Canada where the species is common.

The Svalbard material of Poa hartzii belongs to the main arctic subspecies, ssp. hartzii, which is the subspecies of Svalbard, Greenland and Canada. Two other subspecies are described from the Beringian areas: ssp. alaskana Soreng (P. alaskana (Soreng) Tzvelev) in Alaska and ssp. wrangelica (Tzvelev) Soreng & L.J.Gillespie (P. wrangelica Tzvelev) in westernmost Canada, Alaska and NE Asia.

A hybrid origin of the P. hartzii group of races or species is suspected. Tzvelev (1976) suggested that it could be derived from hybrids between P. glauca and either P. abbreviata or P. arctica. Edmondson (1980) tentatively accepted the former hybrid hypothesis. Investigating the Svalbard plants, Haugen (2000) found indications neither in morphology nor in isoenzymes of an origin involving P. abbreviata or P. arctica (ssp. arctica and ssp. caespitans). An involvement of P. glauca could not be excluded by her data. Gillespie et al. (1997) found support for a hybrid origin of Canadian P. hartzii from the circumpolar P. glauca and the North American P. secunda. She has later (in comment) partly revised her opinion but still finds evidence (cpDNA) of its female genome coming from P. glauca and the other part from P. secunda and the arctic American P. ammophila (see Soreng 2007). Poa hartzii is of hybrid origin but certainly not a recent hybrid in view of its large range and comparative uniformity throughout Svalbard, Greenland and much of the Canadian Arctic Archipelago, and in view of its presence far outside the range of some of its assumed parents (P. secunda and P. ammophila).

Edmondson, J.R. 1980. Poa L. – In: Tutin, T.G. et al. (eds.), Flora Europaea. 5. Alismataceae to Orchidaceae (Monocotyledones): 159–167.

Gillespie, L.J., Consaul, L.L. & Aiken, S.G. 1997. Hybridization and the origin of the arctic grass Poa hartzii (Poaceae): evidence from morphology and chloroplast DNA restriction sites. – Canadian Journal of Botany 75: 1978–1997.

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.

Soreng, R.J. 2007. Poa L. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico 24. Magnoliophyta: Commelinidae (in part): Poaceae, part 1: 486–601.

Tzvelev, N.N. 1976 [1977]. Zlaki SSSR. – Nauka, Leningrad.