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Mat-forming graminoid herb growing in often extensive stands due to horizontal, branched rhizomes, typically with rhizome branches of 4–10 cm between shoots. Aerial shoots ascending from rhizome, at base with several prophylls (reduced leaves without or with a short blade). Culms 12–25(33) cm, erect, smooth. Base of culms with a few withered leaves forming a loose sheath. Leaves and culms glabrous.
Leaves keeled, flat to weakly folded, smooth, with bluish bloom (wax), especially on the upper surface. Basal leaves 4–10(14) cm long, relatively broad (2–4 mm), tapering at the very apex. Culm leaves usually 2, 1.5–3(5) cm long, similar to basal leaves but blade slightly broader, 3–5 mm. Flag leaf blade usually attached near middle of culm. Ligula 2–4 mm, subacute.
INFLORESCENCE AND FLOWER
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 3–5(7) cm long, with patent branches; panicle occupying 1/4–1/5 of culm length. Panicle branches at 5–6(7) nodes with 2–4 branches at each of the lower nodes. Branches 10–25 mm long, smooth or sparsely scabrous, the lower ones with 4–8(10) spikelets mainly in the distal half of the branch. Spikelets 4.5–7 × 2–3 mm with 3–4 flowers. Bracts (glumes and lemmas) with keels, Glumes of nearly the same length, 4–5 mm, more than 1/2 as long as spikelet, broadly lanceolate, acute, with 3 more or less distinct veins, glabrous, scabrous on the keel in distal parts, violet with a narrow bronze yellow and white hyaline margin. Lemmas 4–5 mm, broadly lanceolate, acute, with 5 distinct (but not sharp) veins, with long and dense wavy (curly) hairs on 1/2–2/3 the length of veins and keel but glabrous between the veins, with a large tuft of cottony hairs at the base of the lemma, violet with a broad hyaline margin variegated in bronze yellow and white. Paleas shorter than lemmas, with pubescent veins. Anthers and pollen well developed (probably functional).
Fruit an achene (with one seed). Fruits not observed in Svalbard.
Reproduction by seeds, probably both sexual and asexual (agamospermy), but no seed reproduction observed in Svalbard; restricted local vegetative reproduction by short rhizomes. Anthers, pollen and stigmas well developed in early September.
Fruit dispersal (inside florets), if ever realized, is probably by wind and animals.
For comparison with other species of Poa and other races of P. pratensis, see P. pratensis ssp. alpigena. The three best diagnostic characters of ssp. irrigata compared with the other subspecies are the bluish green colour (bloom, wax) of the leaves, culms and spikelets, the broadly pyramidal panicle with comparatively few branches at the nodes, and the large and comparatively broad spikelets.
On road verges in a settlement, forming large patches along several roads, on gravelly, dry ground.
Introduced. Found in the settlement Longyearbyen (Adventfjorden at middle Isfjorden, Nordenskiöld Land). The plant has been sown to stabilize road verges, most probably in the early 1990s (Hagen 2001; Alsos et al. in press) and has since spread along roads, probably by vegetative means (detached runners transported in soil). It flowers very late, at the end of August and into September, and it is improbable that any reproduction can take place.
This is a widespread race or species in the P. pratensis group in W and NW Europe, mainly coastal (dry salt marshes, coastal cliffs) but also on inland shores and on trampled ground where its low sward is not over-grown by taller competitors. It reaches the Arctic on the European mainland but probably only the shrub tundra zone (zone E in Elven et al. 2011).
The late flowering is probably the reason why it was discovered as late as in 2009, after it had been present for probably ca. 15 years and a few tens of meters from the University Centre building in Longyerbyen. It was then found in large quantities between the University Centre (UNIS) and the shopping district, along the roads where every biologist visiting Longyearbyen walks every day. When looking for it at the end of July in 2010, it was seen only as an anonymous grass sward without any signs of flowering culms yet; however, in the good summer of 2014 it had developed panicles at the end of July.
Alsos, I.G., Ware, C. & Elven, R. In press (2015). Past Arctic aliens have passed away, current ones may stay. – Biological Invasions.
Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF).
Hagen, D. 2001. Botanikk og byfornyelse. Å plukke blomster med bulldoser. – In: Arlov, T.B. (ed.), Fra company town til folkestyre: samfunnsbygging i Longyearbyen på 78° nord: 155–163. – Svalbard samfunnsdrift, Longyearbyen.