Robert J. Soreng

Plants annual or perennial; usually synoecious, sometimes monoecious, gynodioecious, dioecious, and/or asexual; with or without rhizomes or stolons, densely to loosely tufted or the culms solitary. Basal branching intravaginal, pseudointravaginal, or extravaginal; prophylls of intravaginal shoots 2-keeled and open, of pseudointravaginal shoots not keeled and tubular, of extravaginal shoots scalelike. Culms 1–150 cm, hollow, usually unbranched above the base. Sheaths from almost completely open to almost completely closed, terete or weakly to strongly compressed; auricles absent; ligules membranous, truncate to acuminate; blades 0.4–12 mm wide, flat, folded, or involute, adaxial surfaces with a groove on each side of the midvein, other intercostal depressions shallow, indistinct, apices often prow-shaped. Inflorescences usually terminal panicles, rarely reduced and racemelike. Spikelets 2–12 mm, usually laterally compressed, infrequently terete to subterete, usually lanceolate, sometimes ovate; florets (1)2–6(13), usually sexual, sometimes bulb-forming; rachillas usually terete, sometimes prolonged beyond the base of the distal floret; disarticulation above the glumes and beneath the florets. Glumes usually shorter than the lowest lemma in the spikelet, usually keeled, 1–3(5)-veined, unawned; calluses blunt, usually terete or slightly laterally compressed, sometimes slightly dorsally compressed, glabrous or hairy, hairs often concentrated in 1(3) tufts or webs, sometimes distributed around the calluses below the lemmas as a crown of hairs; lemmas usually keeled, infrequently weakly keeled or rounded, similar in texture to the glumes, 5(7–11)-veined, lateral veins sometimes faint, margins scarious-hyaline distally, apices scarious-hyaline, truncate or obtuse to acuminate, unawned; paleas from 2/3 as long as to subequal to the lemmas, distinctly 2-keeled, margins and intercostal regions milky white to slightly greenish; lodicules 2, broadly lanceolate, glabrous, lobed; functional anthers (1–2)3, 0.1–5 mm; ovaries glabrous. Caryopses 1–4 mm, ellipsoidal, often shallowly ventrally grooved, solid, with lipid; hila sub-basal, round or oval, to 1/6 the length of the caryopses. x = 7. Name from the Greek poa, ‘grass’.

Poa includes about 500 species. It grows throughout the world, principally in temperate and boreal regions. Sixty-one species and five hybrid species are native to the Flora region; nine species are introduced.

Poa is taxonomically difficult because most species are polyploid, many are apomictic, and hybridization is common. A variety of sexual reproductive systems are represented within the genus, but individual species are usually uniform in this regard. Apomicts derived from bisexual species usually have functional anthers; they require fertilization to stimulate endosperm (and hence seed) development. Apomicts derived from dioecious species do not require fertilization; they are normally pistillate with vestigial anthers 0.1–0.2 mm long.

Herbivores find most species of Poa both palatable and nutritious. Poa fendleriana, P. secunda, and P. wheeleri are important native forage species in western North America; P. alpina, P. arctica, and P. glauca are common components of alpine and arctic vegetation. Species of Poa sect. Abbreviatae are found near the limits of vegetation in both arctic and alpine regions.

Several introduced species of Poa are economically important. Poa pratensis is commonly cultivated for lawns and pasture, and is a major forage species in cooler regions of North America; P. compressa and P. trivialis are widely planted for soil stabilization and forage; P. annua is one of the world’s most widespread weeds. Poa bulbosahas been cultivated; it is now widely established in the Flora region.

Vegetative characteristics that may be useful for distinguishing Poa from other morphologically similar genera are: the more or less straight, rather than curly, roots; two-grooved, prow-shaped blades; partially or wholly closed flag leaf sheaths; and isomorphic collar margins. Useful spikelet characteristics include: terete rachillas; multiple, relatively small, unawned florets; webbed calluses; well-developed palea keels; and the greenish or milky white intercostal regions of the paleas.

There is a strong correlation between the type of basal branching, prophyll structure, and blade development of the initial leaves. Extravaginal shoots have scalelike prophylls 0.5–3 mm long and initial leaves that are bladeless; intravaginal shoots have prominently keeled prophylls 10–50 mm long that are open on the abaxial side and initial leaves with well-developed blades; pseudointravaginal shoots develop intravaginally but have tubular, indistinctly keeled prophylls, and initial leaves with rudimentary blades.

In bulbiferous spikelets, the upper florets form a single tardily disarticulating offset or bulb, each lemma being thickened at the base and leaflike distally. The bulb falls as a unit, with or without the basal floret. The basal floret(s) may have pistils and stamens, and occasionally sets seed. Generally, there is a progression within an inflorescence, the earlier spikelets being bulbiferous and the later spikelets normal.

Callus hairs in Poa follow one of three patterns. In the most common pattern, there is an isolated dorsal tuft of crinkled or pleated hairs, the web, below the lemma keel. In a few species, additional webs may be present below the marginal veins. In the second pattern, crinkled hairs are distributed around the lemma base, but are somewhat concentrated and longer towards the back; this pattern is called a diffuse web. Webbed calluses are found only in Poa. In the third pattern, the hairs are straight to slightly sinuous, and more or less evenly distributed around the lemmas bases; calluses with such a pattern are described as having a crown of hairs.

Three named infrasectional hybrids are included in this treatment. One, Poa arida, is accounted for in the key. The other two are not. Poa ×limosa is too variable, and P. ×gaspensis is known from too few specimens to make their inclusion in the key helpful. All three are described at the end of this treatment, with comments on the probable parental taxa.

Unless stated otherwise, sheath closure is measured on the flag leaf, and ligule length on the upper 1–2 culm leaves; spikelet, floret, callus, lemma, and palea measurements are on non-bulb-forming florets; floret pubescence is evaluated on the lower florets within several spikelets; length of the callus hairs refers to their length when stretched out; anther measurements are based on functional anthers, i.e., those that produce pollen, as indicated by their being plump or, after the pollen is shed, by their open sacs. For hair lengths, puberulent is to about 0.15 mm long, short-villous to about 0.3 mm long, and long-villous from 0.3–0.4+ mm long, but these are only guidelines, not discrete categories; some species are only on one end of the range, and ranges have not been confirmed for every species. Many species key more than once, due in part to infraspecific variation.

SELECTED REFERENCES Bowden, W.M. 1961. Chromosome numbers and taxonomic notes on northern grasses: IV. Tribe Festuceae; Poa and Puccinellia. Canad. J. Bot. 39:123–138; Duckert-Henroid, M.M. and C. Favarger. 1987. Contribution à la cytotaxonomie et à la cytogéographie des Poa (Poaceae = Gramineae) de la Suisse. Mémoires de la Société Helvétique des Sciences Naturelles No. 100. Birkhäuser, Basel, Switzerland. 130 pp.; Gillespie, L.J. and R.J. Soreng. 2005. A phylogenetic analysis of the bluegrass genus Poa based on cpDNA restriction site data. Syst. Bot. 30:84–105; Gillespie, L.J., A. Archambault, and R.J. Soreng. 2006. Phylogeny of Poa (Poaceae) based on trnT-trnF sequence data: Major clades and basal relationships. Aliso 23:420–434; Hiesey, W.M. and M.A. Nobs. 1982. Interspecific hybrid derivatives between facultatively apomictic species of bluegrass and their responses to contrasting environments. Experimental Studies on the Nature of Species 6, Publication No. 636. Carnegie Institution of Washington, Washington, D.C., U.S.A. 119 pp.; Hitchcock, A.S. 1951. Manual of the Grasses of the United States, ed. 2, rev. A. Chase. U.S.D.A. Miscellaneous Publication No. 200. U.S. Government Printing Office, Washington, D.C., U.S.A. 1051 pp.; Hultén, E. 1942. Flora of Alaska and Yukon [in part]. Acta Univ. Lund, n.s., 38:1–281; Kellogg, E.A. 1985. A biosystematic study of the Poa secunda complex. J. Arnold Arbor. 66:201–242; Marsh, V.L.1952. A taxonomic revision of the genus Poa of the United States and southern Canada. Amer. Midl. Naturalist 47:202–250; Munz, P.A. 1959. A California Flora. University of California Press, Berkeley. California, U.S.A. 1681 pp.; Soreng, R.J. 1985. Poa L. in New Mexico with a key to middle and southern Rocky Mountain species. Great Basin Naturalist 45:395–422; Soreng, R.J. 1991a. Notes on new infraspecific taxa and hybrids in North American Poa (Poaceae). Phytologia 71:340–413; Soreng, R.J. 1991b. Systematics of the “Epiles” group of Poa (Poaceae). Syst. Bot. 16:507–528; Soreng, R.J. 1993. Poa L. Pp. 1284, 1286–1291 in J.C. Hickman (ed.). The Jepson Manual of Higher Plants of California. University of California Press, Berkeley, California, U.S.A. 1400 pp.; Soreng, R.J. 1998. An infrageneric classification for Poa in North America, and other notes on sections, species, and subspecies of Poa, Puccinellia, and Dissanthelium (Poaceae). Novon 8:187–202. Soreng, R.J. 2005. Miscellaneous chromosome number reports for Poa L. (Poaceae) in North America. Sida 21:2195–2203; Soreng, R.J. and D. Keil. 2003. Sequentially adjusted sex-ratios in gynomonoecism, and Poa diaboli (Poaceae), a new species from California. Madroño 50: 300–306; Tsvelev, N.N. 1976. Zlaki SSSR. Nauka Publishers, Leningrad [St. Petersburg], Russia. 788 pp.; Tutin, T.G. 1952. Origin of P. annua. Nature 169:160.

©Trustees of the Royal Botanic Gardens, Kew; reproduced with permission.

Plants annual or perennial. Leaf blades usually flat, often with a blunt or hooded tip. Inflorescences open or contracted panicles of spikelets. Spikelets with 2-several florets; glumes slightly unequal, 1-3-veined, keeled, glabrous;lemas deeply concave, keeled, 5-7-veined, membranous with hyaline margins and tips, unawned, often hairy on the keel and veins, rarely so on the back; calluses short, often with a web of fine, cottony haors; rachillas glabrous; palela keels scabridulous to stiffly ciliiolate; ovaries glabrous. 

Poa includes about 570 species. They are native in cool termperate regions throught out the world, including high elevations in the tropics with cool temperate climates. There is 1 species, Poa leptoclada, known from Somaliland. None was reported for Somalia in the Flora of Somalia. 

Global distribution of Poa.

Note: GBIF records include introduced and cultivated plants. Consequently, the distribution shown often differs from statements about a taxon's native distribution.