Poa arctica
Poa pratensis
arctic bluegrass
Kentucky blue grass
7.5-60 cm, slender to stout, terete or weakly compressed, bases usually decumbent, not branching above the bases;
nodes terete, proximal nodes usually not exserted, 0-2 exserted above.
5-70(100) cm, erect or the bases decumbent, not branching above the base, terete or weakly compressed;
nodes terete or weakly compressed, 1-2(3) exposed, proximal node(s) usually not exserted.
closed for (1/6)1/5 – 2/5 their length, terete, glabrous, smooth or sparsely scabrous, bases of basal sheaths glabrous, distal sheath lengths 1.4-4(5.3) times blade lengths;
collars smooth, glabrous;
ligules (1)2-7 mm, glabrous, smooth or sparsely to infrequently moderately scabrous, apices usually rounded to obtuse or acute, rarely truncate, entire or lacerate;
blades 1-6 mm wide, flat or folded, somewhat involute, smooth, glabrous, apices broadly prow-shaped, cauline blades subequal or gradually reduced distally, flag leaf blades 0.7-9 cm.
closed for 1/4-1/2 their length, terete to slightly compressed, glabrous or infrequently sparsely to moderately hairy, bases of basal sheaths glabrous, not swollen, distal sheath lengths 1.2-5(6.2) times blade lengths;
collars smooth, glabrous;
ligules 0.9-2(3.1) mm, smooth or scabrous, truncate to rounded, infrequently obtuse, ciliolate or glabrous;
blades of extravaginal innovations like those of the culms, those of the intravaginal shoots sometimes distinctly narrower, 0.4-1 mm wide, flat to involute;
cauline blades 0.4-4.5 mm wide, flat, folded, or involute, soft and lax to moderately firm, abaxial surfaces smooth, glabrous, adaxial surfaces smooth or sparsely scabrous, frequently sparsely hairy, hairs 0.2-0.8 mm, erect to appressed, slender, curving, sinuous or straight, apices usually broadly prow-shaped, sometimes narrowly prow-shaped, blades subequal, the middle blades longest, the flag leaf blades 1.5-10 cm.
mainly extravaginal.
mainly extravaginal or evenly extra- and intravaginal.
(2)3.5-15 cm, ovoid to broadly pyramidal, usually open, sparse, with 10-40(60) spikelets, proximal internodes shorter than 1.5(3) cm, with (1)2-5 branches per node;
branches 1.5-6 cm, spreading soon after emergence from the sheath, thin, sinuous, and flexuous to fairly stout and straight, terete, smooth or sparsely to infrequently moderately scabrous, with (1)2-5 spikelets, the spikelets not crowded.
2-15(20) cm, narrowly ovoid to narrowly or broadly pyramidal, loosely contracted to open, sparse to moderately congested, with (25) 30-100+ spikelets and (1)2-7(9) branches per node;
branches (1)2-9 cm, spreading early or late, terete or angled, smooth or sparsely to moderately densely scabrous, with 4-30(50) spikelets usually fairly crowded in the distal 1/2.
(3.5) 4.5-8 mm, lengths to 3.5 times widths, laterally compressed, sometimes bulbiferous;
florets (2)3-6, infrequently bulb-forming;
rachilla internodes smooth or muriculate, proximal internodes glabrous or sparsely softly puberulent to long-villous.
3.5-6(7) mm, lengths 3.5 times widths, laterally compressed, sometimes bulbiferous;
florets 2-5, usually normal, sometimes bulb-forming;
rachilla internodes usually shorter than 1 mm, smooth, glabrous.
lanceolate to broadly lanceolate, distinctly or weakly keeled, keels usually smooth, sometimes sparsely scabrous distally, lateral veins usually moderately pronounced;
lower glumes (3)3.5-5(6) mm, 3-veined;
upper glumes 3.5-5.5(6.5) mm, nearly equaling to slightly exceeding the lowest lemmas, or distinctly shorter;
calluses glabrous or webbed, hairs sparse and short to over 1/3 – 2/3 the lemma length;
lemmas (2.7)3-6(7) mm, lanceolate to broadly lanceolate, usually strongly purple, distinctly keeled, keels, marginal veins, and lateral veins long-villous, hairs on the lateral veins sometimes shorter, lateral veins prominent, intercostal regions short-villous to softly puberulent at least near the base, glabrous elsewhere, smooth to weakly muriculate and/or usually sparsely scabrous, infrequently moderately scabrous, margins broadly hyaline, glabrous, apices acute;
palea keels usually short- to long-villous for most of their length, rarely nearly glabrous and scabrous, intercostal regions broad, usually at least sparsely softly puberulent, rarely glabrous, apices scabrous;
anthers 1.4-2.5 mm, sometimes aborted late in development.
unequal to subequal, usually distinctly shorter than the adjacent lemmas, narrowly lanceolate to lanceolate, infrequently broadly lanceolate, distinctly keeled, keels usually sparsely to densely scabrous, infrequently smooth;
lower glumes 1.5-4(4.5) mm, usually narrowly lanceolate to lanceolate, occasionally sickle-shaped, 1-3-veined;
upper glumes 2-4.5(5) mm, distinctly shorter than to nearly equaling the lowest lemmas;
calluses dorsally webbed, sometimes with additional webs below the marginal veins, hairs at least 1/2 as long as the lemmas, crimped;
lemmas 2-4.3(6) mm, lanceolate, green or strongly purple-tinged, distinctly keeled, keels and marginal veins long-villous, lateral veins usually glabrous, infrequently short-villous to softly puberulent, lateral veins prominent, intercostal regions glabrous, lower portion smooth or finely muriculate, upper portion smooth or sparsely scabrous, margins narrowly to broadly hyaline, glabrous, apices acute;
paleas scabrous, keels sometimes softly puberulent, intercostal regions narrow, usually glabrous, rarely sparsely hispidulous;
anthers usually 1.2-2 mm, infrequently aborted late in development.
= 36, 42, 56, 60, 62-68, 70, ca. 72, 74-76, 78-80, 82-84, 86, 88, 99, 106.
= 27, 28, 32, 35, 37, 41-46, 48-147.
Poa arctica
Poa pratensis
Poa arctica is a common circumboreal species of arctic and alpine regions, growing mainly in mesic to subhydric, acidic tundra and alpine meadows, and on rocky slopes. It extends south in the Rocky Mountains to New Mexico. The frequency of sterile anthers in plants of the high arctic suggests that P. arctica is sometimes apomictic in that region. Over most of the rest of its range, P. arctica usually develops normal anthers. This and isozyme data for populations from alpine and low arctic regions suggest sexual reproduction is common in these habitats.
The most reliable way to distinguish Poa arctica from P. pratensis (p. 522), particularly subsp. alpigena, is by the wider paleas and the presence of hairs between the palea keels. Bulbiferous forms of P. arctica differ from P. stenantha var. vivipara (p. 594) in not being glaucous, and in having rhizomes and terete, smooth panicle branches. Poa xgaspensis (p. 601) also resembles P. arctica, but it has sharply keeled, more scabrous glumes and a spikelet shape that is intermediate between P. pratensis and P. alpina (p. 518). Poa arctica forms natural hybrids with both P. pratensis and P. secunda (p. 586).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Poa pratensis is common, widespread, and well established in many natural and anthropogenic habitats of the Flora region. The only taxa that are clearly native to the region are the arctic and subarctic subspp. alpigena and colpodea. Outside the Flora region, P. pratensis is native in temperate and arctic Eurasia. It is now established in temperate regions around the world.
Poa pratensis is a highly polymorphic, facultatively apomictic species, having what is probably the most extensive series of polyploid chromosome numbers of any species in the world. Poa pratensis is a hybridogenic species, i.e., it comprises numerous lineages with the same basic maternal genome, but different paternal genomes. The lineages are perpetuated by agamospermic and vegetative reproduction. Some major forms are recognized as microspecies or subspecies. These have some correlated ecological and morphological differences, but the morphological boundaries between them are completely bridged, and in some cases the taxa (e.g., subspp. agassizensis and colpodea) may represent environmentally induced plasticity.
Natural hybrids have been identified between Poa pratensis and P. alpina, P. arctica, P. wheeleri, and P. secunda. Many other artificial hybrids have been made; these involve many different, often distantly related, species. In addition, there are many cultivated forms of the species; these have been seeded widely throughout the Flora region for lawns, soil stabilization, and forage. Most cultivated forms favor subsp. irrigata morphologically; others tend towards subspp. pratensis and angustifolia, the latter occurring most commonly in xeric sites.
Poa rhizomata (p. 546) resembles P. pratensis, but has acute ligules arid sparse inflorescences, florets that are usually unisexual, and generally larger spikelets; Poa macrocalyx (p. 527) looks like a robust P. pratensis with large spikelets, and lemmas and paleas that are generally hispidulous between the veins and palea keels. Poa confinis (p. 552) also resembles P. pratensis, but differs in having glabrous or sparsely hairy lemmas, and diffusely webbed calluses.
Cultivars of Poa pratensis
Plants densely to loosely tufted, often forming turf, shoots clustered. Basal branching intra- and extravaginal or mainly extravaginal. Culms 8-50 cm. Innovation shoot blades usually shorter than 45 cm, (0.4)1-4 mm wide, usually flat, sometimes some involute, usually soft, sometimes somewhat firm, adaxial surfaces usually glabrous; cauline blades flat or folded. Panicles 3-15 cm, broadly pyramidal, open or somewhat contracted, with 2-7(9) branches per node; branches ascending or widely spreading, sparsely to densely scabrous, with few to many spikelets per branch. Spikelets lanceolate to broadly lanceolate, not bulbiferous; florets normal. Glume keels strongly compressed, sparsely to moderately scabrous; upper glumes shorter than to nearly equaling the lowest lemmas; lemmas 2.8-4.3(6) mm, finely muriculate, lateral veins glabrous; palea keels scabrous, glabrous, intercostal regions glabrous. 2n = 41-45, 48-59, 62, 64-74, 76, 78, 80, 81, 84-90, 95.
More than 60 cultivars of Poa pratensis have been released in the Flora region. Plants grown from commercially distributed seed have generally been placed in subsp. pratensis by North American authors, but they appear to include genetic contributions from at least three major subspecies, e.g., subspp. angustifolia, pratensis, and irrigata. These and intermediate forms, especially those favoring subspp. irrigata and pratensis, are best simply referred to as Poa pratensis sensu lato or labeled as cultivated material.The chromosome counts listed here are numbers reported for the species that are probably not subspp. alpigena, angustifolia, or colpodea; they may represent subspp. irrigata or pratensis.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Plants lacking well-developed rhizomes; anthers aborted late in development; plants of the high arctic | subsp. caespitans |
1. Plants usually with well-developed rhizomes; anthers normal or plants not of the high arctic. | → 2 |
2. Panicles erect, the branches relatively stout, fairly straight; longest branches of the lowest panicle nodes 1/4-1/2 the length of the panicles; culms wiry, usually several together; calluses glabrous or shortly webbed; paleas sometimes glabrous; plants glaucous, growing in the southern Rocky-Mountains and adjacent portions of the Intermountain region | subsp. aperta |
2. Panicles lax to erect, the branches slender, flexuous to fairly stout and straight; longest branches of the lowest panicle nodes 2/5 – 3/5 the length of the panicles; culms slender to stout, varying from solitary to several together; calluses glabrous or webbed, the hairs usually more than 1/2 as long as the lemmas; paleas pubescent; plants sometimes glaucous, widespread in distribution. | → 3 |
3. Calluses glabrous; spikelets not bulbiferous | subsp. grayana |
3. Calluses webbed, often copiously so, sometimes glabrous in bulbiferous spikelets; spikelets sometimes bulbiferous. | → 4 |
4. Spikelets (5)6-8 mm long; lemmas 4-6 mm long; blades 2-6 mm wide; rachillas usually hairy; plants primarily of the western arctic, extending to northwestern British Columbia | subsp. lanata |
4. Spikelets (3.5)4-7 mm long; lemmas (2.7)3-4.5 mm long; blades 1.5-3 mm wide; rachillas commonly glabrous; plants widespread | subsp. arctica |
1. At least some spikelets bulbiferous; plants of the high arctic tundra | subsp. colpodea |
1. Spikelets not bulbiferous; plants widely distributed. | → 2 |
2. Panicle branches smooth or almost smooth. | → 3 |
3. Basal branching primarily extravaginal; blades flat or folded, soft, adaxial surfaces usually glabrous, sometimes sparsely hairy; plants of alpine and tundra regions | subsp. alpigena |
3. Basal branching both intra- and extravaginal; blades folded or involute, somewhat firm, adaxial surfaces often sparsely hairy; plants widely distributed, but not in alpine or tundra regions | subsp. agassizensis |
2. Panicles branches more or less scabrous. | → 4 |
4. Intravaginal innovation shoots present, intra- and extravaginal blades alike, 0.4-1 mm wide, folded to involute, somewhat firm, adaxial surfaces often sparsely and sofly hairy; plants of dry meadows and forests | subsp. angustifolia |
4. Intravaginal innovation shoots present or absent, if present then differentiated or alike, at least some with blades 1.5-4.5 mm wide, flat or folded, adaxial surfaces rarely hairy; plants widespread, often of more mesic sites. | → 5 |
5. Culms 8-30(50) cm tall, often somewhat glaucous, particularly the glumes; blades flat; intravaginal shoots absent or present and with blades similar to those of the extravaginal shoots; panicles with few spikelets per branch and 1-2(5) branches per node; plants of low, wet, often sandy ground | subsp. irrigata |
5. Culms to 100 cm tall, not glaucous; blades flat or folded; intravaginal shoots present, with blades similar to those of the extravaginal shoots or distinctly narrower; panicles with several to many spikelets per branch and 3-5(7) branches per node; plants of various habitats, including those of subsp. irrigata | subsp. pratensis |
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