Poa is an easily recognized genus, most of the species having a tuft of crinkled, cobwebby hairs on the callus below each lemma, and the leaves ending in a boat-shaped tip (as they do, however, also in several other genera). The species, on the other hand, often cause considerable difficulty in identification. The lemma must be very carefully examined, for hairs along the margin and keel may be so closely appressed as to be nearly invisible until disturbed with a needle. Specimens of some species of Festuca might run here in the key, but have firm glabrous lemmas nearly or quite nerveless. In some species of Poa, the young sheaths may be closed, splitting later into the characteristic open sheaths.
1. Plants with swollen, bulbous bases; inflorescences usually bulblet-bearing.
1. Plants without bulbous bases, inflorescences with normal spikelets.
2. Callus at base of lemma without a cobwebby tuft of crinkled hairs (although the keel may be villous).
3. Lemmas mostly with 5 distinct nerves (sometimes 3-nerved in the annual and small-anthered P. annua); lower glume 1-nerved.
4. Anthers not over 1 mm long; lemma glabrous between the villous nerves; usually a low annual, less than 30 cm tall, the panicle 2–8 (–11) cm high.
4. Anthers mostly over 1 mm long; lower part of lemma sparsely and finely pubescent between the nerves; a tall perennial, 30 cm or more in height, the panicle ca. (5–) 6–15 cm high.
5. Panicle narrow, much longer than wide, the branches ascending; plants colonial from elongated rhizomes; ligules of upper leaves 2–4 mm long.
5. Panicles broad, about as wide as long, the lax branches spreading or nodding; cespitose; ligule less than 2 mm long.
3. Lemmas with only 3 distinct nerves (the intermediate nerves very obscure or absent); lower glume 3-nerved.
6. Lower part of lemma villous between the keel and the margins; glumes broadly ovate to oblong or obovate (more than half as wide as long); panicle about twice as long as wider or even somewhat wider.
6. Lower part of lemma glabrous, scabrous, or at most short-pubescent between the keel and the margins; glumes ± narrowly ovate-lanceolate (about half as wide as long or narrower); panicle usually ± contracted, 3 or more times as long as wide (open in some forms).
7. Culms strongly flattened (nodes normally compressed); exposed nodes (those exserted beyond the sheaths) mostly 2–3 (–4) per culm; plants rhizomatous; larger glumes ca. 2–2.8 (–3) mm long; rachilla smooth and glabrous.
P. compressa (in part)
7. Culms slightly if at all flattened (nodes terete or nearly so, even on pressed specimens); exposed nodes mostly 0–1 per culm; plants not rhizomatous; larger glumes (2.7–) 3–4.4 mm long; rachilla usually ± pubescent.
8. Lemma scabrous or minutely pubescent at base between and on the keel and margins; spikelets roundish on back, the glumes and lemmas rather obscurely keeled.
8. Lemma glabrous except for the usually strongly villous keel and margins (and sometimes a few long hairs at the base); spikelets ± flattened, the glumes and lemmas distinctly keeled.
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2. Callus at base of lemma with a small or large cobwebby tuft of crinkled hairs.
9. Margins of lemma completely glabrous.
10. Lower branches of panicle 1–2 (very rarely 3) per node; keel of lemma glabrous and smooth (lemmas therefore completely glabrous except for web on callus).
11. Longer ligules of uppermost culm leaves 0.6–2.5 (–2.7) mm long; lemmas acute (as seen from the side, the margin meeting the tip of the keel at an angle of about 45° or less); anthers ca. 0.9–1.5 mm long.
11. Longer ligules of uppermost culm leaves 2.6–4.9 mm long; lemmas obtuse (margin on most or all of them meeting the tip of the keel at an angle greater than 45°); anthers ca. 0.7–1 mm long.
10. Lower branches of panicle 4–8 per node; keel of lemma scabrous or minutely or strongly pubescent near the base.
12. Ligules 0.7–2.2 (–3) mm long; lemmas mostly 3-nerved (intermediate nerves obscure); anthers 0.4–0.7 mm long.
12. Ligules (3.2–) 4–11 mm long; lemmas distinctly 5-nerved (intermediate nerves prominent); anthers ca. 0.8–1.8 mm long.
9. Margins of lemma ± hairy, at least toward base.
13. Lemmas 5-nerved (the nerves between the keel and margin prominent); ligules 0.6–2 (–2.6) mm long; anthers 1.2–1.8 (–2.1) mm long; rachilla glabrous.
14. Keel of lemma hairy on about the basal 2/3, only scabrous or smoothish on distal third; lemmas glabrous between keel and margins (or marginal nerves); plants rhizomatous.
14. Keel of lemma hairy nearly to the end of the green portion; surface and intermediate nerves of lemma usually ± pubescent; plants without rhizomes.
13. Lemmas mostly 3-nerved (intermediate nerves obscure or absent); ligules, anthers, and rachilla various, but not combined as above (except in P. compressa, with flattened nodes and culms).
15. Upper ligules (2.3–) 2.6–5 mm long; anthers 0.8–1.2 (–1.5) mm long; rachilla glabrous.
15. Upper ligules 0.2–2 (–2.5) mm long; anthers mostly less than 0.8 or more than 1.2 mm long; rachilla glabrous or pubescent.
16. Rachilla smooth and glabrous; branches of panicle 1–4, usually 2, per node; larger glumes ca. 2–2.8 (–3) mm long.
17. Panicle ± stiff and contracted, the branches mostly bearing spikelets nearly to the base; culms strongly flattened (note especially the nodes); plants rhizomatous; anthers 1.1–1.7 mm long; upper ligules (0.5–) 0.8–1.8 (–2.5) mm long.
P. compressa (in part)
17. Panicle ± lax and open, the elongate, very thin branches bearing branchlets or spikelets mostly above the middle; culms not flattened; plants not rhizomatous; anthers ca. 0.5–0.7 mm long; upper ligules ca. 0.5–1 (–1.5) mm long.
16. Rachilla ± puberulent, short-hairy, or strongly scabrous [30–40×; a black background will help reveal this]; branches of panicle mostly 2–5 per node; larger glumes (2.5–) 3–4.4 mm long.
18. Longest ligules (0.9–) 1.2–1.8 (–2.5) mm long; exposed nodes of culm 0–1; leaf blades ± strongly ascending, height to uppermost node 0.1–0.4 times the plant height (except on immature material).
18. Longest ligules 0.3–1.2 (–1.5) mm long; exposed nodes usually 1–4; leaf blades often divergent from culm, height to uppermost node 0.25–0.8 times the culm height.
19. Longest ligules 0.3–0.8 mm long; height to uppermost culm node 0.4–0.8 times the plant height; glumes narrowly lanceolate, the widest clearly narrower than the lemmas.
19. Longest ligules 0.7–1.2 (–1.5) mm long, height to uppermost culm node 0.25–0.4 times the plant height; glumes lanceolate to somewhat broadly lanceolate, the widest often nearly the same width as the lemmas.