Immature specimens of Poa palustris are sometimes hastily identified as an Agrostis or are included in mixed collections. They may readily be distinguished by having at least the rudiments of a second floret in the spikelet and a cobwebby tuft at the base of the villous-margined lemma.
1. Palea present, about half as long as the lemma or longer; anthers ca. 0.8–1.5 mm long.
2. Larger ligules less than 2.2 mm long; leaf blades not over 4 mm wide; panicle ± open (the branches diverging from their bases).
2. Larger (upper) ligules ca. 2.5–6 (–7) mm long; leaf blades usually more than 4 mm wide or the panicle branches ± strongly ascending at their bases.
3. Plants rhizomatous but not stoloniferous, i.e., culms arising from underground rhizomes, straight or curved at the very base, otherwise erect and nearly or quite straight; larger leaf blades mostly 3–7 (–10) mm wide; spikelets usually flushed with red or purplish; bases of middle panicle branches mostly meeting the axis of the panicle at an angle of 30–45° (except when very immature).
3. Plants stoloniferous but not rhizomatous, i.e., culms usually decumbent at their bases, the lower nodes often strongly bent and/or rooting, but underground rhizomes absent; larger leaf blades 1.7–3 (–4) mm wide; spikelets pale, greenish; bases of middle panicle branches usually strongly ascending or appressed to axis of panicle, at most diverging about 15° (but panicle branches often spreading distally).
1. Palea absent or vestigial; anthers ca. 0.6 mm long or shorter (except in A. canina).
4. Anthers ca. 0.8 mm or more long; lemma usually with a stiffish bent awn.
4. Anthers ca. 0.6 mm long or shorter; lemma awnless (very rarely awned).
5. Longest panicle branches less than 6 (–12) cm long and the uppermost leaf blade more than 5 cm long; leaf blades flat, the wider ones 1.5–3.5 (–6) mm broad; panicle branches forked about or below the middle, often smooth or only sparingly hispidulous-scabrous; panicle pale, greenish (very rarely tinged with reddish).
5. Longest panicle branches more than 6 cm long or uppermost leaf blade less than 5 cm long (or both conditions); leaf blades usually ± involute, the widest up to 1.5 (rarely 3) mm broad; panicle branches often not forked until beyond the middle, copiously hispidulous-scabrous; panicle ± flushed with reddish.
6. Lemmas 1.3–1.8 mm long.
6. Lemmas 0.9–1.2 mm long.