The giant species G. maxima (Hartm.) Holmb. is known from adjacent parts of Ontario and may enter Michigan at some point. It is very aggressive, capable of forming large monodominant stands 3–4 m tall from coarse rhizomes. It would key to G. grandis, but differs in having stiffer panicle branches, scabrous lower sheaths, and larger glumes (the lower ca. 2–3 mm long). Glyceria grandis has lax panicle branches, smooth lower sheaths, and shorter glumes, the lower less than 2 mm long.
1. Spikelets (7–) 9–31 (–38) mm long, linear-cylindric, on erect, straightish, strongly ascending pedicels mostly shorter than the spikelets; ligules mostly 6–16 mm long (often infolded and/or strongly lacerate apically).
2. Lemmas acute at the tip, exceeded ca. 1.5–2.5 mm by the palea.
2. Lemmas obtuse at the tip (sometimes the very tip irregularly erose or dentate, but the outline rounded), at most exceeded 0.5 mm by the palea.
3. Back of lemma smooth or nearly so, at least between the nerves; anthers 0.5–1.1 mm long; grain 1.2–1.6 mm long; wider leaf blades 2–5 (–9) mm broad.
3. Back of lemma scabrous to hispidulous; anthers 1–1.6 (–1.8) mm long; grain 1.8–2.6 mm long; wider leaf blades 5–11 mm broad.
1. Spikelets 2–6.5 (–7) mm long, ovoid to oblong or short-cylindric, mostly on spreading, lax, or strongly undulate pedicels often longer than the spikelets (except in G. melicaria); ligules 0.3–5 (–6.5) mm long.
4. Lemmas smooth, not corrugated (the nerves visible but not strongly raised), the larger ones (2.8–) 3–3.5 (–4) mm long; spikelets becoming 3–5 mm broad at maturity.
4. Lemmas with prominent raised nerves, giving a ± corrugated appearance, the larger ones 1.5–2.6 mm long; spikelets becoming 1.5–2.7 mm broad at maturity.
5. Ligules less than 0.8 mm long; panicle branches (even at maturity) strongly ascending or appressed to the axis; larger (second) glume ca. 2.1–2.5 mm long.
5. Ligules over 1 mm long; panicle branches (except when very young) widely spreading; larger (second) glume various.
6. Larger (second) glume 1.6–2.2 (–2.5) mm long; smaller (first) glume 1.2–1.7 (–1.8) mm long; anthers ca. 0.7–1 mm long.
6. Larger (second) glume less than 1.5 mm long; smaller glume less than 1 (–1.2) mm long; anthers ca. 0.3–0.5 mm long.