1	Spikelets linear to narrowly oblong, terete, 1–4.2 cm; palea keels narrowly winged in upper half		(2)
+	Spikelets usually ovate to narrowly oblong, laterally compressed, up to 1(–1.4) cm; palea keels wingless		(4)
2 (1)	Spikelets 2.5–4.2 cm; palea distinctly longer than lemma.		1 G. acutiflora subsp. japonica
+	Spikelets 1–2.5 cm; palea as long as or only slightly longer than lemma		(3)
3 (2)	Leaf blades 2–3 mm wide; lower panicle branches 2 per node, bearing 1(–3) spikelets.		2 G. chinensis
+	Leaf blades 4–10 mm wide; lower panicle branches 3–5 per node, bearing up to 15 spikelets.		3 G. notata
4 (1)	Culms 20–50 cm tall, tufted or with slender rhizomes; leaf blades 1.5–3.5 mm wide		(5)
+	Culms robust, 50–150 cm tall, with thick spreading rhizomes; leaf blades 3–16 mm wide		(6)
5 (4)	Plant loosely tufted; lowest panicle branches bearing up to 14 spikelets; spikelets elliptic-oblong, 0.6–0.9 cm.		4 G. tonglensis
+	Plant with filiform rhizomes; lowest panicle branches bearing 1–4 spikelets; spikelets linear, 0.8–1.4 cm.		5 G. leptorhiza
6 (4)	Stamens 2, anthers 0.5–0.8 mm; plants mainly of swampy forest		(7)
+	Stamens 3, anthers 1–2 mm; plants usually of wet meadows and other open wet habitats		(8)
7 (6)	Culms hard, 5–8 mm in diam.; leaf blades firm; ligule 0.3–1 mm; spikelets pale green maturing yellowish brown.		6 G. leptolepis
+	Culms soft, 3–5 mm in diam.; leaf blades soft, thin; ligule 2–3 mm; spikelets bright green or purple tinged.		7 G. lithuanica
8 (6)	Leaf blades 3–5 mm wide; upper glume 3.5–4.5 mm, 3/4 as long as adjoining lemma or more, acuminate.		8 G. spiculosa
+	Leaf blades 5–16 mm wide; upper glume 2–4 mm, 3/4 as long as adjoining lemma or less, obtuse or subacute		(9)
9 (8)	Adaxial surface of leaf blades grayish green, minutely papillose; panicle open, branches spreading; upper glume 2–3 mm.		9 G. triflora
+	Adaxial surface of leaf blades green, smooth; panicle somewhat contracted, branches obliquely ascending; upper glume 3–4 mm.		10 G. maxima