Leaves long, linear, flat, channelled, or inrolled; ligule papery or long-ciliate.
Inflorescence panicle-like, with several clusters of branches at each node, each branch subtended by a ± leafy bract. Spikelets with 1 or 2 flowers, upper bisexual, lower when present sterile or male. Glumes arranged spirally. Stamens 3–6, filaments usually persistent at base of nut, and often elongated and holding mature nut in inflorescence. Style 2–5-fid.
Note: the glumes and nuts of immature material are often much paler than at maturity.
| | Key to the species | |
| 1 | Ligule papery or membranous, narrow, glabrous or fimbriate on upper margin (hairs more or less 0.1 mm long) | 2 |
| Ligule long-ciliate or woolly | Gahnia lanigera |
| 2 | Lowest 6–12 glumes of spikelet much shorter than upper glumes (less than a third the length of spikelet), so that nut-bearing spikelet is pyriform or clavate | 3 |
| Glumes similar in length or 1–3 lowest or uppermost shorter than rest; nut-bearing spikelet not pyriform or clavate
Back to 1 | 5 |
| 3 | Stamen filaments to 20 mm long; mature nut 3.0–4.5 mm long, red-brown to dark brown | Gahnia grandis |
| Stamen filaments to 8 mm long; mature nut 2.5–5.5 mm long, orange to red or straw-coloured to dark brown
Back to 2 | 4 |
| 4 | Mature nut 5.0–5.5 mm long, 2.2–2.8 mm diam.; anthers 3.3–3.5 mm long (including apical appendage c. 0.5 mm long); spikelets 8–13 mm long | Gahnia erythrocarpa |
| Mature nut 2.5–3.0 mm long, 1.5–1.8 mm diam.; anthers 1.5–2.1 mm long (including appendage c. 0.2 mm long); spikelets 4–6 mm long
Back to 3 | Gahnia clarkei |
| 5 | Mature spikelets yellow-brown or red-brown; mature nut straw-coloured to dark brown, grey or black | 6 |
| Mature spikelets dark grey-brown to black; mature nut red, straw-coloured to dark brown, grey or black
Back to 2 | 10 |
| 6 | Spikelet with 2–5 glumes, 1-flowered | 7 |
| Spikelet with 6–9 glumes, 1- or 2-flowered
Back to 5 | 9 |
| 7 | Anthers 0.9–2.3 mm long (including apical appendage 0.2–0.5 mm long); culms erect, slender or stout but few-noded; inflorescence obvious even if leaves are numerous | 8 |
| Anthers 3.0–3.5 mm long (including appendage 0.3–0.4 mm long); culms decumbent, slender but many-noded; inflorescence inconspicuous amongst numerous leaves and leafy involucral bracts
Back to 6 | Gahnia insignis |
| 8 | Anthers 0.9–1.3 mm long (including appendage 0.2–0.3 mm long); nut 2.5–3.5 mm long, 1.5–2.0 mm diam.; glumes 4 or 5, uppermost 2 much shorter than the rest | Gahnia melanocarpa |
| Anthers 2.2–2.3 mm long (including appendage 0.4–0.5 mm long); nut 5.0–6.0 mm long, 1.0–1.5 mm diam.; glumes 3 or 4, similar in length
Back to 7 | Gahnia filum |
| 9 | Spikelet 1-flowered; nut broad-ovoid to globose, 4.5–6.0 mm long, 2.5–4.0 mm diam., dark red-brown, shining | Gahnia aspera |
| Spikelet 2-flowered; nut narrow-ellipsoid, 1.5–2.5 mm long, 0.8–1.0 mm diam., dark grey-brown, glistening
Back to 6 | Gahnia microstachya |
| 10 | Mature nut red | 11 |
| Nut straw-coloured to dark brown, grey or black
Back to 5 | 12 |
| 11 | Nut 2.5–4.0 mm long; anthers 2.7–3.2 mm long (including apical appendage 0.3–0.5 mm long); culms 5–12 mm diam., hollow; forming big tussocks to 3 m high | Gahnia sieberiana |
| Mature nut 4.0–4.5 mm long; anthers 1.9–2.5 mm long (including appendage c. 0.2 mm long); culms 3–5 mm diam., solid; forming small tussocks to 125 cm high
Back to 10 | Gahnia subaequiglumis |
| 12 | Nut 3.0–3.5 mm long; anthers 4.5–5.5 mm long (including apical appendage 0.2–0.5 mm long) | Gahnia filifolia |
| Nut 1.5–2.5 mm long; anthers 1.5–2.2 mm long (including appendage 0.1–0.2 mm long)
Back to 10 | 13 |
| 13 | Culms to 70 cm high, 2.5–5 mm diam.; spikelets with uppermost 2 or 3 glumes much shorter than the lower 2–4 | Gahnia radula |
| Culms to 45 cm high, 1–2 mm diam.; spikelets with all glumes subequal
Back to 12 | Gahnia microstachya |
