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 | 12 |
| 6 | Slender decumbent culms, often rooting at the nodes, with short, slender leaves; anthers 3.0–3.5 mm long including apical appendage; glumes 2 or 3 | Gahnia insignis |
| Erect culms forming tussocks, with broader leaves sometimes exceeding the inflorescence; anthers 0.8–2.5 mm long including apical appendage [anthers of G. howeana not known]; glumes 3–9
Back to 5 | 7 |
| 7 | Stamen filaments strongly elongated (10–35 mm exserted from glumes), persistent and obvious at fruiting stage; culm diameter 5–10 mm; Lord Howe Island endemic | Gahnia howeana |
| Stamen filaments not markedly elongated (1–5 mm exserted) and not obvious at fruiting stage; culm diameter 2–5 mm; mainland NSW species
Back to 6 | 8 |
| 8 | Culms 15–65 cm long; glumes 6–8 | 9 |
| Culms 60–250 cm long; glumes 2–6
Back to 7 | 10 |
| 9 | Spikelets 5–8 mm long, with uppermost 3 glumes much shorter than the lower; inflorescence 10–35 cm long; culms 2–4 mm in diameter | Gahnia aspera |
| Spikelets 1.5–3 mm long, with all glumes subequal; inflorescence 20–60 cm long; culms 1–2 mm in diameter
Back to 8 | Gahnia microstachya |
| 10 | Involucral bracts longer than inflorescence; culms 4–5 mm diam.; anthers 0.9–1.3 mm long (including apical appendage 0.2–0.3 mm long); chiefly rainforest areas on coast and tablelands | Gahnia melanocarpa |
| Involucral bracts shorter than inflorescence; culms 2–4 mm diam.; anthers 1.7–2.7 mm long (including apical appendage 0.3–0.5 mm long); usually in coastal saline or brackish swamps, often fringing saltmarsh
Back to 8 | 11 |
| 11 | Nut 5.0–6.0 mm long, 0.8–1.5 mm diam., linear to very narrow-ellipsoid, triquetrous, with surface very finely granulate, glistening; spikelets 6–8 mm long, with 2–4 glumes; leaves and involucral bracts smooth to finely scabrous | Gahnia filum |
| Nut 2.0–2.6 mm long, 0.8–1.0 mm diam., narrow-obovoid to narrow-ellipsoid, trigonous, with surface smooth and minutely reticulate, shining; spikelets 4–5 mm long, with 3–6 glumes; leaves and involucral bracts coarsely scabrous
Back to 10 | Gahnia trifida |
| 12 | Mature nut red | 13 |
| Nut straw-coloured to dark brown, grey or black
Back to 5 | 14 |
| 13 | 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 12 | Gahnia subaequiglumis |
| 14 | 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 12 | 15 |
| 15 | Culms to 70 cm high, 2.5–5 mm diam.; spikelets with uppermost 2 or 3 glumes much shorter than the lower | Gahnia radula |
| Culms to 45 cm high, 1–2 mm diam.; spikelets with all glumes subequal
Back to 14 | Gahnia microstachya |
