Australia has about 22 of the 40 odd species of the genus Gahnia (saw-sedge). Gahnia is part of the sedge (Cyperaceae) family. Generally Gahnias are found in wet and boggy environments in the Pacific Islands, Australia, the Malay Archipelago and southern Asia. All are perennial herbs with grass-like leaves and tiny flowers. The leaves often have fine ‘saw-teeth’ margins which can inflict nasty cuts if handled carelessly. The flowers are produced on numerous blackish spikes at the end of leafy inflorescences. The flowers are followed by, usually, red to black nuts.
All the Gahnia family provide favoured nesting sites for small birds.
Gahnia sieberiana or red-fruited saw-sedge is a large, clumping variety found through most of Australia and also in New Guinea and New Caledonia. The stems may reach 3 metres in height. The narrow leaves are rough and often pendulous and up to 2 metres long. Brown and cream flowers are followed by shiny, egg-shaped nuts which are a red-brown colour. It is a hardy plant, suited to moist, sunny conditions. It is a good choice for seaside gardens and is not affected by frost.
Another of the clumping species is Gahnia aspera or the rough-leafed saw-sedge which grows to 0.9 metres and has a spread to 1.2 metres. It is common along the east coast of Australia and is also found on the islands of the South Pacific, in Malaysia and in New Guinea. It frequents damp areas and has narrow leaves with rough undersides. The flowers appear in summer densely clustered on stalks. The red-brown nuts are glossy with a small pointed end.
The tall saw-sedge (Gahnia clarkei) grows to 3 metres and has a similar spread. It is found along the banks of creeks and in damp gullies along the east coast of Australia. The leaves are narrow and have sharp edges. The leaves are up to 3 metres long. Flowers are produced in summer followed by fruits in late summer to autumn. The glossy nuts are a red-brown with small pointed ends.
Gahnia species require an open sunny position and moist acid soil. Like many native plants, some are intolerant of excessive phosphorous. Although some species are ornamental, there has been little cultivation of these grasses as additions to home gardens. Propagation is by division or from seed. Although seed is generally easily gathered, germination can be difficult. Smoke treatments may be applied to seeds from drier habitats and this often improves germination rates.