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Gahnia species
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This article was published in The Fringe, July 2016

Cutty Grass or Gahnia - species host for Forest Ringlet Butterflies

Cutty Grass or Gahnia is the essential host species for the rapidly disappearing forest ringlet butterfly. Gahnia are where the adult female of the rare forest ringlet butterfly (Dodonidia helmsii) deposits her eggs. It is the food source for the emerging caterpillars. If we are ever to have our native butterflies in our cities we need to provide the host plants for them to feed on, even if the plants are lacerating if you approach too closely.

This year the Moths & Butterflies Trust of NZ is embarking on a campaign to save NZ's forest ringlet. It is only found in New Zealand, is the only one of its kind in the world and, sadly, it's on the decline.
With your help we can reverse this situation. Join the MBTNZ's special campaign they will be launching this month.

The April 2013 Fringe article was about Gahnia and it mentioned that the forest ringlet depended upon the various plant species. This article provides a bit more detail.

Most New Zealanders know and are cautious of cutty grass for obvious reasons - it cuts. Technically speaking, however, Gahnia species are sedges not grasses. In New Zealand we restrict the 'cutty grass' label to Gahnia species, despite both toetoe and pampas being true grasses which can also cut.

There are 6 species of Gahnia. In the Waitakere Ranges three of them are common and one, Gahnia pauciflora, is less commonly found there. Of the two species that do not occur within the Waitakere Ranges, Gahnia procera is found in mountains from Coromandel south, and Gahnia rigida is found in the Central North Island and the top of the South Island.

For much of Blockhouse Bay, Green Bay and Titirangi it is Gahnia lacera, or coastal cutty grass, that is most common. This description has been adapted from one by Dr Rhys Gardner, an eminent Auckland botanist and erudite 'Westie'.

"The 1 - 2 metre coastal cutty grass, Gahnia lacera, is easily found by those on the coast as it is abundant in most pieces of dryish coastal forest and scrub here. Gahnia lacera is different from the dense tussocky growth of the other species in that it forms comparatively open colonies like 'a small yellow green bamboo' with the new rhizome segments being more or less horizontal and breaking through the bases of the scale leaves that enclose them. The upper surface (adaxial) of the leaf blade is hardly if at all ridged and is quite smooth (i.e. papillae and teeth are lacking). Notably, a distinct straw-coloured midrib is present on the adaxial surface, a feature which none of the other NZ species have. The small black nut of Gahnia lacera is also very distinctive."

The name 'lacera' suggests it is the most lacerating of the species, yet it is pleasant to handle and walk through and scarcely cuts even when put to the test.

The two other common species in the Waitakeres are much larger, standing 2 or 3 metres tall when flowering, and the foliage is equally long but droops elegantly, being longer and sharper than Gahnia lacera.  Probably in the Waitakere Ranges the most common is Gahnia setifolia which tends to grow on the poorer soils such as on ridges where impoverished clay soils predominate, which can be wet in winter but are particularly dry in summer. The second common species is Gahnia xanthocarpa which prefers richer soils, generally found in the wetter valley floors, swamps and bogs. They are very similar in appearance but generally G. xanthocarpa has dark, glossy green leaves and glossy black seeds, whereas G. setifolia is a paler green and its seeds are a reddish brown colour. All of which is totally confusing as xanthocarpa means yellow fruit. And there is not much that is bristling about the leaves of setifolia, although the name means bristle leaves.

So why should I encourage you to plant such vicious plants? The answer as always is that they are the natural species for the area, and deserve a place in the garden. But more than that, they are part of the ecosystem which occurred prior to man's arrival and in most of suburbia we have totally displaced them. If they go, so do the many species that depend upon them, such as the copper butterflies that flit so elusively in the sunlight, and the forest ringlets which will have to be reintroduced to the Waitakeres from elsewhere, as they have disappeared from our rainforest and now are generally seen only in beech forests.

Plant some cutty grass, join the MBTNZ and bring back the ringlets.






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