This article was published in The Fringe, August 2013
Cook's Scurvy Grass - Nau
During Captain Cook's first circumnavigation of the world (1768-1771), he anchored in the Marlborough Sounds and sent a small crew ashore to reconnoitre for a source of fresh water. In addition, they had instructions to bring back some green vegetable foliage suitable for making into a salad, which Cook then fed to his officers. The crew, seeing 'favoured' treatment being handed out to the 'Upper Decks', wanted some for themselves, and so Cook was able to induce the crew to eat their greens without making an issue of it. Amongst the range of plants brought back there would almost certainly have been the true native puha (Sonchus kirkii), native celery (Apium prostratum), native spinach (Tetragonia tetragonioides), a native cress (Rorippa divaricata) and a second cress-like plant that has become known as Cook's Scurvy Grass (Lepidium oleraceum) or nau.
Nau is not a grass, but a dicotyledonous herb growing to about half a metre by one metre. It is predominantly a cliff dweller growing around areas where seabirds roost or nest, ensuring the plant gets a continuing supply of good rich guano.
As human activity increasingly impacted the coastlines around the country, the occurrence of both seabirds and Lepidium oleraceum has declined to the point of being extirpated along most mainland areas. Now, largely confined to outer islands, the Cook's Scurvy Grass is not making the return to mainland sites that some of the other species appear to be achieving.
Sixteen exotic species of Lepidium have been identified as established in the wild, and recent studies have identified a total of 16 native species of which two are now considered extinct. The widely distributed forms of Lepidium oleraceum occur from the Kermadec Islands, through coastal North Island, to Westland/Marlborough and the Chatham & Subantarctic Islands.
One of the extinct species was an endemic to the Waitakere Ranges coastline from Karekare to Whatipu and it is sad to relate that one of the primary causes of its demise may well have been over-collecting by the director of the Auckland Museum, Thomas Cheeseman. His collecting of this particular Lepidium was probably as a result of his recognition that it was somewhat different from collections of other Lepidium species he had, but he never formally recorded the form. The chances of the newly-named but extinct species Lepidium amissum being re-found are slim, but it is entirely possible that somewhere along the rugged coastline there is a last remaining plant struggling to compete with more aggressive species, both native and exotic.
In addition to the lack of fertile sites and competition from weeds, the Lepidium species is threatened by the fungus-like white rust disease, Albugo candida, and a range of introduced pests and diseases, including snails, aphids, leaf miner, diamondback moth and cabbage white butterfly. They are also greedily consumed by cattle and other livestock.
The most recent, and perhaps most serious threat to their survival is the accidental introduction to the Nelson region of the great white butterfly which has greater potential to spread and devour isolated populations of Lepidium than even the common white cabbage butterfly.