Friday, September 24, 2010

Golly, it’s gorse!

I’ve known gorse a while. I don’t remember much of it about when I was growing up in Palmerston North but when we made our infrequent trips to Wellington there it was, clothing the hills in a yellow blanket, for it flowered much of the year. Wainuiomata I think was gorse’s spiritual home, it’s Turangawaiwai.

Gorse was an early introduction, for hedging. It does this remarkably well. It grows rapidly in all manner of climates, it can be cut back mercilessly and still thrive and within a couple of years it forms an impenetrable barrier to stock. Those are the good things about it, added of course, to its suitability for honey and pleasant smell.

The bad thing about gorse is its ability to spread. At one year old it flowers and the seeds can stay viable in the soil for forty years or longer. Because of its rapid growth and unpalatability and tolerance of disturbance, drought and flood, it colonises habitats such as wetlands, tussock grasslands and riverbeds where it rapidly displaces the native vegetation.

The problems with gorse would have been recognised within a few years of its arrival but control techniques were limited then, and once it had a toehold, its spread was speedy and predictable. Cutting, grubbing and burning, work to some extent but continual follow-up work is needed as cut stumps sprout and the seed in the soil germinates in the high light conditions and fertile, black ash following a fire.

Rapid planting with other species such as pines or poplars may limit gorse a little but no native species will outgrow it in its early years. Up in the far north gorse is spindly and weak. It’s too warm there for it to thrive but it does shelter native seedlings which soon overtop it. In Hinewai Reserve on Banks Peninsula the gorse-covered hillsides were left untouched, much to the surprise of many involved with habitat restoration. This proved the right answer because natives have come through the gorse which dies off as it is shaded out.

Gorse is a good nursery crop there but is this true for Southland? Massive, dense and lightless gorse forests seem to exclude native seedlings and add decades of gorse seeds to the environment before anything like a break in the canopy allows a native to sneak through. The succession may work eventually but tolerating gorse means that it spreads far from the uncontrolled area in the meantime. One place where gorse has been replaced is at Omaui. The council reserve had been cleared of bush and converted to pasture. Gorse took over but as you walk up to the lookout you can see where manuka has beaten it. Huge gorse trees five metres high and 20cm through the trunk are struggling to keep their foliage in the canopy. Everywhere native seedlings are growing where the manuka canopy has started to break up and eventually forest will return.

Very old gorse at Omaui, struggling to
keep up with the surrounding manuka

The Gorse weevil was imported in one of New Zealand’s first efforts at biological control. The weevil lays its eggs in the seed pod and the growing larvae devour the seeds. When the mature pod bursts open it flings little weevils everywhere instead of gorse seeds. If an insect is capable of experiencing fright, this surely a good test. You are settling down to a quiet evening in the pod with your brothers and sisters. Someone is telling spider jokes, someone else is weevling a wall-hanging and another is reading ‘A History of Beetles, a who’s-huhu of the Coleoptera’. A framed homily on the wall says, ’See no weevil, Hear no weevil, Do no weevil’, the latter referring to the strict code of ethics observed by weevildom. Another decoration has the national anthem ‘Wee vil overcome’ – not a vain sentiment considering ten percent of insects are weevils.

Suddenly, with no warning apart from a couple of creaks, the home cracks in half and weevils, furniture, toothbrushes, everything is shot out into space. Soaring through the air like miniature Weevil Kneivels, the family scatters. Post traumatic shock syndrome takes most of them but the survivors – pale, unsettled and showing a range of addiction problems – eventually pair up and lay eggs in a new pod, but perish with the first hint of winter.