Dialogue: Wild kiwifruit worse than an alien invasion

By ROSALEEN MACBRAYNE

They creep relentlessly across our land and waterways. No, the alien invasion is not triffids but unwelcome weeds threatening the environment.

The noxious nuisances take root surreptitiously. And along with the silent aggressors, such as woolly nightshade and old man's beard, are the wildcards, such as kiwifruit.

Yes, indeed, the furry berry that has been such a bonanza, earning New Zealand hundreds of millions of dollars a year, has spread its tentacles well beyond the orchard gates.

Left unchecked, kiwifruit vines can run rampant and destroy native bush and forests. The rogue variety springs up mainly from seeds spread by waxeyes, blackbirds and rosellas. It grows vigorously around Te Puke, near old-established orchards that were at the forefront of the commercial evolution of the famous fruit three decades ago.

Over the years more and more kiwifruit are germinating where they shouldn't be - and they are notoriously difficult to kill. Hardy wild vines flourish in bush, pine plantations and scrub areas, particularly around rural localities in the Bay of Plenty, where 80 per cent of the country's commercial crop is grown.

In some places, 25-year-old vines are well-established in native bush, growing up to 20m tall and as thick as a man's leg. Chop them down with a chainsaw and they grow again.

The search is on for the most effective, environmentally acceptable and economical method of eradicating wild kiwifruit. Several herbicides have been tested in Taskforce Green control trials, and a 10-week control programme is under way in the Te Puke area.

Community cooperation can play an important part in halting the march of the free-growing fruit.

The Bay of Plenty regional council, Environment BOP, reminds farmers who collect reject kiwifruit for stock to avoid feeding out excess amounts. If birds are able to mass-feed on piles of soft fruit, vines can be expected to establish in bush or forests through the seeds carried in bird droppings.

Kiwifruit lies dormant in pine blocks and germinates in large numbers when light and temperature conditions increase at logging time. In native bush, kiwifruit plants seem to prefer warm slopes and stream banks.

Farmers are urged to be sensible and cover large piles of reject kiwifruit with material such as wind-mesh. They should feed out only what stock can eat at one time.

Weed introductions are surprises to most people. Unwanted plants are not usually brought in intentionally and their establishment can be avoided.

Alligator weed is another plant pest that can be restricted with a bit of common sense. Since 1996, it has been identified at five sites throughout Bay of Plenty and might have been introduced with other plants intended for wetland development.

Anyone bringing wetland plants from outside the region should check where they have come from and ensure no foreign matter is attached.

Alligator weed, native to South Africa, can grow well either in an aquatic or terrestrial environment. An extreme risk to canals and drains, the weed forms extensive floating mats which can effectively cover a water surface. Its prolific growth restricts water flow, increases sediment, aggravates flooding and acts as a barrier, accumulating debris that would otherwise be flushed downstream.

These are but two examples of a wealth of unwanted weeds that have been able to colonise parts of the country through human carelessness.

To get rid of them once they have invaded is a difficult and expensive exercise. Better to think first, and nip them in the bud before the mass migration can begin.