Conservation comment: Flax to control erosion

Whanganui has a serious problem with our lovely river silting up and threatening the boatbuilding activities that are a large part of our economy.

Environmentalists will be more concerned with the living organisms and fish that are wiped out every time we get heavy rain. Tourists will be put off by the muddy-looking water.

I believe we have a major conservation issue here to stop the millions of litres of toxic, yellow water running into the river from slips in our catchment area. Much of the cause of these slips was things that were done badly years ago, old bulldozed tracks up the hillsides and badly planned road cuttings.

There are still far too many slips, however, caused by landowners with no knowledge of soil conservation. We should have a system where sensitive land is zoned and all work should be checked by Horizons - certainly work like felling trees and heavy machinery work.

Horizons Regional Council has certainly helped me on my block with willow plantings and old man's beard control, etc.

One of my seemingly successful trials has been using New Zealand flax (Phormium tenax) to stop "tunnel erosion" on a hillside grazed by sheep.

My drinking-water tank on the hilltop was being threatened by continuing niggling erosion from the face below. Shelter belt trees like willows were out of the question, because high-voltage power lines pass overhead.

Out of desperation, I tried two rows of flax plants across the hillside. I had thought that flax were swamp plants and would not tolerate the dry, windy conditions. They thrived right through the two drought summers we had and have tamed the whole hillside.

Even in the June storm, this area stayed in perfect condition. I never even had to fence them off, as my sheep don't touch flax.

Pushing my luck, I also "mass planted" giant flax plants on another steep face that had slipped every year for three years. I had to replace most of this boundary fence each year.

A year after these flaxes became established that paddock never slipped again. This was one of the few paddocks facing Roberts Ave left intact after the June slips.

My latest trial is to use flaxes as sort of battens in a "living fenceline" to protect another problem slip face.

Finding flax plants is no problem, as they grow so fast and a sharp spade is all that is needed. The leaves can be trimmed off with shears or left on for tying into the fence line.

I don't know why they are so good at holding the soil together.

They don't seem to have any deep roots or large mass, but they certainly stop that water flow. If it helps to clean up our river, it has my vote!

-Rob Butcher is a conservationist, retired engineer and beekeeper.