Mulching the Waipoua River

Mechanical tree-trimming and mulching is standard maintenance for hedges and an efficient way for trimming willows that are hanging over the Waipoua River in Masterton.

“A tree-mulcher operating in the middle of a river is not what most people would expect to see,” says Graham Reidy, Greater Wellington flood protection engineer, based in Masterton.

“Although it might seem a bit novel, we will be asking people stand well back to avoid the risk of getting hit by a stray chip or piece of branch.”

Over the week of March 22 to 26 a tractor with a trimming and mulching arm will be working in the Waipoua River from the railway bridge to the Colombo Road Bridge, trimming the riverside willows to keep them at a useful size for protecting the banks from erosion.

“Willows are a great tree for holding riverbanks together,” says Mr Reidy, “but they need to be well maintained.”

“If they get too big and top heavy they start leaning over the river and getting weak in the roots. The next thing you know they are falling into the channel and the river is eroding the bank behind them and putting the stopbanks at risk.”

Mr Reidy says using a mulching arm on a digger has proved to be a quick and efficient way of trimming willows in shallow rivers.

“The machine trims and mulches the branches at the same time. Most of the debris falls on the bank, while some falls in the river and is washed away in the next flood.

“We’ve been successful with this technique to maintain trees in the Mangatarere River, west of Carterton, and also in the Monganui in South Wairarapa and it has been used to maintain trees along side the Manawatu River around Palmerston North.”

Mr Reidy says the alternative is for staff to trim the trees with chainsaws and have the branches dragged back onto the bank, stacked into piles and burned, which takes more time and money and tends to make more mess.