Willows - the river bank stabilisers

by John Walsby June 1994

All around the world, wherever there has been wholesale clearance of native forest for agricultural development, subsequent problems have arisen with land stability and with rivers flooding or changing course. Wherever there is no forest cover to absorb heavy rainfall or delay run-off, water gushes down cleared uplands, stripping away topsoil and starting landslips. River channels, unable to contain sudden torrential flows, burst their banks and the lowlands developed for pasture or cropping become flooded. Frequently they are left blanketed with thick layers of silt.

Tropical countries like Bangladesh, are particularly vulnerable to these so-called "natural disasters" (which are actually not natural but the result of human interference) because they are subject to long periods of extremely heavy rain during the monsoon season. The consequences are often catastrophic but even in New Zealand major flood and landslip damage has occurred regularly since European settlers started large scale bush clearance. In the top half of the North Island, the river banks along the lower tidal reaches are protected by fringing mangrove forest but these trees cannot survive in freshwater upstream or in colder regions further south.

To stabilise river banks which had earlier been stripped of native vegetation, pioneer farmers started planting European willows; trees with long, wiry, slender branches and narrow leaves. In winter most lose their leaves and the orangy-brown bark of the most recent growth gives the bare branches a golden hue on sunny days. They have subsequently become so widespread that willow flanked waterways are now a part of the character of the New Zealand countryside that we take for granted. But it has not always been this way.

The first species introduced to New Zealand was the weeping willow, one of many species of tree willows which usually have single trunks, broad crowns and long lance shaped leaves with serrated margins. The first couple of weeping willow cuttings were planted about 150 years ago. When they grew to sizable trees they in turn became the source of cuttings of many other weeping willows throughout the country.

Tree willows (distinct from osier and shrub willows) are well suited for river bank stabilisation because their extensive root systems form dense fibrous mats above and below the waterline. These protect soft muddy banks from being eroded when rivers are in full spate.

The crack willow, Salix fragilis, was originally the most popular species as it was easy to propagate. Its scientific name alludes to the ease with which young shoots break off and its common name, to the sharp sound made when they are removed. Propagation involved nothing more than pushing freshly removed shoots into muddy ground where they quickly rooted. This made it easy to plant up long stretches of slumping river banks but it also resulted in widespread accidental plantings.

When twigs are snapped off by cattle, possums, wind or rushing flood water, they take root easily wherever they become stranded. In places this has led to serious problems when the interlaced roots and branches of maturing, self-planted crack willows have completely choked small streams.

When travel to New Zealand was only possible by sea, all willows introduced here were brought as cuttings because willow seed is only viable for a short time. Today many of the common species can still only be propagated from cuttings because they are unable to set seed here. All willows are single sexed trees and only the osier and grey willows, which have both male and female plants in New Zealand, can set seed. By chance, there are only female plants of golden, bitter and weeping willows, and only male plants of the crack and golden-weeping willows.

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