Flood protection on four South Canterbury rivers has been boosted with the planting of about 50,000 willows.

The rivers to benefit from this season’s pole planting have been the Rangitata, the Ōpihi, the Waihī and the Ōrāri, Environment Canterbury’s rivers manager, David Aires, said.

“We traditionally plant 10-15,000 willows each year in South Canterbury as part of our flood protection works. This season’s additional planting is a result of recent flooding events.”

Aires said work was carried out on 130 separate work sites on the river systems as the regional council played catch up after significant flooding events in recent years.

“This work is essential flood protection infrastructure and, as they establish, will make our southern rivers stronger and more resilient to future events.”

ECan’s river works supervisor Ben Macleod said it was the biggest planting year “we’ve had in the 12 years I’ve been at the regional council”.

“Along with our own people, at one point we had six to seven local contractors working across all our sites. It’s been an amazing team effort from start to finish.”

ECan said the project was possible through operational funding/rates, partner contributions, and central Government funding.

The willow “poles” were young tree stems, older and more robust than seedlings.

ECan said the willow trees, when mature, strengthened the region’s rivers and increased safety and resilience for river communities in the face of climate change.

Macleod said the willows were fast-growing, even in the most challenging conditions, as their dense root structures formed a fibrous mat that protected bare soil from erosion and stabilised the riverbank.

“Willows act as a buffer, slowing water flow and protecting the river edge from the full brunt of the current,” Macleod said.

“They remain one of the most reliable options for stabilising banks in erosion-prone areas or where flooding poses a significant threat.”

ECan said it used modern varieties that were sterile cultivars that had ground-stabilising benefits, without the risk of habitat loss due to invasive spread.

However, the council was aware that, in the wrong place, willows could also be a flood risk, taking up space in the active channel of a river.

“So, while we are planting willows in some places, in other places we are removing pest species like crack willow, which can spread rapidly where it’s not wanted.

“Willows are one of the best and most cost-effective forms of flood protection we have,” Macleod said, with several methods using cut willows also playing an important role.

This included anchored tree protection which involved cut willows being anchored to the riverbank with wire rope and concrete blocks, drums or railway irons, depending on the location of the site.

“The trees continue to grow and form a stable mass of plant material, which helps to stabilise the bank and prevent further erosion and slumping,” Macleod said.

Layering involved felling willows but keeping them attached to the stump.

“This allows it to put down roots along the length of the felled willow, which stabilises the bank, catches silt and debris to build the bank back up again and also creates areas of still water to slow down the erosion.”