Rakaia Gorge weed control project off to a great start

The first stage of a major weed control project in Rakaia Gorge is progressing well, with some impressive gains in the first 12 months.

The four-year operation will help protect native biodiversity and significant ecosystems from being infested with exotic weeds. The initial focus is on the control of cherry, sycamore, wilding pines and cotoneaster across key sites within the gorge area.

The project is a collaboration between us, Selwyn District Council, landowners, Manawa Energy's Rakaia Fund - administered by the Rakaia Catchment Environmental Enhancement Society, and Toitū Te Whenua Land Information New Zealand.

Ashburton District Council has also contributed by removing several old-man pines to curtail wilding spread into the wider project area.

A joint effort

Throughout the 2022 season, the team collectively logged more than 1,500 hours on the tools, controlling well over 12,000 cherries and sycamore.

More than $133,000 of funding from the various agencies was delivered in the 2021/22 financial year.

Several weed-control methods were proposed for the project - with a "drill and fill" treatment selected as the preferred option, due to its reduced impact on surrounding vegetation. Holes were bored into the base of the trees and filled with chemicals.

It was also agreed that the felling of large seeding trees near the Rakaia Gorge Walkway was the best course of action for that particular area, as dead-standing trees would pose a significant risk to recreational users in the future.

Technology lends a hand

Rakaia Gorge is a hazardous environment. Its steep terrain and bluffs make it a challenging area to safely carry out a control operation.

To help mitigate these risks, a high-tech drone was used to survey the land beforehand, providing an insight as to where the biggest hazards were. The drone survey also gave the team a better understanding of the density and location of specific weed species, which helped with a targeted approach.

The drone flight was carried out during autumn, when the species charged colour (for easy identification) and it will double as a monitoring tool to measure changes in canopy composition over time.

Garmin GPS units kept track of the work completed and helped avoid double-ups of coverage, or areas getting missed.

Progress towards protection

An ecological assessment of the gorge in 2019 found at least 16 threatened and at-risk native plant species, including Canterbury pink broom and rōhutu/myrtle - both nationally critical - along with others that are regionally uncommon.

Controlling the large, shade-tolerant weed trees will help stop them from outcompeting the native species.

Project lead Billy Bartrum is thrilled with the progress so far.

"12,000 cherries and sycamore controlled is a tremendous effort. This work couldn't have been achieved without everyone’s contribution to this project. I would like to extend a huge thank you to the landowners and agencies involved."

Selwyn District Council project lead Andy Spanton agreed.

"2022 represented the start of this important programme and as such, will give the project team essential information that will be used to help guide the project over the next few years.

"We are in a good place now to plan for operational activities in 2023 and 2024. By that time, we should have a clear idea of the further resource required to complete the project."

Next steps

It's likely that a key focus for the next control season will be on the cotoneaster that dominates a large area of land in a site referred to as 'Zig Zag', adjacent to the Rakaia Gorge Walkway.

Landowner David Harper is also working on a control operation for this site, involving the stands of pines within this area, and wetland enhancement works on his property. 

He said he's proud to be involved in this project and is grateful to everyone involved.

"The Rakaia Gorge holds a special place in our hearts. We feel very excited that this is just the beginning of looking after this very important place. This will be something that future generations will carry forward."