COW CRESS TRIAL - KAHAHAKURI
SUMMARY
Farmers in the Kahahakuri sub-catchment have embarked on a collaborative effort to address the pressing issue of cow cress, which is impacting waterways across the catchment.
In 2023, the Kahahakuri Catchment Group secured a TLC Demonstration Grant to investigate effective options to control this invasive weed. TLC has worked closely with the catchment group on this project and has also partnered with Access to Experts (A2E), a Ministry for the Environment–supported service that connects catchment groups with freshwater specialists. Through A2E, the project has been supported by expert advice from Dr Ranvir Singh, Associate Professor in Environmental Hydrology and Soil Science at Massey University.
Through this collaborative approach, the project has moved from investigation into on-the-ground action, with a practical trial focused on suppressing cow cress while improving riparian resilience.
BACKGROUND
Cow cress, also known as water celery, is an invasive species which is similar to watercress but with a larger leaf and an unpleasant bitter taste. It is identified as a pervasive threat across the catchment, encroaching upon tributaries and streams. Farmers are concerned about its adverse effects on stream health and the heightened risk of flooding and damage due to the bulk of the weed.
It is understood that cow cress arrived in the catchment about eight years ago, but over the last five years, local farmers have observed a concerning proliferation, likely caused by milder winters, legislation around the fencing of waterways and elevated nitrogen.
INVESTIGATION
A2E support for the project has been led by Dr Ranvir Singh who undertook a number of site visits and research to assess the extent of cow cress and potential management approaches.
In May 2024, local farmers attended a presentation at the Ongaonga Community Hall where findings were shared and next steps discussed. The session also included insights from Richard Frizzell of Nelson City Council, who spoke about his experience managing cow cress in the Nelson region.
Dr Singh noted that managing cow cress is likely to be a long-term challenge requiring a coordinated and consistent approach. “Spread of cow cress appeared abundant in the visited farm drains and streams,” he said. “Cow cress appears to favour nutrient-rich environments but is intolerant to dense shading. A combination of targeted spraying, riparian planting, and mitigation of nutrient losses should help reduce and manage its growth in streams.”
Dr Singh also introduced several edge-of-field nutrient loss mitigation practices for consideration, including managed drainage, woodchip bioreactors, and runoff detainment bunds.
REPORTS
Following a presentation to farmers in the Kahahakuri catchment, a report was prepared by Dr Singh which includes potential mitigations for managing the spread of cow cress and stream bank erosion in streams and drains in the Kahahakuri catchment.
Please note: this report also contains information on stream bank erosion in the Kahahakuri sub-catchment prepared by Prof. Ian Fuller (Physical Geography) at the School of Agriculture and Environment, Massey University.
WEEDMAT AND RIPARIAN PLANTING TRIAL
Building on the research phase, the project has now moved into action with a trial using a combination of weedmat and carex plantings to suppress cow cress growth and improve riparian cover.
The trial has been designed by Nathan Burkepile from NZ Landcare Trust and will test two different weights of wool/hemp weedmat, alongside spot spraying to support grass establishment. Planting is being carried out by the team at Mauri Oho.
This trial will provide practical, locally relevant insights into methods that may help manage cow cress while strengthening stream margins over time.
WITH THANKS
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Access to Experts (A2E)
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Dr Ranvir Singh, Massey University
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The Kahahakuri Catchment Group
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Richard Frizzell of Nelson City Council
FURTHER INFORMATION & CONTACT
To find out more about the cow cress trial or the Kahahakuri sub-catchment, head to the Kahahakuri sub-catchment page on this site.
To get in touch, please contact TLC Catchment Coordinator Pixie Seligman:
Email: tukicoordinator@gmail.com
Phone: 027 721 2423