COW CRESS TRIALS- KAHAHAKURI | Tukituki Land Care

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.

KAHAHAKURI COW CRESS
MITIGATION REPORT

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.

DOWNLOAD REPORT

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.

Results will be shared once the trail is complete.

August 2026 Update

We are now starting to see the results of the planting trial. Sedges planted in 2024 are well established and have reached a good size, while those planted in 2025 continue to grow well.

The mountain flax planted in 2025 remains relatively small, so this year we are trialling swamp flax harvested from mature plants elsewhere on-farm. This could provide a cost-effective option for landowners wanting to establish larger plants without the expense of buying mature nursery-grown plants.

The harvested swamp flax has been planted on a flatter ‘beach’ area alongside the stream. This site was considered suitable because the stream does not generally experience the extreme flooding or high water flows that could topple larger flax plants before they become established.

Carex geminata has also been planted on a steep, bare bank opposite the established sedges. The aim is to establish vegetation on both sides of the stream, providing greater shading of the waterway as the plants mature.