Weed control
We manage weed infestations to protect our communities from flooding, enhance the natural character of waterways and support native plants and animals.
Healthy waterways are central to a healthy environment. Our rivers are important places of cultural and ecological significance ki uta ki tai/from the mountains to the sea.
We control weeds with herbicides, mechanical clearance and hand clearance.
Why we control weeds
Unmanaged weed growth dramatically alters the natural character of rivers and poses a significant flood risk. Weed growth also severely impacts natural habitats, which affects the diversity and abundance of native plants and animals within the river environment.
Flood control
Ashley river after weed control
Across the region, we manage about 2,000km of waterways and 671km of drains in 59 river rating districts. Our flood protection assets are valued at $850 million (2023), and this includes 1,601km of managed river berm and 647km of stopbanks.
Left unchecked, invasive weeds in these areas are a significant flood risk.
- Weeds bind river gravels into dense islands within the channel, which can obstruct natural water flow and redirect floodwaters onto riverbanks or stopbanks. This can cause erosion and put critical flood protection infrastructure at risk.
- Weeds can also undermine the structural integrity of stopbanks and can smother and kill trees planted for flood protection.
By removing weeds, we can increase the water-carrying capacity of the river and reduce flood risks.
Read more about projects we’ve completed that highlight the importance of weed control:
- Ashley River/Rakahuri vegetation clearance project
- Rangitata River flood recovery (2019)
- Regionwide planting and berm transition
Biodiversity protection
Banded dotterel nest with eggs
Weeds are also a major threat to biodiversity, preventing the river system from naturally braiding and contributing to the loss of habitats for native fauna, including native fish, invertebrates and our iconic braided river birds.
Managing weed growth takes the pressure off our indigenous plant life, creating opportunities for native plant regeneration and supporting the restoration of natural habitats.
- Learn more about the importance of braided rivers.
- Learn more about how we protect Canterbury's biodiversity
- Protecting Waitaha Canterbury rivers from invasive weeds
- View current river works.
Climate change and invasive weeds
The National Climate Change Risk Assessment states that “risks to indigenous ecosystems and species from the enhanced spread, survival and establishment of invasive species due to climate change” is one of the top two risks to the natural environment that must be addressed urgently.
Climate change may cause increased growth due to warmer conditions and provide opportunities for weeds to expand into areas they couldn’t previously survive in.
On the other hand, a potential increase in flood events that clear out weeds in alpine riverbeds may reduce the need for spraying in more rivers in the years following events.
How we control weeds
Key priorities in our weed control work include:
- The prevention or removal of weed species that have severe impacts on braided rivers
- Targeting weed species that are highly invasive
- Consideration of the range of values being protected
- The long-term commitment of resources to maintain cleared areas.
Read our current annual spray programme report (PDF file, 1.1MB).
Herbicides
Using an unmanned aerial vehicle (UAV) for targeted spraying
Herbicides are a vital tool in our work. Time, budget, and physical constraints related to the large areas we manage mean it is often the most effective and economical method for controlling weeds along Canterbury’s waterways. However, its use is carefully considered and only used where it is the best practical option.
Methods of spraying differ depending on the area, the scale of infestation and site accessibility.
Larger-scale fairway spraying operations are typically carried out aerially by helicopter or drone, via spray boom or targeted spraying of individual trees. This technique is used where there are significant areas to cover or where access is limited. Aerial spraying is the fastest and most cost-effective method for large spray programmes.
Ground methods include using vehicle-based spray units, handheld spray guns or using a knapsack carried by the operator.
Chemicals used
Herbicide use is strictly controlled through the conditions of resource consents.
Our current consents, CRC222040, CRC222041, CRC222043, authorise us to use herbicide formulations of glyphosate (a general broadleaf weed killer) and triclopyr (brush weed killer).
To make sure we are following best practices, we asked a specialist agronomist to review our herbicide use. They found that the products we are currently using (glyphosate and triclopyr) are fit for purpose, and there are currently no better alternatives available.
Our consents allow for the use of any herbicide approved by the New Zealand Environmental Protection Agency, if identified as being a better alternative. This gives us the flexibility to adopt new and potentially better herbicides, based on specialist advice, if they come to market.
Managing herbicide risks
Herbicides are regulated in New Zealand by the Environmental Protection Agency (EPA), and there are laws around their use. Our use of herbicides is also guided by our current consent conditions.
We have developed a detailed Spray Handbook (PDF File, 545.94KB) that sets out the conditions for spray operations to manage risks, including herbicides entering the water, spray drift, and sprays entering sensitive areas such as surface water intakes, known beehives, rūnanga sensitive sites and spawning habitats.
Reducing herbicide use
Long-term, we are committed to reducing the volume of herbicides we use.
In some areas, such as drainage scheme waterways, it is possible to reduce herbicide use through riparian planting and shading. We support private landowners or community groups who want to enhance riparian planting by providing them permission and guidance for undertaking this. It is a long-term approach, as it takes time for plants to produce adequate shade for this to be effective.
We are also actively using native planting on river berms, with the natural spread of these plants helping to suppress weed encroachment. In river fairways and berms, we are already looking to more holistic river management and more strategic weed maintenance throughout the whole length of the river to reduce the seed source of weed species. This work has already started through the Braided River Revival programme of work and the regionwide berm transition project, which is one of our climate resilience programme of flood protection projects.
We have proposed the development of an Agrichemical Strategic Management Plan that would outline a pathway for a reduction in herbicide usage. This plan will be jointly developed with our partners - ngā papatipu rūnanga, with input from stakeholders.
We will also continue to monitor and adopt new technologies and tools such as biocontrol, that could be used as alternatives for widescale weed control where appropriate.
Mechanical clearance
Large pest plants (for example, willows) can require mechanical intervention to remove, using large machinery such as diggers and grapple saws. Mechanical clearing is expensive and areas left unmaintained after clearance may require ongoing intrusive works. Routine spraying can help prevent this.
This technique is also used when creating small islands within the river to enhance the area for river nesting birds. When carefully considered in a site-specific manner and employed in a controlled and managed way, mechanical clearance can improve biodiversity and habitats.
In drainage systems, we employ a range of mechanical clearance techniques, including:
- a weed-cutting boat
- an excavator with a weed rake
- line and hedge trimmers
- chainsaws
- a dragline with a grapple attachment.
Hand clearance
Hand clearance of weeds is used on a limited basis, for example, in planting sites or areas with existing native plants where access for machinery is restricted, and sprays cannot be applied due to the surrounding land use or sensitive habitats.
Some community groups and volunteers remove weedy growth from braided rivers by hand (for example, pulling small lupines out). This option is not often practical over the thousands of hectares of riverbeds we are responsible for managing, often containing mature plants that are hard to remove by hand.