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 is a significant flood risk. By severely impacting habitats, it affects the diversity and abundance of plants and animals in the river. 

Flood control

Across the region we manage about 2,000km of waterways and 671km of drains. This includes 1601km of managed river berm and 647km of stopbanks protecting a total asset value of $850 million across 58 river rating districts. Left unchecked, invasive weeds in these areas are a significant flood risk.

There are three main locations where we spray for flood protection and drain maintenance.

  • Fairway spraying controls the growth of weeds in the active part of the river channel called the ‘fairway’, which carries fast flowing water during floods.
  • Berms are the buffers along each side of the fairway. Weeds are sprayed here as they kill trees planted there for flood protection. Berm spraying also includes maintenance spraying of stopbanks and access tracks.
  • Weed control in drainage schemes is needed because weeds restrict the flow of water, impeding drainage and causing localised flooding.

Ashley River/Rakahuri vegetation clearance project 

The Ashley River/Rakahuri is a good example of the importance of weed control, as this waterway had no weed management prior to our extensive Ashley River/Rakahuri vegetation clearance project funded by Climate Resilience and Flood Protection funding. In March 2021 we undertook aerial spraying from Ashley Gorge to about 1km east of Cottles Road (about 536 ha).

During the May 2021 flood event, the vegetated island in the areas that hadn’t been sprayed resisted erosion, deflected flows into the river berm and banks, and the riverbeds remained covered in weedy growth, diminishing flood flow capacity.

In comparison, the areas that had been sprayed suffered far less damage, and the weed cover that was on those beds was able to scour out, leaving clear gravel behind.

Biodiversity protection

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 some of our iconic braided river birds which need clear gravels to nest and raise their chicks.

The diversity of aquatic habitats is also impacted by weed infestations that can reduce the variety of native and sports fish that occupy our rivers.

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

We apply some key principles in our approach to weed control. Priorities 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.


Using an unmanned aerial vehicle (UAV) for targeted spraying

Using an unmanned aerial vehicle (UAV) for targeted spraying

Herbicides are a vital tool for managing weeds at a landscape scale. Methods of spraying differ depending on the part of the waterbody that needs weed control, the scale of weed infestation and accessibility to the site. Time, budget, and physical constraints related to the large areas we manage mean that herbicide use is the most effective and economical method for controlling weeds in Canterbury’s waterways.

Herbicide use is carefully considered and only used where it is the best practical option for controlling weedy vegetation.

Larger-scale fairway spraying operations are typically carried out by helicopter via spray boom or targeted spraying of individual wilding trees. This technique is used where there are significant areas to cover or where access to the gravel islands is limited. It is the most time-consuming and cost-effective method for large spray programs. We also apply herbicides to stopbanks, tracks, berm areas and some fairways via ground-based handheld spray guns. Ground methods include using a spray unit hitched to a truck or tractor.

Small-scale targeted spot spraying operations within drain banks or dry drain beds are done using a knapsack carried by the operator. We also use a specialist unmanned aerial vehicle (UAV) for targeted spraying in drainage networks.

Other methods we use include mechanical clearance and hand clearance, but it is not feasible to meet our current flood protection and biodiversity obligations across the 2,000km of rivers and 671km of drainage channels within our 59 river rating and drainage districts across Canterbury without using herbicides.

Herbicide regulation in New Zealand

Herbicides are regulated in New Zealand by the Environmental Protection Agency (EPA) and there are laws around their use. Find out how the EPA regulates herbicides and how it checks that they are safe to use.

Chemicals used

Our current consents (CRC981580 and CRC041535) 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 undertake an independent review of the herbicides we currently use. They found that the products we are currently using (glyphosate and triclopyr) are fit for purpose, and there are no better alternatives available now, nor are there likely to be any available soon. 

Managing potential risks from herbicides

We follow recommended safety precautions including:

  • Wearing personal protective equipment, such as gloves, goggles, and boots.
  • Using sprays in calm and dry conditions.
  • Using herbicides in recommended concentrations.
  • Storing and disposing of products correctly.

Our use of herbicides is guided by the manufacturer’s recommendations, Environmental Protection Agency (EPA) rules as well as our current consent conditions and other regulatory rules. These work practices and rules along with information from rūnanga, and stakeholder groups are combined into a Spray Handbook and guide for the use of herbicides (PDF file, 545.94KB).

Preventing herbicides from getting into waterways

We have developed a detailed Spray Handbook (PDF File, 545.94KB) that sets out the conditions for spray operations to ensure that the potential risk of herbicide contamination in surface water and groundwater is managed.

The handbook sets out measures to avoid herbicides entering water as much as practical, avoiding sensitive areas (for example by avoiding areas around schools, campgrounds, dwellings, drinking water intakes, other surface water intakes, known beehives, rūnanga sensitive areas and spawning habitats etc) and applying herbicides within strict limits.

Managing spray drift

When using helicopters or an unmanned aerial vehicle (UAV) for aerial spraying, nozzles on the spray boom can be adjusted to manage spray drift risk. Additives can also reduce the risk of spray drift.

Aerial spraying will be avoided at certain sites including schools and preschools, dwellings, marae or campgrounds, areas where spray drift may affect organic farms and areas where spray may drift over flood protection vegetation, over water or into non-target vegetation.

Spraying is usually done early in the morning when winds are the lightest. Wind speed and direction are carefully monitored and spraying stops when conditions are likely to cause the spray to drift outside of the target area.

Water quality monitoring

The existing resource consents set limits for herbicide residues in water resulting from spray operations. To determine if water quality standards are being met, we carry out water quality monitoring at a minimum of six representative sites across Canterbury.

Long-term plans to reduce the use of herbicides

We are committed to a long-term reduction in the volume of herbicides we use. Some initiatives include the following:

Monitoring and adopting new technology and methods

We will monitor new technologies and tools such as biocontrol that could be used as alternatives for wide scale weed control where appropriate.

Lower Waitaki River riparian planting

An example of riparian planting

Riparian planting and shading

In drainage network waterways, it is feasible to reduce or even phase out 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 on how that can be undertaken alongside maintenance of the drainage function.

This is a longer-term solution for the 671km of drains that we maintain, as the plants need time to establish and grow to provide adequate shading to the stream.

Holistic river management

In river fairways there may be opportunities for holistic river management and more strategic weed maintenance through the whole length of river in an aim to reduce the seed source. This work has already started through the Braided River Revival programme of work and the “berm transition” project which is part of our climate resilience programme of flood protection projects.

Establish an Agrichemical Strategic Management Plan

We propose 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.

Mechanical clearance

If riverbeds become choked with weeds, typically the only resolution is mechanical intervention – which can be more intrusive to the river and associated ecosystems, and very expensive for local rate payers. 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.

Hand clearance

Hand clearance of weeds is used on a limited basis, for example in the Halswell Drainage Scheme where access for machinery is restricted, and sprays cannot be applied due to the surrounding land use or sensitive habitats.

Some volunteer groups remove weedy growth from braided rivers by hand (ie pulling small lupines out).

This option is not practical over the thousands of hectares of riverbeds currently managed, often containing mature plants that are hard to remove by hand.

Our consent to use herbicides

Herbicide use is strictly controlled through the conditions of resource consents.

Our current consents (CRC981580 and CRC041535) 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 undertake an independent review of the herbicides we currently use.

They found that the products we are currently using (glyphosate and triclopyr) are fit for purpose, and there are no better alternatives available now, nor are there likely to be any available soon. 

Current stage: An application to renew these consents is in process and a hearing is scheduled to begin on 25 March. Until a decision has been made on the new consent application, we can continue to operate under the existing consent conditions, allowed for under the Resource Management Act 1991.