Options for weed control

Aquatic plants are natural components of healthy water systems, but they can grow out of control and become a nuisance. 

Excessive aquatic plant growth can happen for various reasons including introduced weeds invading water bodies, lack of shading and increased nutrients entering the water, eg fertilisers, run off from agricultural land. The causes are not easy to isolate but unless they are addressed you are likely to continue having problems with plant growth. 

Below is a summary of some of the ways you can manage aquatic plant growth. You may need to use a combination of techniques. 

Before using any of these methods you will need to check with your local regional council to see if a resource consent is required. 

Bottom lining 

Bottom lining involves laying weed mat or black plastic over aquatic plants. 

The advantages are:

• can be cost effective at suitable sites
• can last for many years. 

The disadvantages are:

• sediment can build up on top of the lining
• the lining may tear or disintegrate in some environments.  

Drag lining or mechanical clearance 

Drag lining involves dragging a line behind a boat along the bottom of the waterway. Mechanical clearance is where a digger is used to clear weed from the channel. 

The advantages are:

• can achieve total vegetation removal from an area
• is an effective means of keeping waterways open for drainage
• can be a cost effective way to eradicate a wide number of aquatic species for several years.  

The disadvantages are:

• can over-deepen and widen drains affecting margin habitats (causing bank instability and reduced habitat for fish)
• fish are often killed as a result
• can result in a loss of bird habitat
• not all areas may be able to be reached by machinery
• there may be an increase in sediment disturbance
• it may need to be repeated.  

Grass carp 

Grass carp are used to remove all aquatic plants.

The advantages are:

• can eradicate a wide number of aquatic plant species
• can be effective for several years
• can be cost effective.  

The disadvantages are:

• eradication of all aquatic plants in not always desirable or necessary
• grass carp require reasonable water quality for survival
• management of intermediate weed densities is rarely achieved
• removal of grass carp may be difficult and expensive
• the introduction of grass carp may affect the other species in the water body
• security measures are required to confine grass carp. 

Get more information on grass carp.

Habitat manipulation 

Various techniques are used to manipulate the water body, eg changing the water level or flow, native plantings alongside the waterway, fencing and limiting stock access to the area. 

The advantages are:

• may involve minimal cost
• can enhance natural habitats
• may provide long-term and environmentally sensitive control.  

The disadvantages are:

• may take time to be effective.  

Herbicide or chemical control

Diquat or endothal are the only chemicals registered for use in water in New Zealand. 

The advantages are:

• these chemicals are selective in the plants they eradicate and are non toxic to fish and most other aquatic life
• they have a short life in natural waters as they are degraded by microbial organisms.  

The disadvantages are:

• with holding periods for swimming, drinking water, using water for stock and irrigation
• dead plants rotting causes oxygen depletion which could lead to an algal bloom
• may need to be repeated
• may not be accepted by the public. 

Suction dredging 

Suction dredging involves using diver operated dredges modified to suck up macrophytes. The advantage of this method is that the reinvasion of nuisance plants is slow. However it can disturb the sediment in the waterway. 

Weed cutting 

Hand cutting weeds involves diving or using a mechanical cutter behind a boat. 

The advantages are:

• minimal impact on the environment as some of the habitat and bank stability is maintained.  

The disadvantages are:

• can result in total fish removal
• can be obtrusive in water bodies where the bottom is rocky and uneven
• may need to be repeated.  

Silver carp 

Silver carp are not used to control aquatic plants – they are used to control blue-green algal blooms.

Get more information on silver carp.