The best option for controlling aquatic plants in a body of water is to take the necessary steps to prevent the problem from occurring. It is easier and more cost effective to prevent an aquatic weed problem than to treat one. Even when preventative measures are only partially effective, they often promote the effectiveness of other control measures.
Preventative measures include monitoring and early detection of new infestations; the use of booms and fences to prevent spread; hygienic practices when moving boats, trailers, watercraft, fishing nets and traps from one water-body to another; and proper management of a water-body and uses of its surrounding land to minimise nutrient loads and disturbances to banks and riparian vegetation.
At home, if you have aquatic plants growing in a fish tank or garden pond and you wish to dispose of them, place them on plastic and leave them to completely dry out in the sun. Never throw aquatic plants into drains, dams or water courses. Also become familiar with aquatic plants that can become weedy in your area and avoid buying these plants from nurseries and aquarium suppliers.
Monitoring and early intervention
The water-body must be inspected two or three times a week especially during the warmer months, which is the peak growing period for most aquatic weeds. Under favourable conditions salvinia and water hyacinth can double their mass every 7–10 days.
Control small infestations before the plants have reached a mature stage, when their mass can make removal difficult. Early intervention can avoid expensive labour intensive methods. Removal of small infestations by hand may be the only control that is necessary.
Water quality
Maintaining good water quality is essential for users and also inhibits the establishment of aquatic weeds.
For information on maintaining good water quality in farm dams see Farm Water Quality and Treatment available from the NSW DPI at dpi.nsw.gov.au/ag…
Aeration of water is a proven method of maintaining good oxygen levels. It has been shown to keep some aquatic plants dispersed enough to prevent them from forming a dense cover over the water surface.
Maintenance control
Maintenance control is a system of not allowing water courses, dams or any water-body to become completely covered with aquatic weeds. If allowed to get out of control all aquatic weeds can have extreme impacts on the natural aquatic environment. The importance of control, reporting and getting advice if you have an aquatic weed problem cannot be stressed enough.
An example of the devastation that can result from uncontrolled aquatic weeds is the salvinia infestation that occurred in the Hawkesbury–Nepean River in 2004, which cost $1.6 million to control.
Mechanical and physical removal
Mechanical removal involves the removal of the plant biomass from the water body using specially designed harvesters or equipment. Mechanical harvesting can be difficult in and around overhanging trees and can be expensive to implement.
Physical control includes the removal of plant material by hand. Mechanical and physical removals are often a good first option, particularly where the water is used for animal or human consumption.
There are many other advantages of physical removal.
There is less plant material to break down and provide nutrients for aquatic weed growth.
The removed material, except for alligator weed, can be recycled as mulch, provided it is used where it will not be washed back into water.
Mechanical or physical removal has the advantage that the water can be used immediately, compared to the restricted use following herbicide applications.
Booms
The use of booms to catch and concentrate floating aquatic plants and attached floating plants after they have been severed can be effective in appropriate situations. A boom can be placed across a waterway to harvest the bulk of a weed. Commercial booms can be expensive; a simple and relatively less expensive boom can be made from a piece of shade cloth and a few star posts; or un-slotted agricultural pipe fitted with a mesh sleeve and suspended on wire rope.
To be effective, booms must be checked, cleared and maintained on a regular basis.
When considering placing boom on a waterway consult with a Fisheries Conservation Manager. They can help to ensure that the design and layout does not impact on fish movement and can advise whether a permit is needed. This will also ensure the local Fisheries Officer is aware of the boom’s location, in case they receive reports of possible illegal fishing in the area.
Free floating plants
Free floating plants such as red azolla, duckweed, water lettuce, water hyacinth and salvinia begin their growth around the water’s edge. If measures are taken at an early growth stage they can be successfully removed before the growth becomes excessive.
Submerged plants
Where practical, submerged plants such as ribbon weed or cabomba can be cleared by dragging a wire rope, chain or similar device along the bottom of the water-body between two vehicles on either side. The chain or rope follows the bed, severing the plants close to the bottom, and the plant material floats to the surface where it can be removed.
Emergent plants
Emergent plants such as cumbungi (Typha spp.) and phragmites (Phragmites australis) can be cut below the waterline in autumn, but this will only retard their growth for a short time requiring an assessment of water conditions which are favaouring the weeds.
Weeds that are anchored to the banks are difficult to control by mechanical means which may destabilise the bank structure. Mowing may be used in some situations to reduce the bulk of the material. Great caution should be taken when mowing as this practice may also spread the problem. It is important to know about the plant you are mowing to ensure that this practice does not create a bigger problem.
Environmental control
Control can be achieved by altering the water-body in some way to limit the growth of aquatic plants.
For submerged plants, lowering the water level to expose them to the sun can be effective.
Dredge or excavate to a depth where the plants will not grow, or will only grow at reduced densities due to lack of light. This approach is most successful in very turbid water.
Limit the inflow of nutrients by diverting effluent from stockyards or feeding areas.
Do not allow stock direct access to waterways; provide a watering point below the catchment area. Trials have shown this increases water quality and stock health as the water is not polluted by traffic and manure.
• Provide a buffer zone around waterways and between water storages by way of long, dense grass or a strip of native shrubs and trees. This can impede or trap the movement of aquatic plants from one water source to another.
Biological Control
Biological control uses the natural enemies of the plant to attack, weaken or kill it.