Introduction

Aquatic weeds grow profusely in most areas of water, but are particularly troublesome where run-off water containing high levels of plant nutrients feed them. Examples include run-off from golf courses and fertilised pastures, and various kinds of effluent.

Small dams present special problems for vegetation control because they are often used for several purposes. The one dam might provide water for stock, gardens, household needs, recreation, fish and wildlife, therefore extreme caution is necessary when removing vegetation from such waterways or applying herbicides to these. It is common to regard all vegetation as undesirable in small dams, but the vegetation may well be vital in the ecology of the dam, in stabilising banks or trapping silt around the edge of the dam.

Where herbicides are to be applied, the treatment must be carefully selected so that it will not kill fish and wildlife or damage gardens. It might be difficult to find a treatment that does not interfere with some of the uses of the dam.

Any control program will generally need to be repeated over time to be effective. Therefore, before committing yourself to a control program, weigh up the potential benefits against the effects of weed removal, the cost of treatment and, if herbicides are to be used, the possible side effects of using herbicides. Sometimes it is better to do nothing, rather than upset the fine balance in the ecosystem and be faced with a continuing control program.

Identifying the problem

To select the most appropriate control program, it is essential that the problem is correctly identified. Frequently, several species of weeds are present. If one species of weed is removed this might simply lead to the proliferation of other, possibly less desirable, species. The most common aquatic weeds can be divided into four groups:

• Free-floating weeds - salvinia, water hyacinth, duckweed (shown at right), red azolla and water lettuce (shown below left).
• Submerged weeds - lagarosiphon, ribbon weed, elodea, pondweeds and milfoils (red milfoil shown below right).
• Emergent weeds - milfoils, cumbungi, common reed, pondweeds, sedges and rushes.
• Bank weeds - alligator weed, water couch and ludwigia.

Mechanical control

If dam water is used for human or animal consumption or for irrigating a garden or crops, chemical control is often undesirable or not possible. Mechanical control could be the better alternative. The physical removal of plant material deprives any remaining plants of substantial amounts of nutrients and thus regrowth is slower. Also, this method of control, unlike chemical control, does not threaten fish life because it does not deplete the oxygen levels. (The breakdown of vegetation that follows chemical treatment will deplete oxygen levels.)

Floating plants, such as salvinia and azolla, may be removed using a scoop net.

Submerged plants, such as ribbon weed and milfoils, may be effectively removed by a wire rope weighted at intervals. Alternatively these plants can be cleared by dragging the dam using a heavy chain, anchored at a point on the dam bank. A tractor, with the other end of the chain attached, is driven around the dam. The chain follows the bed of the dam, severing the plants close to the bottom. Where this does not work because of locality, or because trees are in the way, use an excavator or scoop.

Emergent plants, such as cumbungi and phragmites, can be cut below the waterline in autumn, severely retarding their growth. This is particularly true of cumbungi. Special underwater cutting machines are available for both submerged and emergent plants, but these are costly and do not eliminate the problem.

Weeds that are anchored to the banks are difficult to control by mechanical means. Mowing reduces the bulk of material but it is essential that mowing is carried out regularly.

Environmental control

Control can be achieved by altering the water body in some way to limit the growth of aquatic plants. Different approaches are given below:

• Empty the dam and dry the bed out for several weeks to dry the plants.
  
• Dredge or excavate to a depth where the plants will not grow, or will grow at reduced densities due to lack of light. This approach is most successful in very turbid water.
  
• Shade the dam using bank-planted trees appropriate for the area. However, this might limit access and cause water loss and/or problems with roots.
  
• Limit the influx of nutrients by diverting effluent from stockyards or feeding areas. Do not allow stock access to the dam — provide a watering point below the catchment area.

Chemical control

Chemical control is suitable where the weed infestation is small and the water is not for stock, for domestic purposes, or for watering gardens and crops. A small infestation means that only a small quantity of chemical is needed, and water contamination is minimal. Also, the decay of small amounts of vegetation will not reduce oxygen levels in the water sufficiently to kill fish. Where the water is for stock, for domestic supplies, or for garden or crop irrigation, it is imperative that you observe the withholding period for the particular chemical.

It is important to adhere strictly to the following procedures:

• Identify the plant or plants correctly.
• Select a chemical registered for use in water and on that particular plant.
• Read the chemical label carefully and observe all special precautions. Take particular note of toxicity to other plants, fish or wildlife, residual activity and withholding periods for treated water.

Chemicals registered for control of noxious weeds are listed in the publication Noxious and Environmental Weed Control Handbook. For further information on chemicals and rates, read the product label or enquire at your local NSW DPI office.

Herbicide applications to water may require approval under the Protection of the Environment Operations Act 1997. Approval must then be obtained from the Environment Protection Authority.

Biological control

Biological control uses the natural enemies of the problem weed and so is regarded as the ideal type of control as it is non-polluting and usually self-sustaining. However, such a program requires several years of overseas exploration and testing of organisms. Before introduction it must also be demonstrated that the organisms do not attack economic crops and native plants.

Biological control is rarely the complete solution for a weed problem and must usually be supported by other measures.

The CSIRO in Brisbane has a special research program on biological control of aquatic weeds. Work has concentrated on water hyacinth, alligator weed, salvinia and water lettuce, the first three of which are discussed below.

Water hyacinth

The water hyacinth control program has been moderately successful in Queensland, but in northern New South Wales it has only given suppression. South of Kempsey it has had little effect. The most successful insect is the Neochetina weevil, which is best suited to large, deep bodies of still water; it is unlikely to provide adequate control in small dams.

Salvinia

Successful control of Salvinia has been obtained in northern Queensland using the weevil Cytobagous salviniae. A moth has also been released. In New South Wales, the weevil has been tested near Sydney and on the north coast. Results are encouraging, especially in the warmer climate of the north coast.

Alligator weed

Excellent results in the control of alligator weed (shown at right) have been achieved on the Georges River (near Sydney) with the two flea beetles Agasicles and Disonycha, and the moth Vogtia. However, these have been unsuccessful on the banks of the Georges River and against terrestrial infestations at Williamtown (near Newcastle). Activity is strictly limited to the aquatic environment.

Integrated control

When used in isolation it is unusual for the control techniques listed above to provide adequate long-term control.

Integrated control combines two or more techniques in a unified program. An integrated program usually provides more efficient and stable control in the long term with fewer undesirable side-effects.

Following is a step-by-step example of integrated control of plants in a small dam:

1. Mechanically remove the most accessible plants.
2. Treat any remnants with spot applications of herbicide.
3. Dredge the dam to deepen it and therefore discourage growth.
4. Divert cattle yard effluent away from the dam catchment.
5. Plant trees to shade the dam and therefore, as with dredging, reduce available light to the plant.
6. Use appropriate biological control agents if available.

Continual monitoring of the situation is necessary.

Implementation of control measures

The successful implementation of any weed control program depends on the resources available, the situation, the weeds present and the ability to carry out the control methods.

Consider the long-term consequences of any control method. Using the wrong method could worsen the problem. Remember, in order to prevent a recurrence of the weed problem, you must be prepared to manage the dam and its surrounds well into the future.

Further information

The following resources are helpful in identifying aquatic weeds:
The National Herbarium, Royal Botanic Gardens, Sydney.
Water Plants in Australia, Australian Water Resources Council.
Water Plants of New South Wales, by GR Sainty and SWL Jacobs.

For related information, see:
Guidelines for the use of herbicides in or near water, Australian Government Publishing Service, 1985.