Introduction

Cabomba is a serious aquatic weed that has great potential to spread in NSW. Cabomba was introduced into Australia from the USA as an aquarium plant and first recorded here in 1967. It is now posing a serious threat to our waterways.

Cabomba is an aggressive invader of freshwater systems, particularly if they are nutrient rich. It is a fully submerged aquatic plant that out competes native freshwater plants. It can impede aquatic recreational activities and drowning is a risk for entangled swimmers. Cabomba is listed as a Weed of National Significance.

Description

Cabomba species are characterised by submerged rhizomatous stems growing up to 5m. There are five species that are recognised in Australia: C. aquatica, C. carolinia, C. furcata, C. haynesii and C. palaeformis. Only C. carolinia is known to be naturalised in Australia. This species now includes the previous separate species C. australis and C. pulcherrima - and several natural and horticultural varieties.

Cabomba is strictly aquatic and completely submerged except for flowers and occasional floating leaves. The root system is attached to the bottom of the water body and it primarily grows in depths up to 3m.The submerged leaves and stems have a thin gelatinous coating, with leaves being opposite and repeatedly divided to form feathery fan shaped structures. Flowers are solitary and raised above the waters surface attached to a long stem.

The main types of cabomba that have been grown and found in NSW and Queensland are:

• Common cabomba (C. caroliniana),
• Pink cabomba (C. furcata).

The distinguishing features that separate Pink cabomba from the other cabomba species is plant and flower colour. The C.furcata plant is pink in colour with purple flowers, whereas C.caroliniana is green to brown with white flowers. C. palae-formis also tends to have white flowers, while C. aquatica is easily distinguished by yellow flowers.

Distribution

Cabomba occurs in several locations in NSW. The most severe infestations are in the upper catchments of the Richmond River and the Burringbar Creek system - both on the NSW far north coast. These infestations are quite significant and have existed for about 6-8 years.

Other infestations have been found recently in the Tweed River near Murwillumbah, Grafton, Port Macquarie, Taree and Forster.

There is also an infestation in Glenbrook Lagoon, near Penrith in the Sydney basin and at Eastlakes in the heart of Sydney.

There is strong evidence to suggest that some of these infestations were deliberately planted for commercial harvesting.

Cabomba will grow in dams, ponds, lakes and freshwater streams. There is great potential for it to spread further in NSW. It has been promoted as a suitable aquarium plant, which poses the greatest threat for its spread.

Why is it a Problem?

Cabomba is regarded as a major threat to freshwater systems and their native plants, because it:

• Disrupts aquatic ecosystems, seriously affecting native plant and animal life. It quickly forms a dense monoculture reducing biodiversity.
• Decreases the quality of water by causing odours, accumulation of organic matter and stagnation of streams.
• Degrades the aesthetic value of streams
• Reduces the use of waterways for recreation and transport.
• Increases the cost of treating potable waters and impairs the sustainable use of drinking water storages.

Spread

Mainly human movement has spread cabomba, for it is a popular plant in the aquarium industry. When it is released into natural waterways its rapid growth dominates the native vegetation.

Regeneration from seed has not been observed in Australia. Once established in a stream, it spreads vegetatively as fragments break off and move down stream.

Declaration

In NSW cabomba is declared a W4g noxious weed. The declaration includes all cabomba species, except Cabomba furcata (Pink Cabomba). The W4g declaration means that:"the weed must not be sold, propagated or knowingly distributed." The declaration does not require any control of existing infestations.

C. furcata is exempt from the declaration as it does not pose the same potential threat as other species. C. furcata is the only species still allowed to be propagated and traded for the aquarium industry.

Control

Prevention is the best means of control, for once cabomba becomes established it is extremely difficult to control. The early identification and control of a small infestation is always desirable. When assessing the weed problem, consider the various control options and adopt a method best suited to your situation. There is no one simple solution.

Mechanical removal of small infestations can be attempted, if practical. All fragments of the weed must be removed and disposed of carefully. Excavators may be used to remove larger infestations, but this technique can be expensive, awkward and not always practical.

Draining or "draw down" of a water body can also be effective. If the weed problem exists in a dam or retention pond, draining and exposing the stems and leaves for several weeks can be a useful technique in appropriate situations.

There are no herbicides registered for cabomba control in NSW. A number of strategies have been attempted in Queensland with limited success. NSW Agriculture is currently undertaking investigations into possible herbicide control techniques.

Water Management

Vigorous growth rates of all water weeds, including cabomba, can be an indication of high nutrient levels in a water body. Nutrient enriched run-off should be minimised as part of a control program for water weeds. Nutrient loading can come from urban and commercial run-off, sewerage outfalls and agricultural sources. Management practices to minimise nutrient enriched run-off should always be considered when implementing a control program for aquatic weeds.

Acknowledgments

The author acknowledges the following sources:

• Queensland Department of Natural Resources (1998) Cabomba Pestfact.
• Mackey, A.P. and Swarbrick, J.T. (1998) Cabomba carolinia,
• A. Grey. In Biology of Australian Weeds, Volume 2.(published by RG and FJ Richardson, Melbourne).
• Parsons and Cuthbertson (1992) Noxious weeds of Australia. (published by Inkata Press, Melbourne)
• Sainty and Associates (1999) Weeddeck.