What is Canadian pondweed?
- Canadian pondweed is a weed of freshwater ponds, lakes, reservoirs and rivers in Tasmania.
- Canadian pondweed is a
declared weed in Tasmania under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of Canadian pondweed are prohibited in Tasmania.
How to identify Canadian pondweed
Image top: Canadian pondweed, © Kiowa FennerImage above: Strand of Canadian pondweed, © Kiowa Fenner
- Canadian pondweed is a perennial (long-lived) freshwater herb, with much branched stems to 3 m long. The leaves are dark green and occur in whorls of three on the middle and terminal parts of the stem, becoming more crowded at the ends of the stem.
- Canadian pondweed is usually firmly rooted in the bottom mud and forms a thick green mass below the water surface. The flowers are white and unisexual, and are carried towards the water surface by a thread-like extension up to 30 cm long. Plants are either male or female; only male plants occur in Australia. Flowering is in summer.
- Stem growth begins in September and October as water temperature rises above 15oC, and a thick mat of intertwining stems forms below the water surface. Growth declines in autumn and the plant over-winters in a semi-dormant state. New plants form in autumn as a result of stem fragmentation.
- For further help in identifying Canadian pondweed, search the
Dennis Morris Weeds and Endemic Flora database for Canadian pondweed illustrations. If you are still in doubt about the weed you are dealing with, contact Biosecurity Tasmania on 03 6165 3777 for help.
Canadian pondweed in Tasmania
- Canadian pondweed occurs at several sites in southern, central and northern Tasmania.
- Canadian pondweed is a serious weed in a variety of freshwater environments, reducing water flow and interfering with recreational activities, hydro-electric operations and irrigation.
What is the legal status of Canadian pondweed in your area?
Detailed management and control guidelines for Canadian pondweed can be found in the Canadian Pondweed Control Guide. Refer also to
Herbicides for Aquatic Weed Control. For further information see Weed Links and Resources.
Herbicides for Aquatic Weed Control
Statutory Management Plan for Canadian Pondweed
Weed Links and Resources
Other useful links
Canadian Pondweed Control Guide
- Plan your control program, this will save time and money in the long-run;
- Consider the impact of your control methods on off-target species, especially if herbicides are used;
- Get in early - For new infestations, eradicate before the plants reach the flowering stage: once plants begin seeding, control becomes more difficult and expensive;
- Carefully time your use of herbicide for best results (see the
Herbicides for aquatic weed control link for more information);
- Coordinate your control program with neighbouring landholders where your weed problem crosses property boundaries;
- Revisit and regularly inspect the site and ensure follow-up is undertaken.
- Don't start your control program without first planning your approach;
- Don't allow an infestation to become established. Get in early!
- Don't rely on just one treatment: follow-up is essential.
Spread of Canadian pondweed
- Only male plants occur in Australia, and all reproduction is vegetative. Infestations of Canadian pondweed increase in size when the over-wintering colony begins new growth in spring.
- New colonies are formed when stem fragments break away from the colony in stream flow and establish downstream. Canadian pondweed stems break easily when disturbed mechanically, and in late summer and autumn large numbers of stem fragments are swept along irrigation channels to take root downstream.
- Hand pulling, cutting and the use of booms and draglines can provide temporary relief, but are expensive and time-consuming and are not effective in the long-term control of Canadian pondweed.
- Physical removal provides only temporary reduction of an infestation, and can encourage spread of the weed by stem fragments.
Lowering water levels
- Lowering water levels in an infested water body or channel can be effective in controlling Canadian pondweed provided the site is completely drained, preferably in summer, exposing the weed to drying out.
Herbicides for Aquatic Weed Control