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Canadian pondweed (Elodea canadensis)

Elodea can form dense underwater infestations in Australian freshwater, reducing water flow and interfering with water supplies, recreation and irrigated agriculture. It can also out-compete native aquatic plants and alter aquatic habitats.

Key facts

Type: Weed, aquatic.

Native to: Temperate North America.

Established in: QLD, NSW, ACT, VIC, TAS, SA.

Other names: American Elodea, American Waterweed, Canadian Waterweed, Oxygen Weed.

Best time to act: New growth appears in spring as water temperatures rise above 15 °C; early summer is a recommended time to control populations approaching nuisance size, with another treatment in late summer before fragmentation.

Identification

  • This is a submerged, perennial aquatic herb with brittle, branched stems usually up to 3 m long, sometimes reaching 6 m, rooted to the bottom.
  • Stems grow towards the surface and can form a thick green mass at and just below the water level.
  • Small, stalkless, mid- to dark-green leaves grow in whorls, usually of three, and are generally 5–15 mm long and 1–5 mm wide.
  • Leaf margins may have fine, dark-tipped teeth or be smooth, and the leaves are often curled.
  • Floating flowers are uncommon and small, with three white, pinkish or purplish petals; only male plants have been recorded in Australia.
  • Look-alike: Dense Waterweed (Egeria densa) has more crowded whorls, often of four or five leaves, with leaves up to 40 mm long and conspicuous white flowers up to about 2 cm wide.
  • Look-alike: Lagarosiphon (Lagarosiphon major) has stiff leaves arranged alternately around the stem and strongly curved downwards; Hydrilla (Hydrilla verticillata) has longer, more markedly serrated leaves, usually in whorls of three to eight.

Where it grows

In Australia it occurs mainly in south-eastern regions, in freshwater lakes, ponds, reservoirs, streams, channels, drains and wetlands, and is recorded in NSW, QLD, SA, TAS, VIC and WA.

Why it matters

  • Dense infestations can reduce water flow by 60–80%, interfere with water supplies and obstruct irrigation and drainage channels, sometimes causing channels to overflow.
  • Thick mats can disrupt waterway traffic and navigation and impede boating, swimming and fishing.
  • Elodea can out-compete or impede native aquatic plants and alter aquatic habitats, with potential impacts on wildlife.
  • Changes to water light, temperature, oxygen and pH can affect water conditions and fish numbers.

How it spreads

  • In Australia, spread is vegetative because only male plants occur here, so sexual reproduction is not possible.
  • Stem fragments and swollen shoot tips can be carried downstream, produce buds and root to form new plants; colonies also expand from buds on runners and stems.
  • The aquarium trade has assisted its introduction and spread, and plants may establish after aquarium contents are discarded in waterways.

Control options

  • Prevention is the most effective approach: do not introduce Elodea to unaffected areas, and do not dump or grow aquarium plants in waterways.
  • Lowering water levels can be effective if channels or water bodies are completely drained and dried, preferably in summer; shading the water surface can reduce growth.
  • Apply control in early summer as populations approach nuisance size, and again in late summer before fragmentation and the formation of overwintering propagules.
  • Approved herbicides can be used; mechanical removal often encourages spread through stem fragments and may provide only temporary relief.

Hygiene and washdown

  • Do not dump aquarium plants or aquarium contents into waterways, and do not introduce Elodea to unaffected areas.
  • Its stems fragment readily and fragments can spread downstream, so avoid transferring plant material between waterways.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • Western Australia: Declared pest (C1), prohibited. C1 Exclusion: to be kept out of WA, or out of the parts of WA where it is not established. Report any sighting to DPIRD. It must not be brought into WA (prohibited organism).
  • Tasmania: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • Northern Territory: Declared weed. Weeds Australia lists it as a declared weed in the NT under the Weeds Management Act 2001. Check the NT declared weeds list for its class and what it requires.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania
  • South Australia: Landscape South Australia Act 2019 (SA), administered by the regional landscape board

Sources