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Dense waterweed (Egeria densa)

Introduced to Australia as an aquarium plant, Dense Waterweed is now patchily naturalised in several states and territories. It forms dense underwater growth that can obstruct water use and harm aquatic habitats and biodiversity.

Key facts

Type: Weed, aquatic.

Native to: Brazil, Argentina and Uruguay in South America.

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

Other names: Dense Water Weed, Leafy Elodea, Egeria, Brazilian Elodea.

Best time to act: Growth resumes in spring and continues rapidly until February to March, and flowering occurs from spring to autumn.

Identification

  • It is a submerged, much-branched perennial freshwater herb, usually rooted in the substrate, with slender green to brownish stems up to 1.5 metres long and sometimes up to 5 metres.
  • Its stalkless, strap-like leaves are usually in crowded whorls of 4 or 5, occasionally 3 or 6 to 8, and are 15 to 40 millimetres long with very fine serrations.
  • The upper stem sections are crowded, often less than 2 millimetres apart, and the leaves cluster densely near the growing tips.
  • Showy white flowers grow at the water surface; male flowers are about 1.5 to 2 centimetres across, and only male plants are known in Australia.
  • Look-alike: Elodea canadensis has shorter leaves, usually up to 15 millimetres long, in fewer and more widely spaced whorls, and has smaller, less conspicuous flowers.
  • Look-alike: Lagarosiphon major has stiff leaves that curve strongly downwards and spiral around the stem, while Hydrilla verticillata has more widely spaced leaf whorls and smaller, less conspicuous flowers.

Where it grows

In Australia it is patchily naturalised in south-eastern Queensland, southern New South Wales, the ACT, Victoria, Tasmania, far south-eastern South Australia and some lakes and rivers around Perth, in nutrient-rich ponds, lakes, reservoirs, ditches and slowly flowing streams.

Why it matters

  • Dense growth can retard water flow, increase siltation, choke irrigation channels and clog equipment.
  • It can impede hydroelectric operations, potable water supplies, navigation, commercial fishing, boating, fishing and swimming.
  • Dense stands can reduce or replace native aquatic plants and animals; thick mats also reduce light, deplete oxygen and cause fluctuations in water quality.

How it spreads

  • In Australia it reproduces vegetatively: brittle stem fragments and small buds can grow into new plants, and colonies expand as fragments take root.
  • Water currents carry fragments and shoots away from parent colonies, while fragments can also be caught on boat trailers, fish traps and other equipment.
  • The aquarium trade is a major means of human dispersal, and the weed has also been deliberately planted for commercial harvesting.

Control options

  • Hand-pulling, cutting and machine digging can provide only temporary relief because fragments may form new plants; control must remove or kill all plants and fragments to prevent regrowth.
  • Removed plants can be dried and burned or buried; water-level fluctuations, such as flushing or draining channels or dams, may help in some situations.
  • Use approved methods for any chemical control, as herbicide use in water bodies and water systems is constrained by environmental requirements.
  • Prevent new infestations by never dumping aquarium or pond plants into waterways and by encouraging suitable aquatic plants instead.

Hygiene and washdown

  • Check boat trailers, fish traps and other equipment for plant fragments, which can spread the weed between waterways.
  • Never dump aquarium or pond plants into 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: Regional priority in 1 of 11 regions. Regional priority weed in North West. 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 (C2), prohibited. C2 Eradication: present in limited areas and targeted for eradication. 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