Quick Facts

Quick facts

• Elodea (Elodea canadensis) is a submerged aquatic long-lived (perennial) herb.
• It differs from the related species Dense Waterweed (Egeria densa), Lagarosiphon (Lagarosiphon major) and Hydrilla (Hydrilla verticillata) by having fewer leaves in a whorl and shorter individual leaves with these other species both having leaves up to 40 mm long in whorls of 3–8, or alternate, in the case of Lagarosiphon.
• In Australia, reproduction and spread is by rooted fragments.
• It is an invasive plant that can become problematic in many freshwater environments, including ponds, lakes, rivers, creeks and irrigation channels, especially in cooler areas.
• Dense infestations reduce water flow, and can interfere with water supplies, recreational activities and irrigated agriculture, as well as out compete native plants, alter aquatic environments and impact on fish.

What Does It Look Like?

What is it?

Elodea (Elodea canadensis) is a submerged aquatic long-lived (perennial) herb. It has brittle, branched stems usually up to 3 m, sometimes up to 6 m long and is rooted to the substrate. The stems are produced off short runners (stolons) and grow towards the surface forming a thick green mass at, and just below, the water level. The stalkless leaves are arranged in whorls (in rings around the stem) of 3 (rarely 4 or 5) along the stems, and are mid to dark green, often curled and more or less narrowly oblong to oval-shaped or egg-shaped. The leaves are 5–15 mm or sometimes up to 20 mm long and 1–5 mm wide, with the margins minutely toothed with fine, dark-tipped, forward-pointing teeth or sometimes smooth-edged. The internodes (the sections of stem between joints/leaf whorls) are 3–25 mm long but become shorter near the tip. The lower leaves are small and egg-shaped, in opposite pairs.

Elodea is dioecious, i.e. with male and female flowers on separate plants, but only male plants have been recorded in Australia. The floating flowers are uncommon, solitary, have 3 petals and are small (3–5 mm in diameter). They arise from the upper leaf axils (joint of leaf and stem) on a long white thread-like hypanthium (a slender stalk or floral tube that elongates to carry the flower to the water surface) that is usually 2–20 cm long but sometimes up to 30 cm long. The petals are white to pinkish or purplish. Male flowers have a hypanthium that usually breaks off to allow the upper part of the flower to float to the water surface and narrowly lance-shaped petals about 5 mm long. Female flowers have spoon-shaped petals, but have not been found in Australia to date.

The fruit is an egg-shaped, beaked capsule 6–9 mm long and 2–3 mm in diameter and contains 1-6 seeds but has also not been found in Australia to date (Aston 1973; Sainty & Jacobs 1981, 2003; Häfliger et al. 1982; Cook & Urmi-König 1985; Jacobs 1993; Bowmer et al. 1995; Holm et al. 1997; Romanowski 1998; Parsons & Cuthbertson 2001; Weber 2003; Richardson et al. 2006).

For further information and assistance with identification of Elodea, contact the herbarium in your state or territory.

Flower colour

White, Pink, Purple

Growth form (weed type/habit)

Aquatic, Herb

Where it currently grows? Preferred habitat

Elodea grows in temperate regions, less commonly in subtropical and tropical regions, in a range of water situations such as clear, cold to warm, still to turbulent or sometimes swift/fast-flowing freshwaters of lakes, quiet ponds, reservoirs, streams, channels, drains and wetlands, at depths to 3 m and, occasionally, to 5 m or more. It is a major weed of irrigation channels. Elodea prefers peaty or muddy silt substrate, is tolerant of cold and can withstand freezing, tolerates battering and tangling in turbulent water, but does not thrive in water deprived of iron. It appears to have a high light requirement for maximum growth, tolerates slightly brackish waters, and grows vigorously once water temperatures exceed 15 °C. Growth is affected by nutrient levels, variation in light intensity, day length, light quality and temperature (Aston 1973; Cook & Urmi-König 1985; Cunningham et al. 1992; Jacobs 1993; Bowmer et al. 1984, 1995; Holm et al. 1997; Parsons & Cuthbertson 2001; Sainty & Jacobs 1981, 2003; Weber 2003; Richardson et al. 2006).

Are there similar species?

There are a number of similar-looking submerged aquatic species that may grow in similar habitats. Differences between Elodea which has small inconspicuous flowers, fewer leaves in its whorls (3, rarely 4 or 5) and are also shorter, generally up to 15 mm long and several other species in family Hydrocharitaceae are summarised as follows:

• Dense Waterweed (Egeria densa), which is also introduced in Australia, has more crowded leaves arranged in whorls of 4 or 5 (sometimes 3-8) with each leaf longer, up to 40 mm, and conspicuous 3-petalled white flowers that are up to about 2 cm wide.

• Lagarosiphon (Lagarosiphon major), which is also introduced in Australia, has stiff, strongly downwardly curved leaves that are alternate (spirally arranged) around the stems, rather than in whorls like the other species

• Hydrilla (Hydrilla verticillata), which is native to Australia, has leaves that are longer and up to 40 mm, arranged in whorls of 3–8, mostly well-spaced along the stem, with leaf margins more markedly serrated with fine translucent teeth, a rough and scratchy feeling when handled in the water, and small flowers (Aston 1973; Cunningham et al. 1992; Jacobs 1993; Bowmer et al. 1995; Flanagan 1998; Romanowski 1998; Sainty & Jacobs 1981, 2003; Richardson et al. 2006)

Elodea is like a smaller version of Dense Waterweed, but its shorter leaves are usually arranged in whorls of 3 and it has much less conspicuous flowers (Romanowski 1998).

Why Is It A Weed?

What are its impacts?

Native ecosystems: Dense infestations of Elodea can reduce water flow by as much as 60–80% and, in reservoirs, interfere with hydroelectric output, making it is one of several water weeds that can cause commercial losses to hydropower stations and urban water supplies in New Zealand. On rivers, lakes and other waterbodies, dense tangled mats disrupt waterway traffic and navigation, limit recreational use by impeding activities such as boating, swimming and fishing and significantly alter the aquatic environment (Holm et al. 1997; Thorp & Wilson 1998; Cronk & Fennessy 2001; Parsons & Cuthbertson 2001).

When Elodea invades lakes, streams and wetlands, it can reduce the abundance of native aquatic plants or replace species as its dense closed stands out-compete or greatly impede the growth of native aquatic plants. Aquatic environments and habitats will be altered by heavy infestations, with potential impacts on wildlife (Cronk & Fennessy 2001; Weber 2003). Light intensity, temperature, oxygen content and pH of the water are reduced which affects the solubility of dissolved salts, the nutrient status of the water and, as a result, fish numbers (Parsons & Cuthbertson 2001). Elodea has also invaded constructed wetlands (Gunasekera 2003) and shows a higher dispersal capacity via fragmentation than its weedy relatives Egeria densa and Lagarosiphon major (Redekop et al. 2016).

Agriculture: Elodea is an obstructive and persistent aquatic weed of irrigation supply and drainage channels. Its spread in irrigation systems can threaten irrigated agriculture by greatly reducing water flow and sometimes by causing channels to overflow (Aston 1973; Mitchell 1978; Bowmer et al. 1979, 1984, 1995; Sainty & Jacobs 1981, 2003; Holm et al. 1997; Flanagan 1998; Richardson et al. 2006).

How does it spread?

Although Elodea reproduces sexually and produces seed in its native habitat, its spread in Australia is wholly vegetative as only male plants occur here and so sexual reproduction is not possible. Spread is by fragmentation and dispersal of stem pieces and swollen shoot apices (bud-like formations of densely crowded leaves at the tips of lateral shoots) that are carried downstream. The stems break off easily into fragments that produce buds, and readily root in a suitable situation to grow into new plants (Parsons & Cuthbertson 2001; Redekop et al. 2016).

Colonies expand locally by extending the vegetative mat and increasing in density when buds on horizontal stolons (short runners) and vertical stems on the edge of colonies produce new growth in spring (Parsons & Cuthbertson 2001).

The introduction and spread of Elodea to new areas has been assisted through the aquarium trade (it is used to decorate aquaria, fish bowls and ornamental ponds). It is likely that plants have frequently become established in the wild due to aquarium contents being discarded in waterways (Bowmer et al. 1995; Holm et al. 1997; Flanagan 1998; Thorp & Wilson 1998 -).

What is its history in Australia?

Elodea spread across much of Europe and colonised its waterways in the late 19th century. It now occurs in most temperate countries and in some tropical areas of the world. It was introduced to New Zealand (where only female flowers have been recorded) more than a century ago and was probably introduced through the aquarium trade (Romanowski 1998). It was introduced to Tasmania in the 1960s. The date of introduction to mainland Australia is uncertain due to confusion of identity with similar-looking related species. Elodea was recorded near Mildura, Victoria, in 1949 and at Yarrawonga, as well as in the Berriquin Irrigation District of New South Wales in 1958. It spread rapidly to become, at one period, the dominant submerged weed of the Murray Valley Irrigation Districts and it has also spread to the Murrumbidgee Irrigation Area (Aston 1973; Sainty & Jacobs 1981, 2003; Bowmer et al. 1984, 1995; Cook & Urmi-König 1985; Holm et al. 1997; Parsons & Cuthbertson 2001; Weber 2003).

How To Manage It?

Best practice management

Once established, Elodea is extremely difficult to eradicate. Mechanical removal measures such as using booms, draglines, cutting, hand-pulling and cultivation of beds tend to encourage spread by fragmentation of the stems. Such actions are also often slow and expensive, and will only provide temporary relief (Sainty & Jacobs 1981, 2003; Holm et al. 1997; Parsons & Cuthbertson 2001; Weber 2003).

Non-chemical control: Land management: Lowering water levels in channels, small dams or other water bodies can be an economical, safe and effective control for Elodea if they are completely drained and dried, preferably in summer to expose the weed to desiccation by sun and wind, or exposing the sediments to winter frosts (Bowmer et al. 1995; Parsons & Cuthbertson 2001). Shading the water surface of infested waterbodies can substantially reduce the growth rate (Holm et al. 1997; Parsons & Cuthbertson 2001; Weber 2003).

Studies in irrigation systems show that control measures should be applied in early summer when populations are approaching nuisance size, and again in late summer before fragmentation occurs and over-wintering propagules are initiated (Bowmer et al. 1984).

Prevention is the most effective means of control and Elodea should not be introduced to unaffected areas and aquarium plants must not be dumped or grown in waterways. Management should aim to prevent the introduction of the weed to new areas and also focus on early detection and eradication (Flanagan 1998). Growing more suitable aquatic plants in ponds and aquaria should be encouraged (Lawrence et al. 2005).

Chemical control: is possible through the use of approved herbicides (Bowmer et al. 1979, 1984, 1995; Cronk & Fennessy 2001; Parsons & Cuthbertson 2001; Peirce & Pratt 2002; Weber 2003; Western Australian Department of Agriculture and Food undated). Methods include the treatment of flowing water by injection of chemicals, treatment of ponded or static water, and post-drainage treatment of channel beds. In irrigation channels, where draining and drying are not feasible, biomass is successfully reduced by widespread use of herbicides (Bowmer et al. 1995; Parsons & Cuthbertson 2001).

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

In Australia, Elodea overwinters as vegetative organs, dormant or semi-dormant shoots and axillary or apical buds or 'turions' that can be produced in large numbers in autumn. These reproductive units begin active growth in spring, and prolific and vigorous growth produces a thick mat of vegetative material during summer. Growth can continue rapidly until about February-March, when growth rate declines. In cool temperate areas this vegetative material dies off, over-wintering as stolons or turions in the bottom mud, or it becomes dormant in winter. Plants can remain in a leafy semi-dormant state until the following spring. New growth occurs in September or October (e.g., in south-western New South Wales) as water temperatures rise above 15 °C. In late summer and autumn, stems readily break into pieces to form new plants. Flowering occurs mainly in summer, from November to February (Bowmer et al. 1984, 1995; Cook & Urmi-König 1985; Cunningham et al. 1992; Holm et al. 1997; Flanagan 1998; Parsons & Cuthbertson 2001; Sainty & Jacobs 1981, 2003).

Where Is It Found?

Which states and territories is it found?

NSW, QLD, SA, TAS, VIC, WA

What areas within states and territories is it found?

Elodea occurs mainly in south-eastern Australia, in New South Wales (coastal regions, the Hunter Valley, southern regions and inland irrigation districts) and Victoria (near the Murray River and widespread in the central part of the state, from Cairn Curran and Eppalock Reservoirs south to Colac, Melbourne and southern parts of that state). There are localised or isolated infestations in Tasmania. It has also been reported to occur near Perth in Western Australia, in south-eastern Queensland and the Southern Lofty Region of South Australia (Aston 1973; Cunningham et al. 1992; Jacobs 1993; Henderson 1997; Parsons & Cuthbertson 2001; Barker et al. 2005; AVH 2008). While no infestations have been recorded in the Northern Territory it has been found there in aquarium shops (Flanagan 1998).

Where does it originate?

Elodea is native to temperate North America (Cook & Urmi-König 1985).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

NT, SA TAS, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Elodea canadensis

Other scientific names (synonyms)?

Anacharis canadensis (Michx.) Planch.

Does it have other known common name(s)?

Canadian Pondweed, American Elodea, American Waterweed, Canadian Elodea, Canadian Pond Weed, Canadian Waterweed, Oxygen Weed, Waterweed, American Duckweed