Quick Facts

Quick facts

• Dense Waterweed (Egeria densa) is a submerged, much-branched freshwater long lived herb with stems 1.5 m or more long; that have slender strap-like leaves that are up to 40 mm long and with minutely serrated margins arranged in whorls of 4 or 5 (sometimes 3-8).
• Its dense foliage resulting from leaf whorls that are close together, larger leaves and distinctive showy white flowers help distinguish it from related species.
• In the absence of female plants in Australia, reproduction and spread occurs vegetatively, mainly by fragmentation where stem pieces break away and shoot and take root.
• It is commonly used in ponds and indoor fish aquaria, but its use should be discouraged.
• It grows best in nutrient-rich, stationary or slow-moving water to depths of around 7 m, in freshwater ponds and other water bodies, and slowly flowing streams.
• It forms dense submerged masses so it can become extremely problematic in a variety of freshwater environments, reducing water flow and interfering with recreational activities, hydroelectric operations, irrigation and water supplies, as well as affecting aquatic habitats and biodiversity.

What Does It Look Like?

What is it?

Dense Waterweed (Egeria densa) is a submerged, multi-branched, perennial aquatic herb growing as an underwater mass in fresh water. It is usually rooted in the substrate, but occasionally it forms a dense free-floating mat just below the water surface. The green to brownish slender stems are up to 1.5 m long, sometimes to 5 m long, rooting at the lower joints (nodes) and buoyant so most of the growth is near the surface. The upper internodes (sections of stem between the nodes or joints) are crowded together and are often less than 2 mm apart) while they are longer near the base because the nodes are wider apart. The stalkless leaves are arranged in whorls (in rings around the stems) of 4 or 5 (rarely 3 or 6–8). The basal leaves are small and in opposite pairs, green to dark green, strap-like, tapering to a pointed or rounded tip, 15–40 mm long and 2-5 mm wide, often bent backwards and occasionally slightly twisted, 1-veined, and the margins are minutely toothed or serrated with fine translucent teeth which can only be seen clearly under a hand lens. They are mostly densely clustered or very crowded towards the ends of the branches near the growing tips.

The plants are dioecious (with male and female flowers occurring on separate plants) and only male plants are known in Australia. The floating flowers grow from the upper leaf axils (angle between leaf and branch) and have a thread-like hypanthium (a floral tube that is like a stalk that elongates to take the flower to the water surface) up to 8 cm long. Male flowers (about 1.5–2 cm in diameter) occur alone or in groups of 2–5. They have 3 boat-shaped sepals (outer 'petals', modified leaves), that are 3–4 mm long, and 3 showy white petals, mostly 9–12 mm long and 6–9 mm wide. Female flowers (not seen in Australia) are solitary and have 3 spoon-shaped sepals about 3 mm long and 3 broad, unequal white petals 6–8 mm long and 5–8 mm wide. The cylindrical berry-like fruit that develops in female flowers is 7–8 mm long and about 3 mm in diameter (Aston 1973; Stanley & Ross 1989; Jacobs 1993; Romanowski 1998; Parsons & Cuthbertson 2001; Weber 2003; Richardson et al. 2006; Jordan 2007; Johnson undated).

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

Flower colour

White

Growth form (weed type/habit)

Aquatic, Herb

Where it currently grows? Preferred habitat

Dense Waterweed occurs mostly in warm temperate to tropical regions, although it is also tolerant of cold. It grows in stationary or slowly flowing water, especially where the substrate has been disturbed. It is an occasional weed of mild to warm freshwater ponds, shallow lakes, water holes, reservoirs, ditches and slowly flowing streams, especially in conditions of high nutrient levels. It grows at water depths of up to 7 m and has very low light requirement. Growth is affected by nutrient status, light intensity, day length, temperature and rate of water flow (Jacobs 1993; Parsons & Cuthbertson 2001; Stephens & Dowling 2002; Weber 2003; Tasmanian Department of Primary Industries and Water 2008; Johnson undated). Dense Waterweed does not appear to persist in intermittently flowing streams or those subject to flooding (Thorp & Wilson 1998 – ).

Are there similar species?

There are a number of similar-looking submerged aquatic species that grow in similar habitats. Differences between Dense Waterweed (with its mostly very crowded leaves with very fine serrations on the margins, and its large, up to 2 cm wide, 3-petalled white flowers) and the following species, all in the family Hydrocharitaceae include:

• Elodea (Elodea canadensis); which is also introduced to Australia, tends to have fewer (usually 3, rarely 4 or 5), shorter leaves (mostly up to 15 mm long) in each whorl, with the whorls more widely spaced and less-conspicuous, smaller flowers
• Lagarosiphon (Lagarosiphon major), which is also introduced to Australia, has stiff, strongly downwardly curved leaves not in whorls as in the other species but are arranged spirally around the stem
• Hydrilla (Hydrilla verticillata), a native species in Australia, which has the whorls of leaves more widely spaced, leaf margins distinctly serrated with fine translucent teeth and has less conspicuous, smaller flowers) (Aston 1973; Cunningham et al. 1992; Jacobs 1993; Miller & Schultz 1998; Romanowski 1998; Thorp & Wilson 1998 -; Sainty & Jacobs 1981, 2003; Richardson et al. 2006.

Dense Waterweed is most similar to Elodea, but the leaves are longer, in whorls of 3–8, crowded along the stems and the flowers are conspicuous, with petals up to 12 mm long (Cunningham et al. 1992; Sainty & Jacobs 2003; Richardson et al. 2006). Aquatic species from other plant families are illustrated by the NSW Department of Primary Industries at http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0005/158162/save-waterways-poster.pdf  and the Northern Territory Department of Natural Resources, Environment and The Arts at http://www.nt.gov.au/nreta/natres/weeds/ntweeds/cabomba/species.html 

Why Is It A Weed?

What are its impacts?

Dense Waterweed can become a nuisance in ponds, shallow lakes, slow-moving streams and irrigation channels (Cunningham et al. 1992; Richardson et al. 2006). It is capable of forming extremely dense, submerged masses that can seriously retard water flow, increase siltation, choke irrigation channels and clog equipment, impede hydroelectric activity, potable water supplies and navigation in water ways (retarding river traffic) and impede commercial fishing operations as well as recreational activities such as boating, fishing and swimming activities (Mühlberg 1982; Thorp & Wilson 1998 -; Cronk & Fennessy 2001; Parsons & Cuthbertson 2001; Global Invasive Species Database 2006; Jordan 2007).

Dense Waterweed impacts on natural wetland and lake communities, where dense stands will reduce or replace native aquatic plants and animals. As well as out-competing native macrophytes, this weed alters habitat conditions. The thick mats of intertwining stems reduce light intensity, deplete oxygen and can cause fluctuations in water quality (Cronk & Fennessy 2001; Champion 2002; Weber 2003; Global Invasive Species Database 2006; PIER 2007; Johnson undated).

How does it spread?

In Australia, propagation is solely by vegetative material (Jordan 2007). As well as having small buds that grow into new plants, small portions of stem will root readily to produce new growth (Stanley & Ross 1989). Existing colonies expand locally when over-wintering crowns and buds on stem fragments commence growth in spring. There is growth by profuse branching, as well as new plants arising from the fragmentation of the brittle stems into pieces that send out shoots. New colonies arise from stem fragments and broken off shoots that are carried away from the parent colony in stream flow (Mühlberg 1982; Parsons & Cuthbertson 2001). Plant fragments will be moved naturally by water currents but can also be spread when caught on boat trailers, fish traps or other equipment (Johnson undated).

Human dispersal of Dense Waterweed through the aquarium trade is a major means of its spread (Parsons & Cuthbertson 2001). It has also been deliberately planted for commercial harvesting purposes (Johnson undated).

What is its history in Australia?

Dense Waterweed was introduced to Australia as an aquarium plant and is now patchily naturalised in several states (Romanowski 1998). It is commonly used as an ornamental in home aquaria and small garden ponds, and its appearance in some water ways has been attributed to the dumping of aquarium contents (Aston 1973; Parsons & Cuthbertson 2001; Jordan 2007). Early herbarium records exists from the Royal Botanic Gardens, Sydney in 1947, from Melbourne in 1950 and from Hobart in 1951 (National Herbarium of New South Wales 2008; National Herbarium of Victoria 2008). In Queensland it was recorded as being a problem on an ornamental lake in Toowoomba in 1969 (Queensland Herbarium 2008).

How To Manage It?

Best practice management

Hand-pulling, cutting and digging with machines can be used to control Dense Waterweed but these methods will usually only provide temporary relief. This is because mechanical control methods will actually encourage its spread as vegetative fragments will form new plants (Parsons & Cuthbertson 2001). All control methods need to be thorough and either kill or remove all of the plants to prevent regrowth from stem pieces (Jordan 2007). After mechanical removal, plants can be destroyed by drying and burning or burying (Miller & Schultz 1998; Johnson undated). Control through water level fluctuations may be possible in some situations, e.g., flushing or draining channels or dams (Stephens & Dowling 2002; Sainty & Jacobs 2003; Washington State Department of Ecology 2003). Long-term management is likely to involve both increased and variable water flows and reduced nutrient loading (Johnson undated).

Chemical control is possible, but herbicide treatments need to be approved and are limited by environmental constraints when used within water bodies, wetlands, water sources and irrigation systems (Cronk & Fennessy 2001; Parsons & Cuthbertson 2001; Peirce & Pratt 2002; Washington State Department of Ecology 2003; Weber 2003; Western Australian Department of Agriculture and Food undated).

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

Aquatic weeds can be extremely expensive to eradicate and so prevention is better than cure, especially with these kinds of plants (Western Australian Department of Agriculture and Food 2006). Preventing this weed escaping into water ways is the best way of managing it and so aquarium or pond plants should never be dumped or grown in water ways (Johnson undated; see also Western Australian Department of Agriculture and Food 2006). It is also important to encourage the growing of more suitable aquatic plants in ponds and aquaria (CRC 2005; Lawrence et al. 2005). Dense Waterweed is prohibited and banned from sale in several states and territories (Western Australian Department of Agriculture and Food 2006 undated; Johnson undated).

Integrated management plans and control measures for Dense Waterweed are outlined by the Tasmanian Department of Primary Industries and Water (2008). Planning and implementation of an integrated control program aims at eradicating all existing populations and preventing future infestations of Dense Waterweed. Such a program should include as a minimum: an inventory of Dense Waterweed infestations in the relevant area, well-defined objectives, practicable control actions, follow-up actions, a realistic timeline and a budget. For more details see http://www.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/lwm_pest_plants_egeria 

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Dense Waterweed reproduces only by asexual (vegetative) methods in Australia, while overseas the plant usually over-winters as seeds and dormant or semi-dormant shoots. These reproductive units commence growth in spring and, in Australia, growth continues rapidly until February-March and a thick mat of intertwining stems is formed below the water surface (Parsons & Cuthbertson 2001). Flowering occurs in the warmer months, from spring to autumn (Stanley & Ross 1989; Richardson et al. 2006). New plants are spread and propagated by stems breaking into segments in autumn and shooting from small buds on the segments the following spring (Sainty & Jacobs 1981, 2003).

Where Is It Found?

Which states and territories is it found?

ACT, NSW, QLD, SA, TAS, VIC, WA

What areas within states and territories is it found?

Dense Waterweed is patchily naturalised, occurring in south-eastern Queensland, mostly coastal and inland southern New South Wales, the Australian Capital Territory, Victoria and Tasmania. It has also has been introduced to some lakes and rivers around Perth, Western Australia, and in far south-eastern South Australia (Aston 1973; Rye 1987; Jacobs 1993; Parsons & Cuthbertson 2001; Barker et al. 2005; Western Australian Herbarium 1998 – ; AVH 2008; Johnson undated). It has been found in aquarium shops in Darwin, but is not known to be naturalised in open water in the Northern Territory (Miller & Schultz 1998). It thrives in ornamental lakes in the Australian Capital Territory, where winter surface temperatures are close to freezing (Sainty & Jacobs 1981).

Where does it originate?

Dense Waterweed is native to Brazil, Argentina and Uruguay, South America (Aston 1973; Sainty & Jacobs 1981; Parsons & Cuthbertson 2001).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

NSW, NT, 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

Egeria densa

Other scientific names (synonyms)?

• Anacharis densa (Planch.) Vict.
• Elodea densa (Planch.) Casp.

Does it have other known common name(s)?

Dense Water Weed, Leafy Elodea, Egeria, Anacharis, Brazilian Elodea