Dark green submerged aquatic plant, scarcely branched, with 4 or 5 leaves per node. It forms “tufts” at the water body bottom, and can grow up to 6 meters, forming floating “carpets” close to the surface.
Scientific name: Egeria densa Planch.
Appearance: Aquatic herb
Risk Assessment Score: 20 | Value obtained according to a protocol adapted from the Australian Weed Risk Assessment (Pheloung et al. 1999), by Morais et al. (2017), according to which values above 13 mean that the species has risk of having invasive behavior in the Portuguese territory | Updated on 30/09/2017.
Synonymy: Anacharis densa (Planch.) Vict.; Elodea densa (Planch.) Casp.; Philotria densa (Planch.) Small; Elodea canadensis var. gigantea L.H.Bailey; Elodea densa var. longifolia Bonstedt; Udora densa (Planch.) M.R.Almeida
Last update: 01/06/2020
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How to recognize it
Submerged, dioecious, dark green aquatic plant. It forms “tufts” at the bottom of the water, and can grow to form floating “mats” close to the surface. The very short internodes make the stems have an overall very dense appearance that resembles cylinders 2 to 6 cm thick and 10 to 90 cm long.
Roots: filamentous, at the base of plants and in some nodes, especially in loose fragments of the mother plant. In shallow water, plants may be anchored to the bottom, in other cases they may float free.
Stems: sparsely branched, with short internodes, fragile, breaking easily. “Double knots” (i.e., two single knots separated by a very short internode) occur along the stem every 6 to 12 knots and produce side branches, vegetative buds, and roots. Stem tissues store carbohydrates.
Leaves: arranged regularly along the stems in dense whorls, usually with 4 leaves per node/whorl, but may have 5 or even 6. sawn (only visible with a magnifying glass), smooth surface. Intensely green in natural light, paler in aquaria.
Flowers: spathes grow from the axils of some leaves and from their interior flower peduncles 2 to 6 cm long emerge, exposing solitary flowers, about 2 cm above the water surface. The male flowers are gathered in groups of 2 to 4 for each spathe and consist of 9 stamens surrounded by a perianth with three green sepals and 3 white petals measuring 15 mm The female flowers are isolated (one per spathe) and also have 3 green sepals and 3 white petals 15 mm long; in the center is a unilocular ovary consisting of 3 carpels, surrounded by 3 staminoids (stamen-like but sterile structures).
Elodea canadensis, Lagarosiphon major and Hydrilla verticillata are very similar in appearance but the number of leaves that unite at each node is different and the flowers of Egeria densa are larger; see the illustrated key (below) to see how they are distinguished. Or download the PDF [here]
Characteristics that aid invasion
This plant has great reproductive potential. It reproduces by seed, but is dioecious, depending on pollination by certain insects. Still, even in areas where there are only male plants, such as New Zealand and the United States, this plant shows great invasiveness, reproducing only vegetatively. Small fragments can be carried by the current in rivers and be accidentally dispersed by boats, fishing artifacts or other human activities and each fragment (provided it contains a double knot) can give rise to a new plant and re-root. When a branch of Egeria densa sinks in autumn or winter, a crown of roots can develop from one or more double-knots.
The fact that herbivorous animals (such as fish, waterfowl, invertebrates, etc.) generally prefer native plants can lead to their progressive elimination in places with the presence of exotic oxygenating species (e.g. Lagarosiphon major, Egeria densa, and Elodea canadensis). For this reason, it is common for only exotic species to remain, forming monospecific communities (with only one species).
It can grow in both shallow and deeper waters, and can live up to 6m deep.
Native distribution area
Distribution in Portugal
Minho, Douro Litoral, Beira Litoral, and the Azores (S. Miguel, Terceira, and Flores islands).
For more detailed locations of this species, check the online interactive map. This map is still incomplete – we need your help! Contribute by submitting records of the location of the species where you can find it.
Europe (France, Netherlands, Italy, Switzerland and United Kingdom), North America (Mexico, United States, Cuba, Puerto Rico), Oceania (New Zealand, French Polynesia, Cook Islands).
The main reason for its introduction is its use in aquariums as an ornamental and oxygenating plant.
Preferential invasion environments
Egeria densa is a submerged fresh-water plant, preferring still or slow current water and temperatures ranging between 15 and 17º C. It occurs in both oligotrophic and eutrophic environments and tolerates a wide pH range. It does not tolerate shading . It occurs in ponds, lakes, dams, reservoirs and rivers and canals with slow current.
Although already introduced, its dispersion in Portugal is still relatively limited.
Impacts on ecosystems
It can create dense floating mats which cause several negative impacts both ecologically and economically. Floating dense mats block light eliminating native aquatic plants and negatively affecting of aquatic invertebrate and vertebrate populations . It can make navigation difficult and limit recreational activities such as swimming or fishing; block hydroelectric systems; reduce the aesthetic value and negatively affect water quality.
In the US removing this species from lakes and reservoirs costs some States several million dollars per year.
Controlling an invasive species demands a well-planned management, which includes the determination of the invaded area, identifying the causes of invasion, assessing the impacts, defining the intervention priorities, selecting the adequate control methodologies and their application. Afterwards it is fundamental to monitor the efficiency of the methodologies and recuperation of the intervened area as to perform, whenever necessary, the follow-up control.
Hand pulling: feasible for small invasions, nets may be used; however, the ability to propagate from small fragments implies that it is necessary to repeat the actions or may even result in an increase in the invasion if the removal of the fragments is not adequate.
Cutting: feasible for small invasions, but may result in an increase in the invasion if the removal of the fragments is not adequate.
Shading: some authors refer to the success of localized control of Egeria densa in swimming areas and docks by shading, with the placement of textile materials that block the light. There are authors who consider that mechanical control should only be used in fully invaded places. In places that are undergoing invasion, the use of these methods can accelerate the rate of dispersion.
Drainage: in reservoirs or lakes where it is possible to lower the water level one or several times in a row it may be enough to eliminate this plant, but the success of this methodology depends on several factors such as the degree of dryness, the type of substrate, the temperature of the air and the presence of snow are also known examples of failure. The studies on fungi of the Fusarium genus indicate that they produce lethal biotoxins for plants of the Egeria and Hydrilla genera that may be useful in their control.
There is no biological control agent studied for Portugal. In the United States, some authors suggest the use of herbivorous fish to control Egeria densa, but there are no publications that confirm the effectiveness and selectivity of these methods. Studies carried out in Brazil point to a fungus, Fusarium graminearum, as a potential biological control agent.
The use of the herbicide Diquat is mentioned, although the use of chemical control in water bodies has obvious ecological risks associated with it.
Visit the webpage How to Control for additional and more detailed information about the correct application of these methodologies.
Center for Aquatic and Invasive Plants, University of Florida, Institute of Food and Agricultural Sciences, https://plants.ifas.ufl.edu/plant-directory/egeria-densa/ [Retrieved 22/11/2017].
Marchante H, Morais M, Freitas H, Marchante E (2014) Guia prático para a identificação de Plantas Invasoras em Portugal. Coimbra. Imprensa da Universidade de Coimbra. 207 pp.
Morais MC, Marchante E, Marchante H (2017) Big troubles are already here: risk assessment protocol shows high risk of many alien plants present in Portugal. Journal for Nature Conservation 35: 1–12
Nachtigal GF, Pitelli RA (2000) Fusarium sp. as a Potential Biocontrol Agent for Egeria densa and Egeria najas. Proceedings of the X International Symposium on Biological Control of Weeds. 4-14 July 1999, Montana State University, Bozeman, Montana, USA, Neal R. Spencer [ed.]. Disponível: http://bugwoodcloud.org/ibiocontrol/proceedings/pdf/10_142.pdf [Retrieved 06/12/2017].
Pheloung PC, Williams PA, Halloy SR (1999) A weed risk assessment model for use as a biosecurity tool evaluating plant introductions. Journal of Environmental Management. 57: 239-251.
State of Washington – Department of Ecology – Non-native Invasive, Freshwater Plants: Egeria densa – http://www.ecy.wa.gov/programs/wq/plants/weeds/aqua002.html [Retrieved 13/07/2017].
The Plant List. Egeria densa Planch. Disponível: http://www.theplantlist.org/tpl1.1/record/kew-309440 [Retrieved 06/12/2017].