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.
Common name: leafy elodea, Brazilian elodea, Brazilian waterweed, common waterweed, dense waterweed, egeria
Family: Hydrocharitaceae
Appearance: Aquatic herb
Status in Portugal: Invasive species (listed in Decree-Law no. 92/2019, 10th July).
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).
Fruits: berry-like, ovate, 7-8 mm long and 3 mm wide, with a membranous, transparent pericarp. The seeds are numerous, fusiform, 7-8mm long, with a 2mm filament at the tip.
Similar species
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.
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Native distribution area
South America.
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.
Other places where the species is invasive
Europe (France, Netherlands, Italy, Switzerland and United Kingdom), North America (Mexico, United States, Cuba, Puerto Rico), Oceania (New Zealand, French Polynesia, Cook Islands).
Introduction reasons
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.
