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Plantwise Technical Factsheet

leafy elodea (Egeria densa)

Description
E. densa is a herbaceous, tender plant, with cauline leaves regularly disposed in close whorls, resembling cylinders 2-6 cm thick and 10-90 cm long. Stems sparsely branched, with short internodes, delicate, breaking easily with the parts forming new plants. In shallow water, plants can be anchored to the bottom, otherwise free-floating. Filament-like roots, at the base of plants and at some nodes, especially in broken pieces. Leaves sessile, lanceolate, 1-3 cm long and 5 mm large, apex rounded or acute, margins finely serrated, surface smooth, intensely green when receiving natural light, more pale in aquaria. E. densa normally presents four leaves per whorl, but can present five or six. Plants are dioecious. From the axils of some leaves arise spathes and from their interior emerge floral peduncles 2-6 cm long, that expose solitary flowers ca. 2 cm above the water surface. Male flowers are in groups of 2-4, from one spathe, the perianth formed by a calyx of 3 green sepals, corolas with 3 white petals, 10-15 mm long, stamens 9. Female flowers one per spathe, perianth like that of males, ovary unilocular formed by 3 carpels, androecium only residual with 3 yellow staminodes. Fruits are berry-like, ovate, 7-8 mm long and 3 mm wide with membranaceous and transparent pericarp. Seeds numerous, fusiform, 7-8 mm long, with a 2 mm filament present at the end.
Prevention and control

Mechanical removal such as cutting, hand pulling or netting is feasible for small infestations, though the ability to propagate from small stem fragments means that repeat clearing will be required, or even that infestations may spread if removal is not adequate. Use of the herbicide diquat has been recommended, although using chemicals in water bodies leads to evident environmental risks. The stocking with certain fish such as grass carp has been suggested, as E. densa is highly palatable, but there are no reports as to the effectiveness of this method.

Impact
Although there are positive economic impacts resulting in the trade in aquarium plants including E. densa, this is strongly countered by the costs of control as exercised in many areas where it has become a serious problem. Removal of E. densa from lakes and reservoirs in the USA costs some states several million dollars per annum.
Summary of invasiveness
E. densa is highly desired in aquaria and small ponds, but has become a serious invasive species in larger bodies of fresh water, where dense mats reduce recreational options and crowd out native species as well as altering the hydrology. The principal means of entry is considered to be disposal of aquaria contents into local waterways, and spread is by vegetative means as many introduced populations comprise only male plants. Further introduction and spread is likely.
Hosts / species affected
E. densa is an environmental weed not affecting cultivated crops to any extent, though may impact on agriculture by the blockage of irrigation channels.
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