Egeria densa (Leafy Elodea)

Scientific name

Egeria densa Planch.

Synonyms

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

Common names

Leafy elodea

Family

Hydrocharitaceae

Origin

Egeria densa is native to southern Brazil, Uruguay and Argentina.

Naturalised distribution (global)

Locations within which Egeria densa is naturalised include North America, Europe, Japan and Australasia, Africa (Ghana, South Africa, Kenya, Uganda).

Introduced, naturalised or invasive in East Africa

In Uganda, Egeria densa is cultivated in ponds at Makerere University Botanic Garden; In Kenya, it is more widespread in ponds around Nairobi, Naivasha and Marsabit District in the North.

Habitat

Still or slow-flowing freshwater at altitude of 360 to 2400m above sea level.

Description

Egeria densa is a submerged freshwater long-lived (perennial) plant growing in shallow water. It grows to 1.5 metres, and rarely to five metres long. E. densa grows in water up to 4 m deep.

Stems are cylindrical, trailing and produce roots at intervals along the stem.

The  leaves are oval- to oblong-shaped, generally 1.5-4 cm long, 2-5 mm wide and with an acute apex, and found in groups (whorls) of 4-5 at the stem nodes. Lower stem leaves may be opposite or in whorls of 3, while the middle and upper leaves can grow in whorls of 4 to 8. Dense clusters of leaves appear at the ends of branches which often grow to reach the water surface.

Male and female flowers of E. densa grow on separate plants (dioecious), although only male plants Flowers are white,1.2-2 cm wide) are found at the water surface on stems up to 8 cm long. The flowers have three large petals centred by a cluster of generally nine yellow anthers.

Seed production has not been recorded.

Reproduction and dispersal

Only male plants of Egeria densa have been observed outside its native range (South America); reproduction occurs entirely by vegetative methods.

The plant is spread through movement of plant fragments, both deliberate and accidental e.g. on boat trailers, fish traps or other equipment. E. densa has also been deliberately planted for commercial harvesting purposes.

Similar species

Egeria densa is superficially similar to species in the genera Lagarosiphon and Hydrilla. However, E. densa is larger and its flowers are relatively large and showy.

Economic and other uses

Egeria densa been widely sold for garden ponds and aquaria, for oxygenation, to abort excessive nutrients and for landscaping. However, this use cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Once dispersed to new areas, Egeria densa often establishes in nature. The plant forms thick mats that obstruct boat passage, trap sediments, crowd out native vegetation, and impede the migration of fish. E. densa has been included in the Global Invasive Species Database (GISD 2006). It has been listed as a noxious weed in South Africa(prohibited plants that must be controlled. They serve no economic purpose and possess characteristics that are harmful to humans, animals or the environment).

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Egeria densa should be prevented from escaping into waterways, especially in areas that are currently free of infestation. The plant should be removed from ponds and aquaria and dried in the sun. Manual control can be difficult for large infestations as the plant can regenerate from small stem fragments.

Legislation

Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.

Notes

Egeria densa species seems to be an emerging problem in ponds around Nairobi.  In view of recent increase in fish farming initiatives in many parts of Kenya, immediate control measures and monitoring efforts should be considered. Stocking water bodies with certain fish has been suggested as a control method though this could have negative consequences.

References

CABI Invasive Species Compendium online data sheet. Egeria densa (leafy elodea). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

GISD (2006). Global Invasive Species Database online data sheet. Egeria densa (aquatic plant). www.issg.org/database. Accessed March 2011.

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Henderson, L. and Cilliers, C.J. 2002. Invasive aquatic plants-a guide to the identification of the most important and potentially dangerous invasive aquatic and wetland plants in South Africa. PPRI Handbook No. 16, Agricultural Research Council, Pretoria.www.arc.agric.za/uploads/images/0_SAPIA_NEWS_No._17.pdf.

Simpson, D. (1989). Hydrocharitaceae. In Polhill  R.M. (ed.). Flora of Tropical East Africa. AA Balkema, Rotterdam.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: eafrinet@africaonline.co.ke