Situation of Alternanthera sessilis in the EPPO region
Alternanthera sessilis (Amaranthaceae) is an aquatic annual or perennial plant which was introduced in the EPPO region for ornamental purposes.
Geographical distribution
EPPO region: Spain.
Asia: Bhutan, Cambodia, China, Christmas Island, India, Indonesia, Japan, Lao, Malaysia, Myanmar, Nepal, Philippines, Singapore, Taiwan, Thailand, Viet Nam.
Africa: Ghana, Nigeria.
North America: USA (invasive) (Alabama, Arkansas, Florida, Georgia, Hawaii, Louisiana, Maryland, South Carolina, Texas), US Virgin Islands.
Caribbean: Puero Rico.
Oceania: American Samoa, Australia (Queensland, Northern Australia, Western Australia), Cook Islands, Fiji, French Polynesia, Guam, Micronesia, Nauru, New Caledonia, Norfolk island, Papua New Guinea, New Zealand, Northern Mariana Islands, Palau, Solomon Islands, Tonga, Tuvalu, Vanuatu, Wallis and Futuna Islands.
South America: Ecuador (Galapagos Islands), Peru.
Note: the species is casual in the United Kingdom.
Morphology
A sessilis is a perennial herb with prostrate stems, often rooting at the nodes, 10 to 100 cm long. Leaves are obovate, occasionally linear-lanceolate, 1-15 cm long, 0.3-3 cm wide, and petioles are 1-5 mm long. Flowers in sessile spikes, 0.7-1.5 mm long. Fruits are utricles 1.8–3 mm long and 1.3–2 mm wide. Seeds are lenticular 0.9–1.5 mm long and 0.8–1 mm wide.
Biology and ecology
A. sessilis prefers places with constant or periodically high humidity, but may however tolerate extremely dry conditions. It often grows in mixed association with several other aquatic species. The plant spreads by seeds, which are wind-and water-dispersed, and by rooting at stem nodes. Seedlings appear in April, and fruits appear during August-October in the Northern Hemisphere.
In which habitats
The species occurs in estuarine habitats, riparian zones, ruderal/disturbed areas, wetlands, gardens, open fields and plantations. According to the Corine Land Cover nomenclature, the following habitats are invaded: coastal wetlands, banks of continental water, riverbanks / canalsides (dry river beds), road and rail networks and associated land, other artificial surfaces (wastelands), green urban areas, including parks, gardens, sport and leisure facilities, arable land.
Pathways
The species is traded for ornamental purposes. Its seeds are also naturally spread by wind and water.
Impacts
A sessilis is a pest of sugarcane in Africa. In tropical areas, it is associated with bananas and other cereal crops. It is also often found as a weed of rice, but is considered of low significance. According to the US national assessment, A. sessilis is an agricultural weed that invades disturbed wet areas in tropical and subtropical areas of the US, and which has low impacts on ecosystems. Very little information is found on the impacts of A. sessilis on native species.
Control
In the USA, the species is listed as a noxious weed.
There is no information available on the management of A. sessilis. In India, the plant is reported to be affected by fungal species.
Considering the low impacts recorded for this species, it does not qualify to be registered on the EPPO Alert List. Nevertheless, A. sessilis is expected to extend its distribution to other Mediterranean countries and could be monitored.
Sources
Global Invasive Species Database (2005) Alternanthera sessilis.
Macdonald IAW, Reaser JK, Bright C, Neville LE, Howard GW, Murphy SJ ; Preston G (eds.) (2003) Invasive alien species in southern Africa: national resources. Global Invasive Species Programme, Cape Town, South Africa.
Tomaino A (2006) Alternanthera sessilis. Invasive species assessment protocol: US national assessments, 4 pp.
USDA (Undated) Alternanthera sessilis. Plants profile.