Invasive of the month: Water Cabbage (Limnocharis flava)

Water Cabbage (Limnocharis flava), a serious aquatic weed which displaces native species, restricts water flow and access, provides breeding grounds for disease vectors and disrupts rice production, has been officially recorded in Ghana for the first although unofficial records state that it may have been present for more than 10 years.

Introduction

CABI, assisted by the International Union for the Conservation of Nature (IUCN), is the International Executing Agency for the Global Environmental Fund/United Nations Environment Programme (GEF/UNEP) “Removing Barriers to Invasive Plant Management in Africa” Project.  Staff from CABI Africa, were alerted to the presence of Limnocharis flava or water cabbage by colleagues from the Council for Scientific and Industrial Research (CSIR) in Ghana, during a routine visit to Ghana.  This aquatic weed had invaded a water body and was present in rice paddies alongside the main road between Kumasi and Accra.  It was apparently first recorded in a fish pond on the campus of the University of Kumasi in 1996.  There are also unofficial records of its presence near Yamoussoukro, Ivory Coast but these have not been confirmed.

Description

Limnocharis is rhizomatous

Limnocharis flava is a perennial, rhizomatous aquatic plant with milky sap that can grow to a height of 1m (Fig. 1).  The pale green leaves are oval to elliptic, 50–100 mm long and 30-250mm wide with distinctive transverse veins between the main longitudinal veins.  The triangular petioles are 12-85cm long with the bases having numerous air chambers.  The flowers (Fig. 3) which can appear 2-3 months after germination, have three pale yellow petals produced in umbel-like clusters, measuring 45-190mm across.  The compound fruit contains many wedge-shaped follicles filled with horseshoe shaped seeds - each plant can produce up to 1 million seeds annually.  It can also disperse vegetatively.

Habitat

Being a tropical species, L. flava is frost sensitive and prefers moist regions.  It has been recorded at altitudes ranging between 0-2000m above sea level growing in saturated, fertile and muddy substrate. In its native range in South America, L. flava is relatively common in wet meadows that remain underwater for most of the year. 

The bright yellow flowers can occur 2-3 months after germination

However, it can grow in terrestrial habitats in humid tropical environments and has been recorded to behave like an annual when desiccation of habitat occurs.  In its introduced range it has been recorded in rice paddies, canals and lakes and although its growth is slightly suppressed when submerged it can form leaves underwater.

Impact

In shallow water L. flava can compete with Eichhornia crassipes and in many respects has a similar impact on water bodies.  It disrupts the ecology of water bodies by displacing native species and changes the hydrology of these systems.  Like water hyacinth it also restricts access for humans and livestock to water and provides favourable conditions for disease vectors.  In cropping systems, particularly rice, it competes for nutrients and space which obviously has an impact on yields.

Management

Water hyacinth is currently considered as the worst aquatic weed in the world, including Africa.  Millions of dollars have been spent in an attempt to control this weed in Africa and biocontrol has formed a big component of most management strategies especially in countries with limited resources to introduce mechanical and chemical control measures.  Unfortunately, no research has been undertaken on the biological control of L. flava to date although some work has been undertaken on the efficacy of various herbicides.  Unfortunately, there are some 2,4-D resistant biotypes which is cause for concern.

In shallow water L. flava can compete with Eichhornia crassipes	Limnocharis is capable of growing to a height of 1 metre

Conclusions

The unfortunate situation has now arisen that areas in which water hyacinth has been effectively controlled may very well be invaded by L. flava, largely negating all of the progress that has been made in reducing the negative impacts of the former.  Rice production will also be compromised and place an additional burden on small scale farmers who already have to deal another invasive, Mimosa pigra.

The discovery of L. flava in Africa, once again highlights the need for structures to be put in place that will prevent the introduction of invasive or potentially invasive species.  This is one of the main goals of the GEF/UNEP Project “Removing Barriers to Invasive Plant Management in Africa” which is currently working with institutions in Ghana, Ethiopia, Uganda and Zambia.  However, additional funds are required to ensure that we build capacity throughout Africa and elsewhere.

References

• Backer, C.A. and Bakhuizen van den Brink, R.C. (1968) Flora of Java, pp.1-2. P. Noordhoff, Groningen.
• Gunasekera, L. (2001) Limnocharis – new threat to Australian wetlands and irrigation. Under Control: Pest Plant and Animal Management News 17: 8-9.
• Haynes, R.R. and Holm-Nielson, L.B. (1992) ‘Flora Neotropica: the Limnocharitaceae’ In: J.L. Luteyn and S.A. Mori (eds). New York Botanical Garden, New York pp.34.
• 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. Plant Protection Research Handbook No. 16. Agricultural Research Council, Pretoria.
• Kostermans, A.J.G.H., Wirjahardja, S. and Decker, R.J. (1987) The Weeds: Description, Ecology and Control. In: M. Soerjani, A.J.G.H. Kostermans and G. Tjitrosoepomo (eds) Weeds of Rice in Indonesia. Balia Pustak, Jakarta. pp. 24-566.
• Kotalawala, J. (1976) Noxious water vegetation in Sri Lanka: the extent and impact of existing infestations. In: C.K. Varshney and J. Rzoska (eds) Aquatic Weeds in S.E. Asia. Dr W. Junk b.v.. Publishers, The Hague, Netherlands. pp. 51-58.
• Stodola, J. (1967) Encyclopedia of Water Plants. T.F.H. Publications, Jersey City.
• Wilder, G.J. (1974) Symmetry and development of Butomus umbellatus (Butomaceae) and Limnocharis flava (Limnocharitaceae).  American Journal of Botany 61: 379-394.

Reference: Arne Witt,   Co-ordintor Invasive Species, CABI Africa, Nairobi, Kenya.