Scientific name

Cymbopogon nardus (L.) Rendle

Synonyms

Cymbopogon afronardus Stapf.; Cymbopogon validus (Stapf) Burtt Davy; Andropogon nardusL.

Common names

False citronella (Zaire), citronella grass (Taiwan), blue citronella grass (Kenya), Tussocky Guinea grass (Uganda), naid grass (India).

Family

Poaceae (Gramineae)

Origin

Its origin is disputed with some sources claiming it is indigenous while others claim that a different biotype was introduced which is now invasive.

Naturalised distribution (global)

Locations within which Cymbopogon nardus is naturalised include southern, eastern and north-eastern tropical Africa.

Introduced, naturalised or invasive in East Africa

Cymbopogon nardus is invasive in parts of Uganda (A.B.R. Witt pers. obs.) and is present in  Kenya and Tanzania.

Habitat

Common in grassland and open woodland of Acacia and Combretum on the hills in Uganda. It is possible that it is a native species which is expanding its range due to overgrazing.

Description

Cymbopogon nardus is a tall tufted long-lived (perennial) grass with narrow leaf-blades. Panicle (flower cluster) narrow, 15-30 cm long with racemes (unbranched inflorescence) 8-10 mm long, often rather covered with long soft hairs; spikelets without stalks, flat or concave on the back with winged keels (resembling the keel of a boat), awn (bristle-like structure) 5-6 cm long (Harrington and Pratchett 1974).

Reproduction and dispersal

Cymbopogon nardus establishes naturally from seed, which are known to germinate prolifically after fire. Too-frequent burning is one of the main causes of its increase.

Economic and other uses

Cymbopogon nardus is commonly grown for citronella, an essential oil. It can also be used for thatching, mulching and erosion control.

Environmental and other impacts

Cymbopogon nardus can take over rangelands where it lowers the yield and quality of the forage. It tends to increase where there has been overgrazing though grazing can be used as a management method (see below). The grass is unpalatable to cattle which have been known to die of starvation when it is available in abundance. Buffalo will eat it sparingly and elephants will accept it during the dry season. It has also been detected in national parks in Uganda and may constitute a threat to biodiversity.

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). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

The manipulation of grazing regimes can be used as a management method. Generally Cymbopogon nardus is avoided in grazing. Light grazing encourages it, but heavy grazing pressure of one bullock per hectare prevented recolonisation of the species. Periodic very heavy stocking converted a slope pasture of 47 per cent C. nardus to a mixed pasture dominated by Brachiaria decumbens. The application of 158 kg N/ha increased the content of B. decumbens still further.

C.  nardus is very resistant to fire and too-frequent burning is one of the main causes of its increase. However, Harrington and Pratchett (1974) found that a late burn in the long dry season (usually late August in Uganda) carried out every third year reduced the biomass of C. nardus and encouraged the somewhat better, associated grasses of Brachiaria decumbens, Themeda triandra and Hyparrhenia filipendula. The burn should be against the wind and in weather which would minimise fire temperatures. This would prune the undesirable associated shrub Acacia hockii. Annual burning reduces the size of the C. nardus plants, but does not improve the sward.

Spot spraying with a suitable herbicide reduces C. nardus without negatively impacting indigenous species and indigenous species respond positively to C. nardus reduction (Ssegawa 2007). When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

Legislation

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

References

Harrington, G.N. and Pratchett, D. (1974). Stocking rate trials in Ankole, Uganda: I. Weight gain of Ankole steers at intermediate and heavy stocking rates under different managements. Kampala.

Ssegawa, P. (2007). Effects of Herbicide on the Invasive grass, Cymbopogon nardus (Franch.) Stapf (Tussocky Guinea grass) and Responses of Native Plants in Kikatsi subcounty, Kiruhuura district, western Uganda. www.naro.go.ug/Invasivespecies/Reports/1-Effects%20of%20Herbicide%20on%20the%20Invasive%20grass_Cymbopogon%20nardus.pdf.

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: [email protected]