Scientific name

Opuntia exalta A. Berger

Synonyms

Austrocylindropuntia exaltata (a. Berg.) Backeb.; Opuntia subulata (Muehlenpf.) Engelm.

Common names

Long-spine cactus, cholla cactus

Family

Cactaceae

Origin

South America

Naturalised distribution (global)

Locations within which Opuntia exaltata is naturalised include southern and eastern Africa.

Introduced, naturalised or invasive in East Africa

Opuntia exaltata is invasive in parts of Kenya (A.B.R. Witt pers. obs.) and Tanzania (Henderson 2002).  (Global Invasive Species Database). It is likely to be found in as an ornamental in Uganda but it is not yet invasive there. Small, isolated infestations of O. exaltata are found throughout Kenya. Larger infestations are found in parts of Kenya including Naivasha, Nakuru and Nairobi.

Habitat

Savanna, grassland, but usually cultivated as an ornamental or as live hedge. Currently, this species is spreading into surrounding wooded grasslands, along roads in disturbed soil.

Description

Opuntia exaltata is a spiny, much branched succulent shrub 2-5 m tall. It is pale bluish-green or greyish colouration (glaucous). Its spines are strong, straight, up to 5 cm long, yellow-brown in colour and arise from the white woolly pits (areoles) in clusters of 1-2-3.

O. exaltata develops a trunk with age and its branches (cladodes) are curved, cylindrical, tuberculate (with small wart-like swellings or small rounded outgrowths) with knobbly projections.

Its leaves are elongated 30-60 or 120 mm long, curved, fleshy and persistent.

Flowers are terminal on cladodes, orange or greenish-yellow. The fruit is green, pear-shaped, about 90 mm long, but usually sterile.

Reproduction and dispersal

Opuntia exaltata reproduces vegetatively from the branches.  It produces fruit but the seeds are usually sterile. It produces fertile seeds in native range.

Similar species

There are no similar looking cacti in East Africa.

Economic and other uses

Opuntia exaltata is cultivated as an ornamental or as live hedge used to exclude livestock, large and small wild mammals. It can be used as a medicinal plant. However, these uses cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Opuntia exaltata is a potential ecosystem transformer species. The spines and glochids can irritate the skin. The plant lowers the value of pastures since it cannot be browsed and it also curtails movement of grazing animals. In addition, the spines can injure people who stray in areas where they are growing and even cause tyre punctures to vehicles. Therefore, they can affect tourism in wildlife parks and ranches. The spines can also injure livestock and wild herbivores, especially when normal pasture is reduced by the invading cactus and these animals are forced to feed on O. exaltata. It displaces native species and prevents the free movement of wildlife.

O. exaltata 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). 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.

Manual control can be effective when numbers of plants are very low but must be done carefully otherwise plant fragments will resprout into new plants, thus exacerbating the infestation. Both the spines make this a difficult and uncomfortable process). Burning of uprooted plants will help minimise this risk if there is enough dry material to ensure that the material burns. Plants can be treated by herbicide stem injections. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

Controlled burns have been used to control Opuntia species. Such burns must be well-timed and coordinated to reduce the risk of creating a bushfire and there must be sufficient material to carry a hot fire. Fire could be used for small, isolate stands but it will not penetrate large stands.

The moth Cactoblastis cactorum feeds on this plant in its larval stages and can help control Opuntia exaltata as part of an integrated control programme. It was introduced to Tanzania but never established. It is likely that some cochineal species which feed on Opuntia species have been introduced to East Africa. This group offers some prospects for biological control of Opuntia species.

Legislation

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

Notes

In some parts of the world, the spineless cultivars of Opuntia are exempt from restrictions associated with the more invasive spiny species.

References

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. (2002). Problem plants in Ngorongoro Conservation Area. Final Report to the NCAA.

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]