Verbena bonariensis (Purple Top)

Scientific name

Verbena bonariensisL.

Synonyms

Verbena bonariensis L. var. conglomerata Briq.; Verbena bonariensis L. var. bonariensis

Common names

Purple top, tall verbena

Family

Verbenaceae

Origin

Native to South America

Naturalised distribution (global)

Locations within which Verbena bonariensis is naturalised include Australia, eastern Africa and some oceanic islands with warm climates.

Introduced, naturalised or invasive in East Africa

Verbena bonariensis is considered to be invasive in parts of Kenya (A.B.R. Witt pers. obs.) and Tanzania (Henderson, 2002). In Kenya purple top  is a widespread species that is particularly common around Nairobi and dry upland forests in Timau, Aberdares, Kitale, Rift Valley and Mau The editors are not aware of records of the presence of V. bonariensis in Uganda, though this does not necessarily mean that it is absent from this countries.

Habitat

A common weed of roadsides, pastures, grasslands, open woodlands, riparian vegetation, crops, orchards, gardens, disturbed sites and waste areas in warmer temperate, sub-tropical and occasionally also tropical environments.

Description

The stems of Verbena bonariensis are square in cross-section (quadrangular) and roughly hairy (scabrous) and sometimes develops a slightly woody base.

The oppositely arranged leaves are stalkless (sessile) with bases that slightly clasp the branches. They have elongated (lanceolate), narrowly oval (elliptic) or oblong blades (4-22 cm long and 6-70 mm wide) with pointed tips (acute or acuminate apices). Their margins are irregularly toothed, particularly towards the tip of the leaf blade. The upper surfaces of the leaves, like the stems, are coarsely hairy and rough to touch (scabrous), while their undersides are densely softly hairy (tomentose).

The numerous small tubular flowers are densely arranged into branched, finger-like, clusters at tips of the stems. These clusters (corymbs) are made up of 4-10 slightly-elongated spikes 1-4 cm long. The stalks (peduncles) of these clusters and the small bracts below each flower are covered with short sticky (glandular) hairs as well as longer stiff hairs. Each flower has five small sepals (2.5-3.5 mm long) that are fused together at the base into a tube (calyx tube). The five bluish, purple or lavender-pink petals are also fused together into a tube (corolla tube) about twice as long as the sepals (6-7 mm long), but their tips are separated into five spreading petal lobes (about 2 mm long). Flowering occurs mainly during the rainy seasons.

The small fruit separate into four  brown 'seeds' (mericarps or nutlets) when mature. These 'seeds' (1.5-1.8 mm long) and are elongated in shape.

Reproduction and dispersal

Verbena bonariensis reproduces mainly by seed and self-seeds readily. The seeds may be dispersed by animals, wind, or in water including storm water in urban areas. They may also be spread in contaminated agricultural produce and farm implements. It has a long-lived seedbank.

Economic and other uses

Verbena bonariensis is an ornamental plant whose flowers are very attractive to butterflies and bees.

Environmental and other impacts

Verbena bonariensis is weed of wasteland and pastures. It is not very aggressive but can be persistent. It may be favoured by prolonged grazing.

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.

Complete clearance of the mature plant before seeding and the use of uncontaminated planting material and farm implements can help to prevent its spread. Small infestations can be cleared by hand pulling and digging. Larger infestations can be treated with herbicide. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

Fire can be used as a management tool, but usually in combination with other methods such as chaining. Fire alone may actually increase densities of Verbena bonariensis by plant regrowth and enhanced seed germination.

The editors do not know of any biological control programmes targeted at this species.

References

Agnew, A. D. Q and Agnew, S. (1994). Upland Kenya Wild Flowers. A flora of the Ferns and Herbaceous Flowering Plants of Upland Kenya. 2nd Ed. EANHS, Nairobi-Kenya.

Beentje, H. (1994). Kenya Trees, Shrubs and Lianas, National Museums of Kenya, Nairobi, Kenya.

Germplasm Resources Information Network (GRIN). www.ars-grin.gov/npgs/index.html. National Germplasm Resources Laboratory, National Genetic Resources Program, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), Beltsville, Maryland, USA. Accessed March 2011.

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

Pacific Island Ecosystems at Risk (PIER). Verbena bonariensis L., Verbenaceae: plant threats to Pacific ecosystems. www.hear.org/pier/species/ verbena_bonariensis.htm. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed March 2011.

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