Click on images to enlarge
close-up of four-angled stem (Photo: Rob and Fiona Richardson)
stem, with branches in the forks of the paired upper leaves (Photo: Rob and Fiona Richardson)
young flower clusters (Photo: Rob and Fiona Richardson)
infestation (Photo: Sheldon Navie)
the flowers are borne above the tops of the flowering branches (Photo: Sheldon Navie)
close-up of old flower cluster showing the long stiff hairs and short sticky hairs on the stalk, bracts and sepals (Photo: Jose Hernandez at USDA PLANTS Database)
habit (Photo: Sheldon Navie)
Verbena bonariensis L. var. conglomerata Briq.; Verbena bonariensis L. var. bonariensis
Purple top, tall verbena
Native to South America
Locations within which Verbena bonariensis is naturalised include Australia, eastern Africa and some oceanic islands with warm climates.
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.
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.
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.
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.
Verbena bonariensis is an ornamental plant whose flowers are very attractive to butterflies and bees.
Verbena bonariensis is weed of wasteland and pastures. It is not very aggressive but can be persistent. It may be favoured by prolonged grazing.
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.
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.
Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.
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).
BioNET-EAFRINET Regional Coordinator: email@example.com