Thunbergia grandiflora (Blue Thunbergia)

Scientific name

Thunbergia grandiflora (Roxb. ex Rottler) Roxb.

Synonyms

Flemingia grandiflora Roxb. ex Rottler; Pleuremidis grandiflora (Roxb.) Raf.; Thunbergia cordifolia(Nees)

Common names

Blue thunbergia, Bengal clock vine, Bengal clock-vine, Bengal clockvine, Bengal trumpet, Bengal trumpet vine, blue sky flower, blue sky vine, blue skyflower, blue trumpet vine, blue trumpetvine, clock vine, giant thunbergia, green trumpet vine, Indian sky flower, large flowered thunbergia, large-flowered thunbergia, sky flower, sky vine, skyflower vine, skyvine, thunbergia, trumpet vine.

Family

Acanthaceae

Origin

This species is native to the Indian sub-continent, southern China and Myanmar.

Naturalised distribution (global)

Locations within which Thunbergia grandiflora is naturalised include tropical Australia, tropical South America, Central America, south-eastern USA and some oceanic islands with warm climates.

Introduced, naturalised or invasive in East Africa

Thunbergia grandiflora is naturalised in parts of Uganda (Dawson et al, 2008) and invasive in parts of Tanzania (Tropical Biology Association 2010).

Habitat

A weed of riparian zones (banks of watercourses), disturbed closed forests, forest margins, open woodlands, roadsides, fence-lines, gardens and plantation crops in tropical and subtropical regions.

Description

Thunbergia grandiflora is a long-lived (perennial), vigorous, climbing plant that can grow up to 15 m in height when supported by a host tree.

Younger stems are green, hairy (pubescent), and square in cross-section (quadrangular). The older climbing stems are quite thick when mature, and they usually turn brown in colour and become somewhat rounded in shape.

The oppositely arranged leaves are borne on hairy stalks (pubescent petioles) 4-12 cm long. These leaves are variable in shape (8-22 cm long and 3-15 cm wide) and may have broad heart-shaped (cordate) bases, be somewhat triangular in shape, or be roughly egg-shaped in outline with broad end at base (ovate). Their margins are also quite variable, and can range from being almost entire, to being irregularly toothed (crenated) or have several irregular, pointed lobes. The leaves are bright green in colour and somewhat hairy (pubescent).

The trumpet-shaped (tubular) flowers are borne in elongated clusters (racemes) on long, drooping (pendent) branches. They are large and showy (3-8 cm long and 6-8 cm across) with five pale blue, violet or mauve coloured petal lobes and a pale yellow or whitish coloured throat. Each flower is borne on a stalk (pedicel) 4-5 cm long and has two leafy bracts (bracteoles) at its base. These bracts (15-40 mm long and 10-20 mm wide) are oblong or ovate and have pointed tips (acuminate apices). The flowers also have hairy (pubescent) sepals that are fused together and reduced to a ring-like structure (calyx tube) that is greenish-coloured and may sometimes be streaked with purple or red. Flowering occurs throughout the year, in the plants around Nairobi.

The fruit is a capsule with a rounded (spherical) base (about 18 mm long and 13 mm across) and a long tapered beak (2-5 cm long and about 7 mm wide). These fruits are only produced in the warmer climates. The large, flattened (compressed), seeds (up to 10 mm across) are smooth on one side and warty on the other side. Seeds (5-10mm wide) contained in seed pods.

Reproduction and dispersal

Thunbergia grandiflora reproduces via seed. Seeds in pods are catapulted several metres when the pod splits.(but fruit are only produced in warmer climates). T. grandiflora is also capable of regenerating from stem fragments or portions of the tuberous roots and vegetatively by stolons. Dispersal of the disseminules may be by stem and tuber pieces carried by water or in dumped garden waste. The tuberous roots may also be spread during soil moving activities (e.g. roadworks) and by flood waters.

It is widely cultivated as an ornamental and hedge plant.

Similar species

Thunbergia grandiflora is not similar, but is related to other Thunbergia species among them Thunbergia alata (black-eyed Susan), Thunbergia battiscombei and Thunbergia fasciculata.

It is also similar to Thunbergia laurifolia (laurel clock vine) and quite similar to Thunbergia fragrans (fragrant thunbergia) and Thunbergia arnhemica  all of which are exotic to the region.

T. grandiflora can be more easily distinguished from T. alata by the conspicuous yellow flowers which are much smaller.

Economic and other uses

Thunbergia grandiflora can be used as a medicinal plant, a green manure, for poles, hedges and for fuelwood. It is widely grown as a garden ornamental and wall covering in Kenya.

Environmental and other impacts

Thunbergia grandiflora has an extensive tuberous root system can be weigh up to 70kg. The root system persistently sprouts from its many buds when cut back or pruned. Tubers can damage river banks, paths, fences and building foundations. It is a vigorous climber and can smother vegetation up to 12 metres above ground, reducing light levels for lower vegetation. The weight of the stems can kill trees during infestations. Although it is not currently regarded as a major threat in Kenya, Tanzania and Uganda, it is an important weed in Australia where is can infest agricultural lands and in some conservation areas can establish 100 per cent ground cover and exclude all native vegetation.

T. grandiflora has been included in the Global Invasive Species Database (GISD 2010).

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.

To control this plant, small plants can be dug out, but more established plants have extensive underground tubers that are difficult to remove from the ground completely and therefore removal has to be done repeatedly.

Spraying or painting cut stumps with herbicides such as glyphosate is an effective control method. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

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

Legislation

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

References

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.

GISD (2010). Global Invasive Species Database online data sheet. Thunbergia grandiflora (vine, climber). www.issg.org/database. Accessed March 2011.

Global Compendium of Weeds. www.hear.org/gcw. Hawaiian Ecosystems at Risk Project. Accessed March 2011.

Lazarides, M., Cowley, K. and Hohnen, P. (1997). CSIRO Handbook of Australian Weeds. CSIRO Publishing, Collingwood, Victoria.

Pacific Island Ecosystems at Risk (PIER). Thunbergia grandiflora (Roxb. ex Rottl.) Roxb., Acanthaceae: plant threats to Pacific ecosystems. www.hear.org/Pier/species/thunbergia_grandiflora.htm. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed March 2011.

Tropical Biology Association (2010). Usambara Invasive Plants - Amani Nature Reserve - www.tropical-biology.org/research/dip/species.htm.

USDA Plants Profile. Thunbergia grandiflora Roxb. Bengal trumpet. The Plants Database. plants.usda.gov/java/profile. National Plant Data Center, National Resources Conservation Service, United States Department of Agriculture, Baton Rouge, Louisiana, USA. Accessed March 2011.

Weber, E. (2003). Invasive Plant Species of the World: A Reference Guide to Environmental Weeds. CABI Publishing, Wallingford, UK.

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