Common name: African wisteria tree

African wisteria is a flowering tree originating from south-eastern Africa, its natural range extending from Zambia, south through Mozambique, to Zimbabwe and Botswana.

It is a slow-growing tree and may reach up to 20 m (66 ft) in the wild, though it is typically 5 to 10 m (16 to 33 ft) tall and usually develops multiple trunks supporting a rounded crown made up of gently drooping branches. The bark is pale grey to grey-brown, deeply grooved and rough.

The leaves are feathery, with seven to fifteen leaflets arranged in pairs along the length and an extra leaflet at the tip. In the dry season, they fall off the tree to conserve water, leaving the branches bare and exposed. New leaf growth is encouraged by rainfall at the start of the rainy season, bringing the tree into flower.

The flowers, which are purple, bloom on grape-like clusters that hang from the tips of the branches, creating a showy and interesting display of colour. They last for around four weeks, then are replaced by slender seedpods that become yellow-brown when mature and persist unopened on the tree for up to twelve months.

Use

The tree's small size and showy sprays of purple flowers make it a sought-after flowering tree for the home garden and urban landscapes. Its attractively ribbed bark and small leaves also make it a good candidate for Bonsai.

The wood is heavy, averaging around 800 to 900 kilograms per cubic meter (50 to 56 lbs per cubic ft), and naturally resistant to rot, decay and wood-boring insects. However, the small trunk diameter limits its use to woodcraft, carvings, durable posts and fuelwood. The branchwood is sought after for turnery, tool handles, firewood and for making charcoal.

Climate

Grows naturally in dry to sub-humid subtropical and tropical grassland or savanna climates, generally areas with annual lows of 10 to 25°C, annual highs of 22 to 35°C, annual rainfall of 500 to 1600 mm and a dry season of 3 to 8 months.

Although African Wisteria is also cultivated in areas receiving up to 1600 mm annual rainfall, flowering is at its best and most spectacular in drier areas receiving only half or less that amount of rain and with a pronounced dry season.

Growing

New plants are usually started from seed that can be encouraged to germinate by placing them in near-boiling water and then leaving them to soak and cool. 

Performs best on free-draining clay-loam, loam and sandy-loam soils of a slightly acid to alkaline nature, generally with a pH of 6.5 to 8.0 and on sites with full sun exposure. A leguminous, nitrogen-fixing tree, it also thrives on nutrient-poor soils. It has good tolerance to drought and limestone soils conditions.

Problem features

The Hawaii Pacific Weed Risk Assessment (HPWRA) project assessed African wisteria as a low weed risk species for Hawaii.

Books

• Allen, O. N. & Allen, E. K. 1981, The Leguminosae : a source book of characteristics, uses, and nodulation, University of Wisconsin Press, Madison, Wisconsin

• Barwick, M., et al. 2004, Tropical & subtropical trees : a worldwide encyclopaedic guide, Thames and Hudson, London

• Dharani, N. 2011, Field guide to common trees & shrubs of East Africa, 2nd ed., Struik Nature Publishing, Cape Town

• Jex-Blake, A. J. 1957, Gardening in East Africa : a practical handbook, 4th ed., Royal Kenya Horticultural Society, Longmans, Green and Company, London

• Krishen, P. 2006, Trees of Delhi : a field guide, Dorling Kindersley Publishers, Delhi

• Lemmens, R.H.M.J., Louppe, D. & Oteng-Amoako, A.A. (eds). PROTA, Plant Resources of Tropical Africa, Volume 7(1) : Timbers 1, PROTA Foundation, Backhuys Publishers, Leiden

• Macmillan, H. F. 1943, Tropical planting and gardening : with special reference to Ceylon, 5th ed, Macmillan Publishing, London

• Mathias, M. E., 1982, Flowering plants in the landscape, University of California Press, Berkeley

• National Research Council (Board on Science and Technology for International Development) 1979, Tropical legumes : resources for the future, The National Academies Press, Washington D. C.

• Oakman, H. 1981, Tropical and subtropical gardening, 2nd ed., Jacaranda Wiley Publications, Milton, Queensland