Plant 233

Robinia pseudoacacia L. (Fabaceae)


Black locust

Leaf, flower and fruit of Robinia pseudoacacia from Joseph de Tournefort's 'Institutiones rei herbariae' (1700; v.3, t.417)

The black locust, a deciduous tree native to southeastern North America, is a legume. Bacteria (Rhizobia) in nodules on the tree's roots can convert atmospheric nitrogen into forms that can be used by plants. Black locust is tolerant of infertile soils, low temperatures and drought, spreads by suckering and as a young tree grows rapidly. These attributes make the tree well suited to erosion control but add to its potential as an invasive species.

In its native range, trees can grow to heights of 30 metres, with very straight trunks. The compound (pinnate) leaves are usually green, but can be bright yellow in some ornamental cultivars. Young trees are often densely covered in spines (modified stipules). The flowers are white and fragrant, and arranged in drooping racemes; they are a major source of nectar for bees. The wood is extremely durable and contains high levels of flavonoids which protect the heartwood against decay. As a result, the wood is valued for fence posts, shipbuilding (particularly 'tree nails') and other construction purposes. Visiting Virginia in the early 1700s, Mark Catesby describes 'little hovels' constructed from Robinia timber. The young, protein-rich foliage can be processed into fodder for cattle. Robinia responds well to coppicing and is also a potential source of biomass for fuel.

Although introduced to Britain around 1636, it was from 1819 that the tree was the subject of enthusiastic marketing campaigns by the English pamphleteer William Cobbett. He wrote extensively about its uses, 'this is, in my opinion, the tree of trees' (The Woodlands, 1825) and claimed to have sold over a million trees. He was criticised for selling the tree as 'black locust' at a higher price, and lower quality, than trees sold by other nurseries as 'robinia'.

Linnaeus named the tree after Jean Robin (1550-1629) who was an apothecary and gardener to Henry III, Henry IV and Louis XIII of France. He grew the first example of the tree in Europe, planted in 1601, on the site now occupied by the Jardin des Plantes in Paris.

The first records of Robinia escaping cultivation appear in Germany in 1824. It is now considered an invasive species in many European countries. While it establishes and spreads in grassland and wasteland, it is intolerant of shade and cannot compete in the long term with climax tree vegetation. Robinia trees growing near rivers contribute to nitrogen pollution and its associated problems.

Further reading

Withers W 1842. The acacia tree, Robinia pseudo acacia: its growth, qualities, and uses. Longman, Orme, Rees, Brown, and Longmans.

Sternberg G and Wilson J 2004. Native trees for North American landscapes. Timber Press.

James Penny