Agroforestree Database

Local names:
 Dutch (robinia,valse acacia,schotdoorn), English (acacia locust,black locust,Chinese scholar tree,false acacia,honey locust,robinia,yellow locust,white acacia,white locust,locust tree), French (acacia blanc,robinier faux-acacia,acacia des jardiniers), Hi

Robinia pseudoacacia is a medium-size tree, 25 m tall, 60 cm dbh; trunk irregular; crown open, irregular; branches short, brittle; the persistent stout spines on young shoots are found on mature wood; the smooth bark becomes reddish-brown and deeply furrowed with age.

Leaves are deciduous, alternate, pinnately compound, composed of 7-19 leaflets (terminal leaflets present) on a central stalk 20-30 cm long; 2 spines (modified stipules) at the base of each leaf; leaflets oval, 30-50 mm long, dull green, bristle tipped, smooth margined.

Flowers showy, white, pealike, fragrant, in loose, drooping clusters, about 14 cm long, arising from leaf axils near the tip of a new shoot.

Fruits are pods, 7-10 cm long, flat; husk thin walled, smooth, dark to reddish-brown, several on a central stalk, remain on the tree during winter; seeds dark, beanlike, 3-5 mm long, 5-8 per pod, with a hard impermeable coat.

The genus was named after Jean Robin (1550-1629) and his son Vespasien Robin (1579-1662), herbalists to Henri IV of France, who 1st cultivated R. pseudoacacia in Europe. The specific name is derived from Greek ‘pseudes’ (false), and ‘acacia’; the tree looks like an acacia though it is not one. The name ‘locust’ was given by early missionaries who fancied that the tree was the one that supported St John in the wilderness. It is, however, an American tree, which is not native to any other part of the world.

Ecology

Apart from its cold tolerance, in ecology R. pseudoacacia resembles many other fast-growing, secondary forest legumes. However, it grows well on poor sites as acidic as pH 4.8. Although native to regions of 1000 mm annual rainfall, it survives on as little rain as 400 mm and withstands long dry periods. The trees are pioneers on disturbed soils or burned sites. It dominates early forest regeneration in many native forest stands. The tree does not tolerate waterlogging or shade.

Native range
United States of America

Tree management

Since its Rhizobium is very specific and growth can be completely stunted by the absence of nodulation, a new introduction of R. pseudoacacia must be accompanied by Rhizobia. It coppices readily and may even be lopped annually.

Seed storage behaviour is orthodox; seeds are safely dried to 4% mc, and a few seeds germinate after 50 years of storage at room temperature; viability is maintained for 10 years or more in air-dry storage at 0-5 deg. C; dry seeds survive overnight in liquid nitrogen. There are about 50 000 seeds/kg.

Apart from its cold tolerance, in ecology R. pseudoacacia resembles many other fast-growing, secondary forest legumes. However, it grows well on poor sites as acidic as pH 4.8. Although native to regions of 1000 mm annual rainfall, it survives on as little rain as 400 mm and withstands long dry periods. The trees are pioneers on disturbed soils or burned sites. It dominates early forest regeneration in many native forest stands. The tree does not tolerate waterlogging or shade.

Direct seeding and transplanting are the preferred methods for establishment. The seeds need scarification to germinate well (mechanical or immersion in boiling water). Tree suckers readily form roots and graft easily. They can be propagated, though with difficulty, from hardwood cuttings 15-30 cm long and 1-2 cm in diameter. Treatment with Indole Acetic Acid improves rooting. The tree responds well to tissue culture and has been mass propagated by this method. In nursery culture, R. pseudoacacia is either direct seeded or root sections (5-8 cm long) are planted.

Poison:  Early studies revealed toxins called robin and robinit in the bark, leaves and flowers that are believed to be associated with animal poisoning.

Rapid growth, dense wood and nitrogen-fixing ability make it ideal for colonizing degraded sites. The tree is used extensively to rehabilitate surface mine tailings and to stabilize road banks and mine spoils.

The species is lopped heavily for fodder. Mature leaves average 20-25% crude protein, 12% fibre and 45-50% nitrogen-free extracts; variation in these values is high. Tannin levels are high in young leaves but decrease with maturity. Despite the presence of toxins in the leaves, the fodder is considered highly attractive to many animals.

Apiculture:  R. pseudoacacia honey is regarded as one of the world’s finest. The slowly granulating honey is water-white, heavy bodied, fine flavoured with high fructose and low enzyme content. Tree improvement for late flowering and nectar sugar content 

R. pseudoacacia wood burns hot and slowly, like coal, and makes good charcoal. Wood energy yield is typical of temperate broadleaf trees, about 19.44 kJ/kg. Fuelwood plantations in South Korea coppice readily and may even be lopped annually, yielding 10-20 t/ha of fuel. In Hungary, R. pseudoacacia is often grown on small private farms for wood.

Fibre:  R. pseudoacacia is a preferred wood for pulp production.

Timber:  R. pseudoacacia is high in specific gravity (0.8) and variable in colour, but it darkens to an attractive golden brown. The wood’s hardness makes it difficult to work, but it has been widely used for fence posts, household furniture, wooden pins, wagon-wheel hubs, panelling, siding flooring, boat building, decking, vineyard or nursery props, fruit boxes and pallets. High levels of the alkaloid taxifloin make the wood very resistant to rot.

Nitrogen fixing:  This tree could be called ‘the grandfather’ of nitrogen-fixing trees. In the 1890s, R. pseudoacacia was the 1st tree to reveal nodules and symbiotic nitrogen fixation.

Ornamental:  Being a Papilionoideae legume with white butterfly flowers, R. pseudoacacia can serve well as an ornamental tree around homesteads.

A dense growth habit makes R. pseudoacacia suitable for windbreaks. It is grown in the Appalachians particularly for fence posts.

Soil improver:  Nutrients such as calcium, magnesium and phosphorus are available through leaf litter fall.

Intercropping:  R. pseudoacacia may prove useful for alley cropping in temperate climates. Researchers at the Rodale Research Center in Pennsylvania, USA, are experimenting with intercropping R. pseudoacacia with vegetables. Numerous reports indicate the beneficial effect of this nitrogen-fixing tree to associated plants through improved soil fertility. Mixed plantings of black locust and conifers, however, can lead to reduced growth or death of the slower growing conifers because of shading and overtopping.