Also known as
Robinia pseudacacia, black locust, R. pseudoacacia var. rectissima
Where is it originally from?
South Eastern USA
What does it look like?
Deciduous tree (<25 m tall). Young saplings have smooth, green bark, while older trees have deep, furrowed, shaggy, dark bark with flat-topped ridges. Leaves (20-30 cm) are made up of 7-21 thin, round leaflets that are dark green above and pale underneath, and attached in pairs along the leaf stalk. Smaller branches have 1 cm long spines at the base of each leaf stalk. Large, drooping clusters of pea-like, fragrant, white to yellow flowers (1 cm long, Nov-Jan) are followed by shiny, smooth, narrow, flat seed pods (5-10cm long) containing 4 to 8 seeds.
Are there any similar species?
Sophora (kowhai) species can look similar but don't have spines. Gleditsia (honey locust, water locust) species have often twisted seed pods, have thorns on the trunk and narrow leaflets.
Why is it weedy?
Reproduces vigorously by root suckering and stump sprouting to form groves of trees interconnected by a common root system, and also seeds heavily every 1-2 years, with lesser amounts of seed produced in intervening years. The large, fragrant flowers also compete with native plants for pollinating bees.
How does it spread?
Seeds and also by suckering.
What damage does it do?
Dense thickets shade out other plants in open or disturbed sites. Large, fragrant flowers compete with native plants for pollinating bees.
Which habitats is it likely to invade?
Prefers full sun and well drained soils and invades dry and sandy sites, ex-pasture, disturbed forest, disturbed shrubland, forest edges, plantation forest and roadsides.
What can I do to get rid of it?
1. Pull or dig out: for seedlings only, suckers from the root system are not controlled this way.
2. Cut and squirt: cut notches on downward angles around the trunk, 1 cut per 10 cm diameter, squirt with 1 ml of glyphosate (undiluted) or triclopyr 600EC (undiluted). Avoid ringbarking the tree.
3. Overall spray suckering growth and saplings 4 m tall or less (spring to early autumn): glyphosate (500ml/10L) or triclopyr 600EC (6ml/L) on a 'spray-to-glisten' rather than a 'spray-to-wet' basis.
What can I do to stop it coming back?
Monitor the site and treat any regrowth or seedlings. Plant a dense cover of native trees or shrubs to produce shade. If possible, track down and remove the seed source.