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Invasive Species Sheet - Black Locust

Invasive Species Identification Sheet

Black Locust (Robinia pseudoacacia L.)

Alternate common names: Locust-tree; False or Bastard Acacia; White, Yellow, Green, Red-flowering, Common, Honey-, Post-, or Peaflower Locust.

Alternate Latin names: Pseudo-Acacia odorata Moench, Robinia Pseudo-Acacia L.
- deciduous tree; 40'-100' tall; may produce thickets from root sprouts
- leaves alternate; each with 7-21 leaflets arranged in pairs along leaf mainstem
- leaflets with smooth margins; dark or bluish green on top, lighter beneath
- twigs/branches with pairs of 1/4"-1" long spines where alternate leaves are/were attached
- except for the pairs of spines, twigs are smooth (with no bristles or glands)
- flowers white, fragrant, sweetpea-shaped; in 4"-8" long drooping clusters; May-June
- fruit flat, bean-like pod; brown and split open in the fall when dry; persists in Winter
- bark deeply furrowed, rough and twisted, with horizontal checks; smooth on saplings

The paired, thorny spines distinguish Black Locust trees, even in the winter. The lack of either sticky glands or a bristly surface differentiates young Black Locust twigs from twigs of related shrubs in the genus Robinia. Sweetpea-type flowers (with their 5 petals arranged as 1 large petal, 2 side-wing petals, and 2 petals fused into a boatlike keel) distinguish all species of Robinia from unrelated species that have paired thorns. Black Locust flowers are normally white, but pink cultivars exist. In mid-summer, leaves may appear brown or lacey because of insect attack. Honeylocust (Gleditsia triacanthos L., sometimes, confusingly, also called Black Locust) lacks
sweetpea-like flowers and has twice-compound leaves and single (sometimes branched) thorns.

Black Locust is native only from Monroe Co., PA, south to GA, and west to IA, MO, and OK. Its rootsprout colonies choke out native vegetation in dry areas as well as along streams. In barren areas, its ability to add nitrogen to the soil can promote the survival of non-native plants.