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;
- fruit flat, bean-like pod; brown and split open in the fall when dry; persists
- bark deeply furrowed, rough and twisted, with horizontal checks; smooth on
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)
sweetpea-like flowers and has
twice-compound leaves and single (sometimes
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.