Black Locust

Not native to Iowa, the black locust (Robinia pseudoacacia) is included here because it has been planted so widely over the state, has escaped from cultivation and is sometimes found growing in mixture with native species. Its natural range extends from Pennsylvania southwest ward to Alabama and west ward to southern Illinois. It has been planted widely in Iowa for post production and erosion control. The tree often suffers extensive damage from the locust borer insect.

two examples of black locust leaves
Black Locust Leaves - Photos by Paul Wray, Iowa State University

Hardiness: Zones 4 through 8

Growth Rate: Aggressive; Rapid

Mature Shape: Upright to spreading ovular shape

Height: 50 feet

Width: 25 feet

Site Requirements: Prefers rich, deep, moist, well-drained, and full to partial sun. However, commonly found in poor soil conditions, especially thin and near limestone outcrops.

Leaves: Alternate, compound, with thorns or spines

Flowering Dates: May - June

Seed Dispersal Dates: September - April

Seed Bearing Age: 10 years

Seed Bearing Frequency: Yearly or bi-yearly

Seed Stratification: Soak seeds in warm water for 24 hours.

The leaves are pinnately compound with 11 to 15 leaflets 1 to1-1/2 inches long, rounded at both ends and with smooth margins. The twigs are crooked and angular with short, stout, single, unbranched thorns, one-half inch long. The fruit is a dark, red-brown, flexible pod 3 to 4 inches long, containing small, reddish brown bean-like seeds. On young branches the bark is smooth and greenish to brown in color. On older branches and trunks it is broken into a network of coarse, deep ridges and is gray to gray-brown in color.

black locust bark
Black Locust Bark, Photo by Paul Wray, Iowa State University

black locust twig
Black Locust Twig, Photo by Paul Wray, Iowa State University

black locust thorn
Black Locust Thorn - Photo by Bill Cook, Michigan State University,

side by side photos of dried black locust fruit with an open pod showing seeds and non-dried black locust fruit
Black Locust Fruit - Photos by Paul Wray, Iowa State University

side by side photos of black locust flowers, one white and one purple/pink
Black Locust Flowers - Photos by Paul Wray, Iowa State University