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Black locust

Quick facts

Black locust is a MDA Restricted noxious weed in Minnesota.

  • Black locust is a nitrogen-fixing tree that prefers disturbed habitat, old fields, thickets and degraded woods.
  • It crowds out native vegetation of prairies, oak savannas and upland forests, forming single species stands.
  • Often forms dense stands where established and can choke out other vegetation.

Black locust should be reported. The Minnesota Department of Natural Resources provides detailed recommendations for reporting invasive species.

How to identify black locust

  • Fast-growing tree, up to 75 feet tall with an open crown.
  • Seedling and root sprouts have noticeably long thorns and grow rapidly.

Stem

  • Mature bark is dark gray-brown with deep furrows between flat-topped ridges.
  • Vigorous sprouts and young shoots are greenish in color and have paired spines up to 1 inch long at the base of leaves.

Leaves

  • Alternate, pinnately compound.
  • Each leaf has 7 to 21 elliptic, untoothed leaflets, with one leaflet at the tip.
  • A pair of short, sharp thorns sit at the base of each leaf where it is attached to the twig.

Flowers

  • Fragrant, drooping white flowers arranged in elongated clusters appear in late May and June.

Fruit and seeds

  • Flat, smooth legume seed pods, 2‒4 inches long, contain 4‒8 seeds.
  • Pods turn brown at maturity in September and persist through winter.
  • Seeds seldom germinate.

Roots

  • Extensive fibrous root system.
  • Spreads vegetatively through root suckering and runners.
  • Nitrogen-fixing legume.

 

Black locust seedling with thorns
Black locust leaves

Angela Gupta, Extension educator; Amy Rager, Extension educator; Megan M. Weber, Extension educator

Reviewed in 2019

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