Black Locust, Robinia pseudoacia
Fabaceae or Pea Family

Tall tree, blooming in mid spring.  Bark rough.  Leaves compound, with 6 to 20 egg-shaped leaflets arranged opposite each other, and one similar leaflet at the tip.  Branches have a pair of thorns at the base of each leaf, which persist after the leaves fall.  (One of the thorns in the picture is actually a camouflaged membracid Treehopper, an insect in the order Homoptera and family Membracidae.)  Flowers in dense drooping clusters, white, fragrant, resembling the flowers of peas.  Fruit a long, brown, flattened pod. 

Occasional in wooded areas of the park. 

Unmistakable in bloom when the flowers may cover the trees.  Easily identified even when not in bloom as the thorny branches and leaves are distinctive, at least in the Park.  Honey locust (Gleditsia triacanthos) is most similar, but the leaflets are smaller, and there are 2 at the end, and there are no thorns on the branches. Pods, when present, are also distinctive. 

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Single flower closeup