Robinia pseudoacacia (Black Locust)
|Also known as:|
|Life cycle:||perennial woody|
|Origin:||Ozarks and Appalachia|
|Habitat:||sun; upland forest edges and openings, urban plantings|
|Plant height:||30 to 60 feet|
|Wetland Indicator Status:||GP: UPL MW: FACU NCNE: FACU|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Showy, hanging racemes 2 to 5 inches long in leaf axils of 1-year-old branches, each with up to 30 white pea-like flowers. The flowers are fragrant, ¾ to 1 inch long, the upper petal (standard) erect and greenish yellow at the base, the lower petal (keel) hiding the 10 stamens and single slender, curved style. The calyx surrounding the base of the flower is tubular, about ¼ inch long, hairy, often reddish or brownish green with 5 triangular lobes that are shorter than the tube.
Leaves and stems:
Leaves are alternate, once compound, 4 to 12 inches long with 7 to 23 leaflets. Leaflets are elliptic to narrowly egg-shaped, ¾ to 2¼ inches long, rounded or tapered at the base and blunt or rounded at the tip, sometimes with a small notch. Edges are toothless, the upper surface smooth or finely hairy, lower surface with short appressed hairs along major veins. Fall color is typically a dirty yellow.
Young shoots are green and hairy with numerous pale lenticels (pores), turning red-brown and smooth. A pair of spines, ½ to ¾ inch long, may persist at the leaf scars or drop off older twigs. Buds are embedded in old leaf scars.
Native to the higher elevations of the southeastern US, Black Locust's fast growth rate, tolerance of harsh growing conditions and its tough, rot resistant lumber gave it a high value during early settlement. It is also quite a pretty tree and horticulturists, especially in Europe, have made numerous selections on form and leaf color. In the past two centuries, humans seemed to have planted it everywhere and it is now widely established throughout much of the US, Europe, China and southern Australia. Unfortunately it is not well behaved, spreading by seed and, once established, produces large colonies through root suckering. It can readily exclude native shade trees important to natural ecosystems and due to its root suckers, difficult to manage without resorting to toxic chemicals. Young trees and saplings may be mistaken for Prickly Ash (Zanthoxylum americanum), which has leaflets with pointed tips, at least a few prickles along leaf stalks, and buds that are clearly visible, where Black Locust leaflets are rounded, has no spines or prickles on leaf stalks, and buds are obscured in the leaf scars. Seedlings and new root suckers may also be confused with any number of other members of the Pea family with similar compound leaves, such as Veiny Pea (Lathyrus venosus) or False Indigo (Amorpha fruticosa), all of which would lack spines.
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- Black Locust tree in fall color
- spreading colony of Black Locust
- a clonal colony of Black Locust in flower
- more leaves
- flowering branch
- spines at the leaf base
- bud embedded in the leaf scar
Photos courtesy Peter M. Dziuk taken in Anoka, Hennepin, Houston, Ramsey and Winona counties.
Have you seen this plant in Minnesota, or have any other comments about it?