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.
Branches become increasingly darker gray to gray-brown, the bark on the trunk coarse and deeply furrowed with forking ridges. The trunk can reach 24 inches in diameter at breast height (dbh).
Fruit is a small, thin, flat pod 2 to 4 inches long, green turning reddish or purplish brown at maturity.
Pods contain up to 10 hard seeds, mottled brown and black and about 1/6 inch long.
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?
on: 2017-07-07 20:50:40
I have spotted it growing along the old railroad track bed on the S.W end of town.
on: 2018-05-02 13:21:14
I have a 5 acre parcel that was once pasture land. These have made a solid establishment on my land. They produce an extremely hard wood that is also very good firewood. The folklore is the Spanish once used it to make their ships. I harvest the larger ones for firewood & have been eradicating the smaller ones ASAP.
on: 2018-07-26 11:40:30
Found some Black Locust growing just off the boat launch.
on: 2019-06-01 23:37:22
Found a young up-and-coming stand in an unmanaged forest on the property, they appear to be relatively quickly reproducing due to root suckering. While this stand isn't blooming as of yet, I have noticed that mature trees produce a highly sweet smelling blossom that is very pleasant. Apparently honey from these trees' nectar is also prized.
on: 2019-09-12 14:58:49
Saw this seedling for the first time on my property. It's beautiful but I know better. Pulled it. Thank you for the information. Your site helps me make informed decisions.
on: 2021-09-14 07:54:09
I have a dozen or so of these trying to get started in my native grass/flower shoreline restoration area and will need to use chemicals to kill them, they can't be pulled out and cutting them results in fast regrowth.
on: 2022-08-03 15:13:19
A large colony of this species grows on some rock outcrops near 15th Ave. West in Duluth. It appears to have been sprayed a few years ago but whether that will be enough I don't know. The plants have been here since the 1980s at least.
on: 2022-08-21 09:55:08
My Seek app identified it along the path on the east side of Medicine Lake this am.
on: 2023-02-28 21:21:30
There is one at the dog park in the small dog enclosure at Zachary Ln, Champlin