Robinia pseudoacacia L.
|Black Locust flowering in mid-June at Baker Park Reserve, Maple Plain, MN.|
Black Locust, also called False Acacia or Yellow Acacia, is
a medium to large tree originally from the Appalachian and Ozark mountains.
Intentionally planted for its strong wood, stabilizing roots and attractive flowers,
it has spread widely from its native range. It’s now found in all lower 48
states, several Canadian provinces and every continent around the globe (1, 2).
Black Locust History and Habitat
Although it isn’t native to the Minnesota, Black Locust has been
here a while. The earliest herbarium record in the Minnesota Biodiversity Atlas
(3) is an 1887 specimen collected in Lake City, along the Mississippi
River in the southeast part of the state.
Black Locust thrives in full sun. It reproduces quickly
from root suckers.
In later years, collectors found the plant farther north and
west. The Atlas documents the plant in Duluth in the 1940s and Mankato and Pipestone in the1960s. Records increased in the following decades: 11 in
the 70s and 80s, 18 in the 90s. Then collections taper off, perhaps because the
plant became so abundant that it was no longer a novelty.
It’s unsettling, this march across the state. Black Locust
has escaped plantings to become naturalized, and where this adaptable plant finds
sun and anything but waterlogged soils, it survives and even thrives. Old
fields, rights of way and degraded woods are now part of its expanded habitat.
So are prairies, savannas and open forests, where single species stands
of Black Locust can challenge restoration (4, 5, 6).
Adapted for Colonization
Black Locust is aided in its spread by its pioneer habit, a
set of adaptations for quick colonization of canopy gaps and other
disturbances. It grows fast and produces abundant seeds carried by wind, water
and gravity. Although seeds may drop and germinate below their parent plants, within
a stand the trees reproduce mainly by vigorous root suckers and stump sprouts, shoots
that arise from roots and trunks. That’s especially true if the trees are
damaged, such as from a storm or from cutting, and explains why Black Locust is
so resilient and persistent.
What’s more, Black Locust is a legume, a member of the bean
family, Fabaceae. Like other plants in that family, it can fix nitrogen. Nodules
on its roots hold bacteria that can convert nitrogen gas in the atmosphere to
ammonia, a usable form. That’s an advantage in nutrient-poor soils, and not
just for Black Locust. Soils enriched with this captured nitrogen support other
plants, including non-native ones that may further displace native species (7).
The Neonative Debate
For these reasons, even inside its native range Black Locust
is known to be weedy. Outside its range it’s often called invasive, but some biologists
stop short of calling it non-native. Because its historical range is in North America, they prefer to call it a neonative, a species that isn’t native (here
before Europeans) but also isn’t non-native (from another continent).
|Black Locust's distribution is in green, from EDDMapS (17). Approximate native range is added and circled in red, based on a map from the USDA Southern Research Station (18).|
As it was originally defined, a neonative species is one
that moves to a new area in response to an environmental change, such as a warming
climate (8). Such species arrive without direct human intervention, and because
they come on their own, they challenge our ideas about which species
belong in an area and which don’t – and therefore which should stay, and which
Black Locust doesn’t quite fit that definition. Humans
brought it here, so it’s not a neonative in the original sense of the word. The label
stuck, though, and now Black Locust, the “nuisance neonative,” has a mixed
reputation. In this area it’s bad for biodiversity, especially in natural areas,
but in its home range it’s good for native insects and other animals (10). In
the Midwest the nitrogen it adds to soils can alter natural communities in undesirable
ways, but where mine lands need to be reclaimed, nitrogen enrichment aids
recovery. And where forests and prairies need protection, Black Locust’s suckering
growth is a problem, but where soils are eroding, the trees’ clutching roots are a solution (19).
Overseas, a Similar Story
The debate is just as vigorous in Europe. Black Locust was
introduced there in the 1600s and now is found in more than 40 countries. It
has been widely planted, and although it’s considered one of the worst
invasive plants in the continent, it’s also valued for biomass production,
erosion control and honey-making (11, 12).
Biologists there recommend a “tiered approach” to managing Black Locust:
Remove it from natural areas but keep it in commercial forests, in urban
plantings and in selected forests where succession, the gradual change in plant
community composition, is allowed to play out (12).
Mature Black Locust hold 3- to 4-inch-long, brown pods through
winter. Each contains 4-8 seeds. Buds are alternate, but they’re under the leaf
scars and barely visible. Nodes often have a pair of stout thorns, ½ to 1 ½
inches long. Branches and young trunks may also have thorns. Mature bark is
dark gray or brown with deep furrows and flat-topped ridges.
|Black Locust pods hang on mature trees through winter. Each pod is 3-4 inches long.|
Black Locust buds are alternate but lie under the leaf scars, which may be cracked on their surfaces.
A pair of stout thorns is found at most nodes.
Mature bark is dark brown or gray with deep furrows
and flat-topped ridges. Smaller trunks bear thorns.
Prickly Ash (Zanthoxylum americanum) is a shrub or
small tree that also has pairs of thorns at the nodes, but the thorns are only
¼ to ½ inch long. Buds are red, fuzzy and clearly visible. Prickly Ash does not produce pods.
From a distance, Kentucky Coffee Tree (Gymnocladus
dioicus) resembles Black Locust. Pods on mature females are larger than
those of Black Locust and filled with a green mash. The tree has no thorns.
Honey Locust (Gleditsia triacanthos), likely
introduced to Minnesota as cultivated varieties, also has persistent pods, but they’re
much longer than Black Locust – about 16 inches. Some trees have large,
three-parted thorns on their trunks (13).
Prickly Ash buds are red and fuzzy. The pair of thorns at the nodes are smaller
than those of Black Locust
|Female Kentucky Coffee Trees carry pods through winter, but they're much larger than those of Black Locust. Seeds are embedded in a sticky, green mash.|
The Minnesota Department of Agriculture includes Black
Locust on its list of Restricted noxious weeds, meaning the plant can’t be
imported, sold or transported except as allowed by state law (14). The rules are similar in Wisconsin (15).
Black Locust Toxicity
Leaves, bark and seeds contain robin and phasin, compounds
that interfere with protein synthesis and can kill cells. Horses are especially
sensitive to these toxins, but humans can also get sick with nausea, weakness,
flushing and lethargy. If any part of the plant is ingested, seek medical help
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Plant Data Team, Greensboro, NC USA.
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Accessed Feb. 2, 2023.
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2013. Ecosystem legacy of the introduced N2-fixing tree Robinia
pseudoacacia in a coastal forest. Oecologia 172: 915-924. https://doi.org/10.1007/s00442-012-2543-1
Essl, F. et al. 2019. A conceptual framework for
range-expanding species that track human-induced environmental change.
Bioscience 69 (11): 908-919. https://doi.org/10.1093/biosci/biz101
Shah, S. 2020. Native
Species or Invasive? The Distinction Blurs as the World Warms. Yale E360.
Jaffe, D. Rethinking
Black Locust. Posted in 2019 on the website for the Ecological Landscape
Sitzia, T., Cierjacks, A., de Rigo, D., and
Caudullo, G. 2016. Robinia
pseudoacacia in Europe: distribution, habitat, usage and threats. In
European Atlas of Forest Tree Species. Ed: San-Miguel-Avanz, J., de Rigo, D.,
Caudullo, G., Durrant, T., and Mauri, A. Publication Office of the European
Vítková, M., Müllerová, J., Sádlo, J., Pergl, J., Pyšek, P. 2017. Black
locust (Robinia pseudoacacia) beloved and despised: A story of an invasive tree
in Central Europe. Forest Ecology and Management 384: 287-302. DOI: 10.1016/j.foreco.2016.10.057
Smith, W.R. 2008. Trees and Shrubs of
Minnesota. University of Minnesota Press, Minneapolis.
Minnesota Department of Agriculture. Minnesota
Noxious Weed List. Accessed Feb. 6, 2023.
Wisconsin Department of Natural Resources. Black
Locust. Accessed Feb. 6, 2023.
Poison Control. Are
Black Locust Trees Toxic? National Capital Poison Center. Accessed Feb. 9,
EDDMapS. 2023. Early Detection & Distribution
Mapping System. The University of Georgia - Center for Invasive Species and
Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed
February 11, 2023.
Huntley, J.C. No date. Black
Locust. USDA Southern Research Station.
Farmer, S. 2020. Black
Locust & Drought. CompassLive, USDA Southern Research Station.