Common Name: Black locust
Scientific name: Robinia pseudoacacia
Identification: Black locust is a large, fast-growing deciduous tree, 40- 60’ tall and 30’ wide. It has an oval-shaped crown. The leaves are opposite and composite; each leaf is composed of 7-19 leaflets. The leaves are 6-14” long with each leaflet being 1-2” long and oval in shape with a small notch at the tip. Leaflets are blue-green in color, turning yellow in the fall. The branches of black locust often bear short, sharp paired spines ½-1 ½” in length. Spines are more prevalent on suckers, and are absent in some cultivated varieties. Flowers emerge in mid-to-late spring and are white and borne in drooping clusters. Each flower is 1” across and the clusters are 4-8” long. The flowers are strongly and pleasantly fragrant. Black locust fruits are clusters of pods which are 2-4” long smooth, dark brown, and which each contain 4-10 seeds. The fruit matures in mid-fall and typically remains on trees into winter. (Dirr and Warren 2019, Hilty 2017)
Look-alikes: Black locust is similar in appearance to the native honey locust (Gleditsia triacanthos). The leaflets of a honey locust leaf are narrower, more than twice as long as they are wide, while the leaflets on black locust are more rounded. Honey locust leaves have an even number of leaflets, while black locust leaves have an odd number with a single leaflet at the end of each leaf. Honey locust leaves are bright green, while black locust leaves are blue-green. Outside of cultivation thorns are common on both species, and the thorns of honey locust are much longer (up to 8” long). Juvenile black locust has similar foliage to the shrubby bristly locust (R. hispida), which is also native to the U.S. but not to the Great Lakes region. However, the young branches of prickly locust are covered in bristly hairs and its flowers are purple.
Black locust is unique in being the only woody invasive species addressed by the WIGL Collaborative to date that is native to North America. Its historic native range was small compared to its current distribution and was constrained to the Appalachian Mountains and Ozark Plateau (Stone 2009, Kartesz 2014). It is not native to any part of the Great Lakes Basin.
Black locust is reported occasionally to frequently throughout the Great Lakes region. It is frequently reported near Toronto and Buffalo, and in northern Wisconsin. It is infrequently reported in northern Minnesota. Nationally, this species has been noted in all of the lower 48 states. Black locust is described by USDA as being hardy to -37 degrees Fahrenheit (Zone 3a), which has contributed to its wide distribution.
Additional invasive species distribution data for specific Great Lakes jurisdictions is available via:
- Midwest Invasive Species Information Network (Michigan)
- iMapInvasives (New York and Pennsylvania)
- EDDMapS Ontario
The seeds of black locust are primarily dispersed by gravity and wind. The seeds are toxic to many livestock and wildlife species (Hilty 2017), so long-distance dispersal by animals is likely limited. The germination rate of black locust seed is generally low due to a thick seed coat, but once seed is introduced, it can remain viable in the soil for decades (Michigan DNR 2012). Its current extensive range is primarily due to human introduction and subsequent escape from cultivation. Once black locust is established in a location, it spreads locally through the root system forming dense clonal colonies.
Black locust thrives nutrient-poor sandy soils in habitats with a lot of sun exposure. As such, it can be a major invader of sand prairies, pine barrens and savannah habitats (Stone 2009). It also frequently grows in disturbed areas like old pastures and fields, forest edges, secondary growth forests, floodplains, and transport and utility rights-of-way. Black locust cannot tolerate full shade and is not found in closed-canopy forests (Stone 2009).
Black locust’s fast growth rate and colony forming habit allow it to out-compete native species and form single-species stands in preferred habitats. Black locust is a nitrogen fixing plant and its leaves and other parts contain relative large quantities of nitrogen and other minerals which transfer to the soil when they fall off the plant. Over time, invasion by black locust can significantly enrich soils leading to establishment of other weedy species, particularly in habitats characterized by low-nutrient soils such as dunes, savannahs, barrens, and sand prairies (Michigan Natural Features Inventory 2012).
The leaves, bark, shoots and seeds of black locust are generally considered toxic to livestock, though the extent of its toxicity is debated (Stone 2009). It is most acutely toxic to horses (Michigan DNR 2012) and may be a hazard where it establishes in horse pastures.
Black locust was used by Native Americans and European colonizers for various purposes, and its movement across the continent began hundreds of years ago. It has had multiple ornamental and utilitarian uses (Greene 2015). Its wood is very dense and rot-resistant lending it to use for fence posts and other structures. It has been used at mine reclamation sites for its ability to grow in and improve depleted soils. It has also been used as a preferred forage species for honey bees. Black locust was also a popular ornamental tree in the early colonies, though its popularity has waned due to its messiness, thorniness and tendency to sucker (Missouri Botanical Garden). Hybrids and cultivars remain in trade (Dirr and Warren 2019).
Please see our Landscape Alternatives landing page for more information about how the WIGL Collaborative selected alternatives for each species.
| Alternatives for Black Locust
Common name (Latin name) |
Compound leaves | Tolerates wide range of soils | Fast growing | Colony forming |
| Thornless honey locust (Gleditsia triacanthos f. inermis) ‘Skycole’ SKY LINE b |
✔ | ✔ | ✔ | |
| Kentucky coffeetree (Gymnocladus dioicus) ‘Espresso’ b |
✔ | ✔ | ||
| Quaking aspen (Populus tremuloides) | ✔ | ✔ | ✔ | |
| Northern pin oak (Quercus ellipsoidalis) | ~ | ~c | ||
| Northern red oak (Quercus rubra) | ✔ | ~c | ||
| Shumard’s oak (Quercus shumardii) | ✔ | ~c |
a Listed alternatives are alternatives to black locust for landscaping purposes, but may not be suitable for other applications where black locust is used (e.g. reclamation, erosion control, etc.)
b Male cultivars of these species, including the ones listed, are often preferred because they don’t produce messy seed pods.
c These species grow quickly relative to other oaks, but moderately fast overall
Green = native to part of the Great Lakes Basin
~ = trait is somewhat present but not as pronounced as in check-marked examples
The following is a brief overview of management techniques shown to be effective on black locust. For more detailed information on how to use these techniques, visit our Management and Control page. For local assistance managing woody invasive species, please get in touch with a cooperative invasive species management group or a university extension program.
Timing and spread concerns: Whenever possible, individual plants should be controlled before they establish extensive root networks to prevent vegetative spread. If control is undertaken when seed pods are present, it is best not to remove the plants from the site to avoid spreading seed.
Physical control: Small seedlings can be removed by hand if they are confirmed to be seedlings and not root suckers from a nearby mature tree. Physical removal of larger individuals and suckers is extremely challenging due to the extensive root network and requires heavy equipment. Top killing methods such as mowing, grazing and prescribed fire are not effective as stand-alone practices, since they leave the root system intact and promote suckering. Mowers may also break black locust seed coats and promote increased germination (Stone 2009).
Chemical control and combined approaches: Herbicide applications are typically required to control mature black locust. Foliar, cut stump, basal bark, and stem injection application methods can all be effective, but timing and careful chemical selection are important. Foliar applications should only be conducted when plants are fully leafed out and are likely restricted to sites with few desirable plants. The Missouri Department of Conservation reports that basal bark application to small trees (<6” in diameter at breast height) results in the best control with the least amount of suckering and regrowth compared to other methods. For populations with large black locust trees, stem injection type applications may be the most practicable.
Biocontrol: Black locust has some biological enemies, including viral and fungal diseases and insect pests, all of which can impact its health. However, none of these organisms are approved for purposeful release as biocontrol agents.
With any treatment it will be necessary to monitor for and treat regrowth and seedling emergence. This is especially true of black locust due to its relatively high potential for regrowth.
Resources on management of black locust:
- Dirr, MA and KS Warren. 2019. Robinia pseudoacacia In: The Tree Book: Superior Selections for Landscapes, Streetscapes, and Gardens. Portland, OR: Timber Press. 778-781.
- Greene, W. 2015. Black locust: the tree on which the U.S. was built. LiveScience. 5/6/2020.
- Hilty, J. 2017. Black locust. In: Illinois Wildflowers. 7/17/2019.
- Kartesz, JT. 2014. Robinia pseudoacacia In: Biota of North America Program. 7/17/2019.
- Michigan Natural Features Inventory. 2012. Black locust. In: Invasive Species Best Management Practices. 7/17/2019.
- Missouri Botanical Garden. Robinia pseudoacacia In: Online Plantfinder Database. 7/17/2019.
- Missouri Department of Conservation. Black locust control. 7/17/2019.
- Stone, KR. 2009. Robinia pseudoacacia. In: Fire Effects Information System. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. 7/17/2019.
Photo: Matthew Beziat via flickr.com
