Are Black Locust Trees Toxic?

The black locust (Robinia pseudoacacia) tree is often found in temperate zones and is native to eastern and central US. The bark of the trunk is coarse and deeply furrowed and bears characteristic stout thorns that extend to its branches. Generally blooming in May or June, the white flowers of the tree droop into clusters about 4–8 inches long. Another characteristic feature of the black locust tree is the light to dark brown legumes (pods) that are found on the tree during the winter. They are usually about 2–4 inches long, flat, straight or slightly curved, and typically contain 3–10 seeds per pod. 

The earliest reports of possible poisoning associated with the black locust involved horses, which either became severely ill or died from swallowing parts of the tree. There have also been a handful of cases in which young children have become sick after chewing on or eating parts of the bark or seeds.

Black locust trees contain the toxalbumins (toxic protein molecules produced by certain plants) robin and phasin in their leaves, bark, and seeds. These toxic compounds inhibit protein synthesis and can cause cell death. Of note, the flowers appear to be nontoxic or substantially less toxic. Based on the reports of ingestions by humans in the literature, the most common symptoms after ingestion include nausea, weakness, flushing, lethargy, and vomiting. Rare reports of more severe symptoms like liver injury and seizures have been reported. There have been suggestions that symptoms appear to be related to the amount of material ingested and the extent of chewing—higher amounts and more chewing leads to potentially more serious symptoms. Most gastrointestinal effects develop within 6 hours after an ingestion, which can also lead to other complications like dehydration. 

Although most cases of exposure described in the medical literature are of people swallowing parts of this plant, there have also been several reports of skin punctures and injury from the thorns located on the bark and branches. Most of these exposures have resulted in local symptoms such as pain, swelling, and rash. However, there have been a few reports of more severe outcomes following punctures from black locust thorns including neurologic (brain related) symptoms and local tissue death (necrosis).

Although severe toxicity after exploratory exposures to black locust seeds and bark is uncommon, the handful of cases reported indicate that all black locust exposures should be taken seriously. If someone has been exposed to black locust, get an immediate personalized recommendation online or call Poison Control at 1-800-222-1222. Both options are free, confidential, and available 24 hours a day.

Kristina Yee, PharmD
Certified Specialist in Poison Information