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Black locust (Robinia pseudoacacia), often incorrectly referred to as acacia, is a deciduous tree in the bean family (Fabaceae) that can pose a toxic risk to horses.
Native to parts of eastern North America, black locust grows rapidly, spreads aggressively and is now common across North America and may form dense stands in open fields, disturbed sites, and pasture margins.
Poisoning in horses is most commonly associated with ingestion of black locust bark, but other parts of the plant are also toxic to horses. Clinical signs primarily involve the gastrointestinal and cardiovascular systems and may range from mild digestive upset to severe cardiac compromise, laminitis, and prolonged recovery.
Learn how to identify black locust, recognize signs of poisoning, reduce exposure risk, and support effective prevention and veterinary management.
Black Locust Poisoning in Horses
Black locust (Robinia pseudoacacia) is a perennial deciduous tree belonging to the genus Robinia within the Fabaceae family.
The species is native to portions of the east-central United States but has become extensively distributed throughout North America due to widespread planting and naturalization. The species adapts readily to clay, loam, and sandy soils and can spread aggressively through underground root sprouts. [1][2][3]
Because of its vigorous growth, black locust frequently forms dense groves or thickets composed of closely spaced trees with interconnected root systems. In some areas, the species is invasive and may dominate the surrounding landscape, particularly in disturbed or unmanaged environments. [1][2][3]
Black locust is found throughout the United States, with notable frequency east of the Mississippi River. The tree is frequently present along pasture edges, open fields, roadsides, and horse properties where ornamental or shelter plantings have been established. [1][2][3]
In horses, poisoning is most often associated with ingestion of bark, although leaves, seeds, root sprouts, and other plant parts may also cause adverse effects. Flowers are generally not considered toxic to horses. [2][3]
Table 1. Black locust poisoning in horses at-a-glance
| Category | Key Information |
|---|---|
| Definition |
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| Primary Cause |
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| Common Causes |
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| Onset |
|
| Symptoms |
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| Severity |
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| Treatment |
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| Recovery Time |
|
| Prognosis |
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| Prevention |
|
Identification
Black locust is a medium-to-large deciduous tree that commonly reaches approximately 80 to 100 ft (25 to 30 m) in height. Mature trees typically have a relatively straight trunk and a broad, irregular crown formed by twisted or spreading branches.
In open environments, the canopy may appear wide and uneven, while trees growing in dense stands are often taller and more slender. [1][3][4]
Bark
Seed pods
Flowers and leaves
The bark is gray to brown and becomes deeply furrowed and ridged with age. Older trunks have thick fissures that create a rough, textured appearance. Younger branches are particularly distinctive because they bear paired stout spines at the nodes, an important identifying characteristic of the species. [1][3][4]
Black locust spreads readily, allowing colonies of interconnected trees to develop over time. Dense groves or thickets are therefore common, especially in disturbed soils, pasture margins, and unmanaged fields. [1][3][4]
The leaves are compound and alternate, with a central stem bearing 7 to 19 leaflets. Individual leaflets are oval to elliptic with smooth margins and rounded tips. Young leaf surfaces may bear fine hairs before becoming smoother with maturity. [1][3][4]
In late spring and early summer, black locust produces clusters of large white flowers. These flowers are highly fragrant and resemble those of other legumes, displaying the characteristic pea-like structure typical of the Fabaceae family. [1][3][4]
Following flowering, the tree develops flattened brown seed pods that can remain attached into early winter. The pods are linear and somewhat papery in texture, containing several flattened kidney-shaped brown seeds. [1][3][4]
Black locust commonly grows along fence lines, roadsides, open woodlots, abandoned fields, pasture margins, and disturbed soils. Because the species spreads aggressively, it may establish dense stands in grazing environments where horses can access trunks, branches, fallen limbs, or root sprouts. [1][3][4]
Table 2. Black locust plant identification at-a-glance
| Plant part | Identifying features |
|---|---|
| Overall tree |
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| Bark |
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| Younger branches |
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| Leaves |
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| Flowers |
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| Seed pods |
|
| Seeds |
|
Toxicology of Black Locust
Most parts of the black locust tree are toxic to horses, including the bark, leaves, seeds, root sprouts, and young shoots. The flowers are generally regarded as non-toxic and have not been associated with poisoning.
Though black locust is a known toxin plant to horses, the precise toxic compound responsible for clinical disease in horses is currently unknown. Multiple active compounds have been isolated from the plant over time, although the relative contribution of each to its toxic properties remains unclear.
One of the earliest identified compounds was a bark-derived protein called robin, which produces severe gastrointestinal effects in experimental conditions. [4]
Additional compounds isolated from black locust include robitin, a nonalkaloid glucoside associated experimentally with diarrhea, weakness, respiratory difficulty, paralysis, and cardiovascular abnormalities in horses and other species. A series of triterpene glycosides known as robinisides has also been identified within the bark. [4]
Lectins found in the bark and seeds may also contribute to toxicity. However, because many different compounds in the plant can affect the body, poisoning is likely caused by a combination of toxins rather than one single substance. [4]
In horses, the toxic effects of black locust primarily involve the gastrointestinal and cardiovascular systems. [3]
Signs of gastrointestinal effects in horses may include: [4][5][6]
- Colic
- Diarrhea
- Constipation
- Decreased intestinal motility
Cardiovascular effects may include: [4][5][6]
- Weak pulse
- Arrhythmias
- Discoloration of mucous membranes
- Cold extremities
Respiratory distress and dyspnea may also develop as the horse’s condition worsens. [4][5][6]
Table 3. Relative toxicity of black locust tissues
| Plant part | Toxicity concern | Notes for horse owners |
|---|---|---|
| Bark |
|
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| Leaves |
|
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| Seeds |
|
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| Root sprouts and young shoots |
|
|
| Flowers |
|
|
Symptoms of Black Locust Poisoning in Horses
Clinical signs associated with black locust poisoning primarily reflect gastrointestinal and cardiovascular dysfunction, although neurologic abnormalities may also occur in some horses. Severity varies depending on the amount ingested, the plant part consumed, and health status of the horse at the time of exposure.
Clinical signs may include: [4]
- Colic
- Diarrhea
- Constipation
- Decreased intestinal motility
- Weakness
- Lethargy
- Hyperexcitability
- Alternating excitement and lethargy
- Weak pulse
- Cardiac arrhythmias
- Tachycardia (rapid heart rate)
- Dark red gums
- Cold extremities
- Dyspnea or labored breathing
- Mydriasis (dilated pupils)
Colic is one of the most commonly reported signs and may occur along with major changes in gut function. In severe cases of cardiovascular compromise, horses may progress to heart failure. [4]
Neurologic abnormalities are thought to occur secondary to systemic disease and gastrointestinal dysfunction. Some horses may alternate between restlessness or excitement and lethargy. In severe cases, high ammonia levels in the blood may affect the brain, causing other neurologic signs like incoordination. [4]
Laminitis has also been reported as a potential complication in horses recovering from black locust intoxication. [7]
What's your top priority with your horse's health?
What's your top priority with your horse's health?
When to Call the Veterinarian
Black locust poisoning can affect multiple body systems and may progress quickly once clinical signs develop. Contact a veterinarian promptly if a horse has had known or suspected access to black locust bark, branches, root sprouts, or seed pods, especially if any signs of illness are present.
Veterinary attention is particularly important if a horse shows:
- Colic, diarrhea, constipation, or reduced manure output
- Weakness, depression, hyperexcitability, or alternating excitement and depression
- Rapid heart rate, weak pulse, cold extremities, or discolored gums
- Labored breathing or signs of respiratory distress
- Reluctance to move, hoof pain, or signs consistent with laminitis
- Known ingestion of black locust bark, seed pods or leaves
A veterinarian can evaluate the horse’s cardiovascular status, hydration, gastrointestinal function, and neurologic signs, while also ruling out other causes of similar symptoms. Do not wait for signs to worsen before calling if exposure is likely, since early supportive care may improve the chance of recovery.
Exposure Risk Factors
Black locust is widely distributed throughout the United States and is particularly common east of the Mississippi River. Extensive planting has expanded its range across much of the continent. [4][8]
Because the tree adapts readily to diverse soil conditions and spreads aggressively, it is commonly encountered in pasture-adjacent environments. Black locust frequently forms dense groves, and horses may encounter substantial quantities of plant material when trees are present within grazing areas. [4][8]
Horses are most often exposed when black locust trees are accessible within turnout areas, fence lines, or dry lots. Poisoning is particularly associated with bark chewing behavior in horses, often resulting from confinement or inadequate forage availability. Horses may also find young shoots and root sprouts palatable and consume them readily.
Importantly, toxicity in black locust trees appears inconsistent. Mature leaves are reportedly consumed by some horses without obvious clinical disease, whereas ingestion of relatively small amounts of bark has resulted in severe intoxication in others.
Clinical signs have been reported following ingestion of as little as approximately 70 grams of bark, although larger exposures are more commonly associated with severe disease. [4]
Diagnosis
Diagnosis of black locust poisoning is based primarily on a history of exposure and characteristic symptoms. Identification of bark chewing, stripped trunks, fallen branches, or accessible root sprouts may strongly support suspicion of exposure.
During a physical examination, a veterinarian may find signs such as colic, arrhythmias, dark or reddened gums, weakness, poor circulation, cold extremities, and neurologic abnormalities.
Bloodwork results can vary depending on how severe the poisoning is and whether complications have developed.
Possible findings on bloodwork include: [4]
- Abnormal electrolyte levels
- Low blood calcium
- Increased muscle enzyme levels in horses that are unable to stand
- High blood ammonia levels related to poor gut function
Electrocardiography (ECG) may reveal irregular heart rhythms, although the specific arrhythmia patterns associated with black locust poisoning have not been well-characterized. [4]
Because clinical signs are nonspecific, differential diagnoses may include other causes of toxic colic, gastrointestinal inflammation, enteral encephalopathy, cardiotoxic plant exposure, and systemic inflammatory disease.
Treatment & Management
There is no specific antidote for black locust poisoning, and treatment is primarily supportive and directed toward management of gastrointestinal and cardiovascular abnormalities.
Immediate removal of access to black locust trees, bark, branches, and root sprouts is important to prevent continued ingestion. Horses should be provided with clean forage, water, and a quiet environment during recovery. Activated charcoal may reduce gastrointestinal absorption of toxic compounds if administered early following ingestion. [4][7]
Supportive treatment focuses primarily on alleviating abdominal pain, maintaining hydration, correcting electrolyte imbalance, and managing cardiovascular complications. Medication may be administered for horses experiencing colic or gastrointestinal discomfort.
Fluid therapy may be administered to balance electrolytes for horses with diarrhea, dehydration, or reduced intestinal function. [4][7]
Additional supportive care depends on the severity of illness and may include intensive monitoring, nutritional support, management of laminitis, pain control, and nursing care for weak or recumbent horses.
Prognosis
The prognosis for black locust toxicosis in horses is guarded in symptomatic animals, particularly in cases involving significant cardiovascular compromise or severe gastrointestinal dysfunction.
Recovery is possible with appropriate supportive care, although improvement may require several weeks. Horses that recover are not expected to develop long-term complications. [4][7]
Prevention
Prevention of black locust poisoning in horses relies primarily on limiting access to trees and preventing bark chewing behavior.
Ideally, black locust trees should be removed from turnout areas, dry lots, and fence lines used by horses. Where removal is not practical, horses should be prevented from accessing trunks, low branches, root sprouts, and fallen limbs.
Providing adequate quantities of good-quality forage and enrichment can relieve boredom and hunger in horses, which can in turn reduce bark chewing behavior. Horses confined near black locust trees without sufficient forage may begin stripping bark from trunks and branches within reach. [9]
Pastures and turnout areas should be inspected regularly for storm-damaged branches, young shoots, and root sprouts, particularly in areas where black locust forms dense groves. Prompt removal of fallen material may help reduce accidental exposure.
Frequently Asked Questions
Here are some frequently asked questions about black locust poisoning in horses:
Black locust is poisonous to horses and should not be accessible in turnout areas, dry lots, or fence lines. Most parts of the tree, including the bark, leaves, seeds, root sprouts, and young shoots, contain compounds that can affect the gastrointestinal, cardiovascular, and nervous systems. The bark is the most clinically important source of poisoning and is involved in many reported cases. Horses may be exposed by chewing bark, eating fallen branches, or consuming young shoots and root sprouts.
The bark is considered the most toxic and clinically important part of the black locust tree for horses. Many poisoning cases involve horses chewing bark from trunks, branches, fallen limbs, or freshly cut wood. Seeds, leaves, root sprouts, and young shoots may also contain toxic compounds, although adverse effects from mature leaves appear less consistent. The flowers are generally regarded as non-toxic, but horses should still be kept away from the tree as a whole.
Black locust flowers are generally regarded as non-toxic to horses and have not typically been associated with poisoning. However, the presence of flowers can help identify a tree that has other toxic parts, including bark, seeds, root sprouts, and young shoots. Horse owners should not treat a flowering black locust tree as safe simply because the flowers are lower concern. The safest approach is to prevent access to the entire tree.
Black locust seed pods can be dangerous for horses because the seeds are considered toxic. The tree produces flattened brown pods that may remain attached into early winter, creating a potential exposure risk in pastures, dry lots, and fence lines. Horses should not be allowed to eat seed pods, especially where black locust trees are growing within reach or dropping plant material into turnout areas. Removing pods and preventing access to trees can help reduce accidental ingestion.
If a horse eats black locust bark, poisoning may cause colic, diarrhea or constipation, weakness, depression, poor circulation, abnormal heart rhythm, and labored breathing. Some horses may also show neurologic signs such as incoordination, dilated pupils, hyperexcitability, or alternating excitement and lethargy. Severe cases can progress to cardiac compromise, laminitis, or heart failure. Any suspected ingestion of black locust bark should be treated as a veterinary concern.
Even a relatively small amount of black locust bark can be dangerous for a horse. Clinical signs have been reported after ingestion of approximately 70 g of bark, although larger exposures are more likely to cause severe illness. Risk depends on the amount eaten, the plant part consumed, the horse's individual susceptibility, and how quickly exposure is recognized. Because a safe amount is not predictable, horses should not be allowed to chew or eat black locust bark.
Horses should not eat black locust leaves, even though toxicity from mature leaves appears inconsistent. Some horses may consume small amounts of foliage without obvious illness, while others may develop clinical signs after exposure to black locust plant material. Young shoots, root sprouts, wilted material, and bark are higher concerns. Preventing access to all parts of the tree is the safest management strategy.
Signs of black locust poisoning in horses commonly involve digestive upset, weakness, and cardiovascular compromise. Affected horses may show colic, diarrhea, constipation, decreased gut motility, depression, weak pulse, rapid heart rate, abnormal heart rhythm, cold extremities, and dark red gums that may progress to pale or blue as shock worsens. Some horses may develop labored breathing, dilated pupils, neurologic abnormalities, or laminitis. Severe, sudden, or persistent signs require prompt veterinary attention.
Black locust poisoning can cause colic in horses because the toxic effects often involve the gastrointestinal system. Affected horses may also develop diarrhea, constipation, reduced manure output, or decreased intestinal motility. These signs are not specific to black locust poisoning, so exposure history is important when determining the cause. If colic occurs after known or suspected access to black locust bark, branches, shoots, or seed pods, contact a veterinarian promptly.
Black locust poisoning can cause heart and circulation problems in horses, especially in more serious cases. Reported cardiovascular signs include weak pulse, rapid heart rate, irregular heart rhythm, cold extremities, discolored mucous membranes, and poor circulation. Severe cases may progress to significant cardiovascular compromise or heart failure. Horses showing weakness, abnormal gum color, cold legs, or signs of shock after possible exposure need urgent veterinary care.
Black locust poisoning can cause laminitis as a possible complication in affected horses. Laminitis has been reported during recovery from black locust intoxication, especially in cases involving severe systemic illness. Warning signs may include reluctance to move, hoof pain, shifting weight, or a stance suggestive of foot discomfort. Horses recovering from poisoning should be monitored closely for laminitis and managed under veterinary guidance.
Horses may chew black locust trees because of boredom, hunger, stress, confinement, curiosity, or limited access to forage. Bark and wood chewing are more likely when horses are confined near trees for long periods or do not have enough good-quality forage available. Many poisoning cases involve horses stripping bark from accessible trunks or branches. Reducing access to the tree, improving forage availability, and providing enrichment are key prevention steps.
If your horse ate black locust, remove the horse from the source immediately and contact a veterinarian for guidance. Prevent further access to bark, branches, root sprouts, young shoots, seed pods, and fallen limbs while you wait for veterinary advice. Watch for colic, diarrhea, constipation, weakness, depression, abnormal gum color, rapid heart rate, labored breathing, or signs of laminitis. Do not wait for signs to worsen if exposure is likely, because early supportive care may improve the chance of recovery.
Black locust poisoning in horses is diagnosed based on a history of exposure and compatible clinical signs. Evidence such as stripped bark, chewed branches, fallen limbs, root sprouts, or black locust trees within turnout areas can support suspicion of poisoning. A veterinarian may assess gastrointestinal function, hydration, circulation, heart rhythm, mucous membrane color, and neurologic status. Bloodwork and electrocardiography may help identify complications such as electrolyte disturbances, muscle damage, high ammonia levels, or irregular heart rhythms.
Black locust poisoning in horses is treated with supportive veterinary care because there is no specific antidote. Management may include removing access to the tree, providing clean forage and water, treating colic, correcting dehydration and electrolyte problems, monitoring heart rhythm, and supporting circulation. Activated charcoal may be considered early after ingestion to help reduce absorption of toxic compounds. Horses with severe signs may need intensive monitoring, laminitis management, nutritional support, and nursing care during recovery.
A horse can recover from black locust poisoning, but the outlook depends on the severity of illness and how quickly supportive care begins. Prognosis is guarded in symptomatic horses, especially when there is significant cardiovascular compromise or severe gastrointestinal dysfunction. Recovery may take several weeks in more serious cases. Horses that recover are not generally expected to have long-term effects, although complications such as laminitis require careful management.
Black locust often grows around horse properties along fence lines, pasture margins, roadsides, open woodlots, disturbed soils, and unmanaged fields. It is especially common east of the Mississippi River, but widespread planting has expanded its range across much of North America. The tree can spread aggressively through underground root sprouts and may form dense groves or thickets. These growth habits can make exposure more likely in turnout areas, dry lots, and pasture-adjacent environments.
Horse owners can identify black locust trees by looking for gray to brown bark with deep ridges, paired stout spines on young branches, and compound leaves with multiple oval leaflets. Mature trees often have a broad, irregular crown and may grow in dense groups because they spread through root suckers. In late spring and early summer, black locust produces fragrant hanging clusters of white, pea-like flowers. The tree also develops flat brown seed pods that may remain attached into early winter.
Black locust is often incorrectly called acacia, but its scientific name is Robinia pseudoacacia. It belongs to the bean family and is distinct from true acacia species. The common name confusion matters because horse owners may underestimate the risk if they do not recognize black locust on their property. Identification should focus on the tree's bark, spines, compound leaves, white flower clusters, seed pods, and tendency to form dense stands.
Horse owners can prevent black locust poisoning by removing black locust trees from turnout areas, dry lots, and fence lines whenever possible. If removal is not practical, horses should be kept away from trunks, low branches, fallen limbs, root sprouts, young shoots, and seed pods. Pastures should be checked after storms or pruning because damaged branches and cut wood may still contain toxic material. Providing enough good-quality forage and reducing boredom can also lower the risk of bark chewing.
Summary
Black locust is a deciduous tree found throughout North America. It is toxic to horses, especially when they ingest bark, leaves, seeds, young shoots, or root sprouts.
- Clinical signs of poisoning mainly affect the gastrointestinal and cardiovascular systems and may include colic, diarrhea, constipation, weakness, arrhythmias, labored breathing, and lethargy
- Severe cases may lead to cardiac failure, neurologic signs, and laminitis in symptomatic horses
- There is no antidote for black locust poisoning; treatment is mainly supportive
- Prevention focuses on restricting access to black locust trees and calling a veterinarian promptly after suspected exposure
References
- Black Locust. Michigan Department of Natural Resources Michigan Natural Features Inventory. 2012.
- Guide to Poisonous Plants – College of Veterinary Medicine and Biomedical Sciences. Colorado State University. 2025.
- Burrows. G. E. Toxic Plants of North America. 2nd ed. John Wiley & Sons. 2013.
- Hovda. L. R. Blackwell’s Five-Minute Veterinary Consult Clinical Companion Equine Toxicology. Wiley Blackwell. 2022.
- Gupta. R. C. Ed. Veterinary Toxicology: Basic and Clinical Principles. 2nd ed. Elsevier. 2012.
- Plumlee. K. H. Ed. Clinical Veterinary Toxicology. Mosby. 2004.
- Novotna. T. et al. Robinia Pseudoacacia Poisoning in Two Horses: A Case Report. Veterinárnà MedicÃna. 2024. View Summary
- McQuinn. S. et al. Horse Owner’s Guide to Toxic Plants. Skyhorse Publishing. 1996.
- Krzak. W. E. et al. Wood Chewing by Stabled Horses: Diurnal Pattern and Effects of Exercise. Journal of Animal Science. 1991. View Summary
