Which Trees are Toxic?

It's hard to imagine that a natural part of our environment could possibly be harmful to our horses. However, many plants are not only poisonous, but potentially deadly to horses. Many plants and trees have strong medicinal qualities; early medicines to treat numerous medical conditions--from lowering a fever to stimulating a strong heartbeat--were often derived from plants, shrubs, and trees.

It therefore should be no surprise that the substances in many common trees can be quite poisonous if consumed by our herbivorous friends in raw form. In this article, we will discuss the most poisonous trees, how to identify them, and how to prevent your horse from becoming a victim of their toxic ways.

Black Walnut Tree

One of the most dangerous of poisonous trees is the black walnut. Although prized by humans for its delicious nuts and beautiful wood, the tree itself can be very dangerous to horses. Exposure to the horse is mainly through shavings used as bedding. Exposure to the tree in this manner results in laminitis or founder to varying degrees.

Originally, the tree was thought to produce a toxin in the leaves, bark, and nuts. However, more recent research suggests that only the heartwood of the tree contains the toxin responsible for causing laminitis in horses; but to be safe, horses should never be allowed direct access to these trees.

After exposure to black walnut shavings, a horse will usually begin to show the hallmark signs of laminitis within 10-12 hours. The lower legs of some horses will begin to swell, and they will be reluctant to move. Other horses might just shift their weight from one front foot to the other, or rock their weight backward. If asked to move, they will try to bear most of their weight on their heels instead of walking flatfooted. These are all classic signs of laminitis (except for the swollen legs). Other signs that might be present include a strong digital pulse and/or palpable heat within the hoof.

If you see any of these signs in your horse, whether or not you suspect black walnut tree poisoning, contact your veterinarian at once as an acute episode of laminitis is always an emergency. If you suspect that your bedding is contaminated with black walnut wood shavings, remove your horse from the bedding at once. Most horses have a very good chance of recovering with treatment once removed from the toxin (bedding).

The black walnut tree is a tall hardwood tree that can reach 90 feet (27 meters). Not a common tree, its habitat ranges from North Florida to New York and west to Texas and North Dakota. Its bark is dark brown with deep ridges, and its dark green leaves are long and spindle-shaped with tiny saw-tooth ridges at the edges. The nuts of black walnut trees are first green, then turn dark brown or black.

Red Maple Trees

Red maple trees are some of the most spectacular trees to behold in the fall. The deep crimson leaves are beautiful, but toxic, particularly to horses. The leaves while alive and on the trees are not poisonous, but once they fall off the tree and wilt, they can be deadly.

Horses are most often exposed as the leaves fall from the trees in the fall, or if a branch is blown off of a tree into a pasture by a storm and the leaves wilt on the broken branch. The toxin present in these wilted leaves is unidentified at this point in time. Despite that, we know very well the damage it can cause.

Once ingested by the horse, the toxin begins to destroy the horse's red blood cells. As the red blood cells continue to burst, severe anemia (lack of red blood cells) can occur. The destruction of the red blood cells causes other problems--once a red blood cell is destroyed, the hemoglobin from inside the cell is free in the bloodstream. The hemoglobin is filtered by the kidney, but the kidney is damaged in the process. Therefore, horses poisoned by the red maple toxin are battling severe anemia (which hampers the ability to carry oxygen to their body's cells) and kidney disease.

If a horse eats red maple leaves, he will begin to act depressed and weak within two days. As the horse's body begins to have trouble transporting oxygen to the cells, his heart and respiratory rates will rise. Affected horses will also have icteric mucous membranes (a yellowish tinge to their gums and sclera, the white around the eye).

A horse which has eaten wilted red maple leaves is treated supportively, but has a poor prognosis for survival. He is given intravenous fluids to flush his kidneys (diuresis) in an effort to keep them working. He might require oxygen, and if he's severely anemic, he might receive blood transfusions.

One reference cites that as little as three pounds of wilted leaves can be fatal to an adult horse. Of course, the best treatment is prevention. Once the offending tree is identified, simply ensure that your horse is not exposed to the wilted leaves. Remove storm-blown branches from paddocks or pastures immediately. If these trees are close to your fenceline, it might be prudent to remove the tree.

The red maple or scarlet maple is found throughout the eastern United States and southeastern Canada. This tree is a tall hardwood with green leaves that have three large "fingers" or points and five prominent veins in the leaves. The leaves of this tree turn a brilliant crimson and sometimes yellow in the fall.

Cherry and Plum Trees

Cherry trees were brought into the spotlight last year with the mare reproductive loss syndrome (MRLS) in Kentucky. One theory during the investigation was that black cherry trees might have been a source of cyanide that led to early and late foal losses. Nothing has been proven in research to this date.

Cherry and plum trees and their relatives contain cyanide-containing compounds, which are found in the leaves, fruit, and pits of the trees. The plants are most toxic when drought or frost stresses them, and young, rapidly growing trees are thought to potentially contain a higher concentration of cyanogenic compounds. Wilted leaves are also quite toxic.

Horses become poisoned by ingesting the leaves or seed pits of the trees. Once the plant material is chewed and exposed to the acid within the horse's stomach, hydrogen cyanide is released and rapidly absorbed into the horse's bloodstream. Cyanide works as a poison in that it prevents normal cellular uptake of oxygen. As a result, an affected horse's blood is bright cherry red because it is overloaded with oxygen that cannot be utilized by the horse's cells.

Horses with cyanide poisoning usually are found breathing heavily with flared nostrils. Their respiratory rates and heart rates might be quite elevated. Diagnosis is often by these clinical signs and the bright red color of the blood. Some horses are found dead from cyanide poisoning, and in those cases tissue samples can be tested for the presence of cyanide. If found in time, the affected horse can be treated with chemicals that: 1) Remove the cyanide that is bound to the red blood cells and unblock cellular oxygen transport; and 2) replenish natural stores of a compound that can bind the remaining cyanide and render it harmless.

Cherry and plum trees are present throughout most of the United States, and there are numerous varieties of each species. Their showy flowers in the spring and fruit during the summer is the best way to identify them.

Oak Trees

Oak trees--more specifically, their acorns, buds, leaves, or blossoms--are toxic to all livestock, including horses. Oak poisoning is not very common in horses, but is seen more commonly in cattle and sheep, which are less discriminate eaters. The acorns, leaves, and blossoms contain tannins, which when digested are converted into toxic metabolites inside the gastrointestinal tract of ruminants and horses.

Horses with oak poisoning can develop colic and bloody diarrhea. They also develop renal disease as the toxic metabolites of the tannins damage the horse's kidneys. Treatment of oak poisoning is aimed at maintaining hydration with intravenous fluids and protecting kidney function. The good news is that horses must eat a large quantity of acorns to become ill. Prevention is better than treatment--remove oak trees from your pastures or paddocks.

Oak trees are found throughout the United States and Canada. There are many different deciduous and evergreen varieties of oak trees found in the United States. The acorns they produce in the summer can identify these trees, along with their leaves, as most deciduous oak trees have leaves with seven to nine lobes and the evergreen varieties have elliptical leaves.

While the trees discussed here are the most common poisonous trees in the United States, there are other less common trees such as black locust that can cause problems in horses. However, there are far more poisonous plants in our environment than just trees. If you are unfamiliar with the plants in your area or are unfamiliar with your horse's habitat, check with your veterinarian and your local cooperative extension agent to see if any plants might pose a problem. Also, for more information on poisonous plants, see "Poisonous Plants" in the May 2001 issue of The Horse, article Quick Find #3058 at www.TheHorse.com.


Barr, A.C.; Reagor, J.C. Toxic Plants. The Veterinary Clinics of North America, Equine Practice. Toxicology. St. Louis, Mo.: W.B. Saunders Co., December, 2001, 529-546.

Knight, A.P.; Walter, R. A Guide to Plant Poisoning of Animals in North America. Jackson Hole, Wyo.: Teton New Media, 2000.

Little, E.L.; Knopf, A. The Audubon Society Field Guide to North American Trees (Eastern Region). New York, N.Y.: Alfred A. Knopf, Inc., 1980.

Schmitz, D.G. Toxicologic Problems. Equine Internal Medicine, Eds. Stephen Reed and Warwick Bayly. St. Louis, Mo.: W.B. Saunders Co., 1998.

Toxic Trees Common to the United States

Poisonous Tree

Geographical Range
Black Walnut
Very tall trees up to 90 feet (27 meters). Dark green leaves are long and spindle-shaped with tiny saw-tooth ridges. Green nuts turn dark brown-black.
North Florida to New York and west to Texas and North Dakota.
Red Maple
Tall hardwood with classic Maple shape (three to five lobes per leaf). Leaves are gree on top, silvery on the underside, and turn crimson red in the fall.
Entire eastern United States and very southeastern Canada.
Cherry or Plum
White or pink flowers in the spring and classic fruit can help identify these plants.
Common throughout most of the United States.
Deciduous varieties have classic seven- to nine-lobed leaves. Acorns are the easiest way to identify this tree.
A variety of oak species are found throughout the United States and Canada.
Black Locust
Medium-sized, spiny tree. Leaves are elliptical, dark blue-green on top and pale on the bottom, and are arranged in two rows along an axis.
Central Pennsylvania and southern Ohio to northeastern Alabama, and from southern Missouri to eastern Oklahoma.

About the Author

Christina S. Cable, DVM, Dipl. ACVS

Christina S. Cable, DVM, Dipl. ACVS, owns Early Winter Equine in Lansing, New York. The practice focuses on primary care of mares and foals and performance horse problems.

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