Toxic Plants Every Horse Owner Should Know

Nibbling on tasty plants in the pasture may be your horse's idea of the perfect day. Unfortunately, toxic plants scattered among favorite forage options could cause serious illnesses if eaten. These plants can be dangerous to horses:

Hemlock

Hemlock plants have small white flowers, purple-spotted stems, and leaves that resemble ferns. Eating the stems, seeds, or leaves of the hemlock plant can cause tremors, nervousness, coordination problems, colic, breathing problems, and low heart rate. Horses can recover if they only eat a small amount of hemlock, but large amounts will kill them.

Buttercups

Buttercups tend to grow in overgrazed pastures and may be toxic if your horse eats the fresh yellow flowers and leaves. Symptoms include mouth irritation, excess saliva, colic, diarrhea, reduced appetite, and death in severe cases.

Pokeweed

Pokeweed looks like a small tree with a purple stem, long leaves, and green berries that turn purple as they mature. The roots are the most toxic part of the pokeweed plant, according to Rutgers New Jersey Agricultural Station, although eating leaves and stems can also poison horses. Symptoms of pokewood poisoning include diarrhea, a burning sensation in the mouth, and chronic colic.

Alsike Clover

Alsike clover is most commonly found in the northern part of the U.S. It grows 1- to 2-feet tall, has small pink flowers, and leaflets with toothed leaves. Horses that eat alsike clover may develop red, crusty, inflamed skin. Big liver syndrome can occur if a horse eats the clover over a long period of time. Big liver syndrome has no cure and may cause weakness, weight loss, abdominal pain, jaundice, diarrhea, cirrhosis, head pressing, teeth grinding, or death.

Jimsonweed

Jimsonweed, also known as stink weed, devil's trumpet, and thorn apple, grows up to 4 or 5 feet tall, has prickly seed capsules, and white or purple flowers that resemble trumpets. Eating any part of the jimsonweed plant can cause dry mouth, coordination problems, dilated pupils, weak pulse, diarrhea, coma, convulsions, or death.

Bracken Fern

These fern plants, found in wooded areas and wet fields, can grow 3 feet tall. Bracken ferns contain thiaminase, an enzyme that prevents absorption of vitamin B1, according to Equus. The resulting B1 deficiency can cause coordination issues, blindness, and depression if the horse eats large quantities of the fern.

Landscape Plants

Yew, azalea, rhododendron, and oleander plants are often used in landscaping and aren't usually found in pastures. However, horses can eat the plants if they're planted just a few feet from the fence line. Yew, an evergreen plant with short needles and red berries is particularly dangerous. Eating a small amount of the plant can kill a horse in minutes.

Azalea or rhododendron plants can cause colic, diarrhea, excess saliva, or death, depending on how much the horse eats. Ingesting any part of the oleander plant can interfere with your horse's heart rate and cause trouble breathing, slow or fast heart rate, colic, or death.

Trees

Do you have red maple trees in your pasture? Eating wilted or dried red maple leaves can make it difficult for your horse's red blood cells to transport oxygen. Eating 1.5 pounds of red maple leaves can be toxic to a 1000-pound horse, while 3 pounds can be deadly, according to the University of Tennessee (UT) Extension.

Signs of red maple toxicity include weakness, depression, dark red urine, increased heart rate, yellow mucus membranes, and trouble breathing.

Wild cherry and black walnut trees can also poison horses. Cyanide produced by the leaves, bark, and seeds of wild cherry trees can kill horses, particularly if the leaves are wilted, or the horse eats the seeds.

Black walnut nuts, roots, bark, and wood may be toxic if eaten or if black walnut shavings are used for bedding. Symptoms include lower limb swelling, laminitis, lethargy, and higher hoof temperature. An affected horse may have a higher temperature, pulse rate, or breathing rate.

Identifying and Preventing Poisoning

Call your veterinarian immediately if you suspect your horse has eaten a toxic plant. The sooner your horse receives treatment, if available, the better the chance of a positive outcome.

Many horses don't like the taste of toxic plants and will avoid them if other foods are available. You can reduce the risk of poisoning by checking pastures for toxic plants, removing plants promptly, rotating pastures to prevent overgrazing, and providing hay during droughts when non-toxic plants begin to die.

Worried that your horse may have eaten a toxic plant? Contact our office immediately.

Sources:

Equus: 10 Most Poisonous Plants for Horses, 5/22/2025

https://equusmagazine.com/horse-care/10-most-poisonous-plants-for-horses-8208

The University of Tennessee Extension: Equine Toxicity Series: Poisonous Plants to Horses

https://utia.tennessee.edu/publications/wp-content/uploads/sites/269/2023/10/W784-A.pdf

Rutgers New Jersey Agricultural Station: Poisonous Weeds in Horse Pastures

https://njaes.rutgers.edu/fs938/

Oregon Veterinary Medical Association: 10 Plants Toxic to Horses, 5/1/2017

https://www.oregonvma.org/care-health/equine-and-livestock/health-safety/10-plants-toxic-to-horses