Common Plants and Trees That Are Poisonous to Horses

Most people realize that certain plants are toxic and should not be consumed by humans. In addition, many owners of small animals typically take care when planting vegetation around their yard or when walking their pets outdoors in parks or while camping. Unfortunately, people are often unaware of the fact that certain plants may be poisonous to horses as well. Like smaller domesticated animals, horses can become ill if they consume certain types of plants that are toxic to equine. When a horse owner is unaware of what these plants are, they cannot take the necessary preventative measures to ensure that their horses do not ingest or come into contact with them. In some instances, horses will avoid eating poisonous plants that they find distasteful or bitter. In addition, because of a horse's great size, it generally takes more than a few bites of some toxic plants to make it ill; however, horse owners should not rely on this to keep the horses in their care safe. There are many different types of poisonous plants and they often affect horses differently and at different rates.

Some include:

  • Buttercups
  • Bracken Fern
  • Red Maple Tree Leaves
  • Black Walnut Tree
  • Yew
  • Oleander
  • Poison Hemlock
  • Yellow Star Thistle
  • Johnson Grass and Sudan Grass
  • Tansy Ragwort

Despite the fact that healthy equine will generally avoid eating toxic plants that they do not find palatable, the possibility of poisoning remains. In some instances, a horse may repeatedly consume small amounts of a poisonous weed or plant over a span of time. Although it may not consume large quantities at any given time, the build-up from months of consumption can make the animal sick or may even be fatal. Horses that suffer from malnutrition or that have access to overgrazed and poor quality pastures may have little choice but to graze on plant life that is toxic. Some toxic plants may even be strong enough that a few nibbles are enough to harm the horse.

To avoid the threat of toxic weeds, trees and plants, people with horses under their care must educate themselves on which plants are toxic to horses and have the potential to harm their equine friends. This knowledge will prevent them from planting these plants anywhere near the horses, and will make them aware of what dangerous plants are already within the animal's reach.

Plants may be toxic to horses in a variety of ways. While most poisonous plants are a danger when consumed, some can have a harmful effect with simple contact. While there are numerous plants that are poisonous to horses there are some that are more commonly found near horse pastures than others.

  • buttercups

    Buttercups are a type of plant that is toxic to horses when the fresh flowers and leaves are ingested. This is one of the types of plants that horses often find unpalatable, but will eat if necessary. Mild to moderate cases of buttercup ingestion results in digestive disturbances such as diarrhea and colic. When a horse has been severely poisoned by the plant it may suffer from convulsions and eventual death.

  • Bracken Fern is a perennial herb that when ingested depletes thiamine, also known as Vitamin B1. All parts of the plant are poisonous and when eaten in large quantities it may cause the horse to lose its appetite and as a result suffer from weight loss. Horses poisoned by this plant may become depressed, uncoordinated and may suffer from paralysis and a slow heartbeat. Like buttercups, most horses will not eat bracken fern unless their food supply is lacking.

  • red maple trees

    Red maple trees are grown around the U.S., but are native to the eastern part of the country. The consumption of leaves, regardless of their state of freshness, may cause negative health problems; however, wilted leaves tend to be more problematic as horse may find them better tasting. Poisoning of a thousand pound horse typically occurs after it has ingested one and a half pounds of maple leaves. Poisoning may cause lethargy, elevated heart rate and an increased breathing rate. Horses may also suffer from jaundice, become depressed, and in severe cases go into a coma or die. The urine of the poisoned horse may also be darker in color.

  • black walnut tree

    The black walnut tree is another tree that is poisonous to equine. Unlike the red maple tree, it is the bark, nuts, roots, and woods that are toxic. Contact with black walnut, typically in bedding that has shavings from the tree, can cause increased temperature, lethargy, hoof temperature, swelling of the lower limbs, and depression. With the exception of laminitis, most of the symptoms resolve on their own when the horse is no longer exposed to tree's shavings.

  • yew

    The yew is distinguished by bright red berries and small leaves. Horses often find these leaves desirable and ingesting only a mouthful can cause cardiac collapse, respiratory failure, and death in as little as thirty minutes.

  • Oleander

    Oleander is another ornamental shrub that can potentially kill a horse within a short period. Oleander can be recognized by its large pink, white or deep red flowers. The shrub ranges in size from five feet tall to nearly as tall as a tree at twenty-five feet. It can be found in the western part of the U.S. and in the southwest. The entire part of the plant is poisonous to horses and horses need not ingest large amounts to become fatally poisoned. A poisoned horse may display tremors and twitching of its muscles, pain, an abnormal heartbeat, labored breathing and symptoms similar to colic. Horses may also have diarrhea that is at times bloody. A horse that is fatally poisoned by this shrub can die of heart failure in as little as twelve hours.

  • poison hemlock

    Poison Hemlock is a perennial weed that has leaves similar to the leaves of a fern. In the late summer months, it will also have white, small flowers. All parts of the plant, including the root is poisonous. This plant can be lethal for horses when the animal ingests four pounds or more. A horse that has ingested this plant and become poisoned by it will become nervous and have tremors. Colic and a weakened pulse rate are also signs.

  • water hemlock

    Another perennial weed is the water hemlock. This weed is, in fact, one of the most poisonous of plants. It can be identified by the purple spots on its jointed stems, the umbrella-shaped clusters of white flowers, and serrated leaves. It is found across the U.S., primarily in waterways and areas that are moist. The roots, as well as the leaves and flowers of this weed, are poisonous, which makes it poisonous year-round. Water hemlock is much more toxic than poison hemlock as death may occur after eating as little as eight ounces of the plant. When poisoned, the central nervous system of the horse is affected. Water hemlock may result in violent behavior from the horse, convulsions, muscle and heart function loss, and excess salivation and dilated pupils.

  • yellow Starthistle

    Yellow Starthistle is an annual weed that can result in a brain disease that is not only irreversible, but also fatal. It is found in Northern California and Colorado. Cotton-like hair covers the leaves of the plant and there are yellow disc-like flowers. Signs of poisoning may not appear until weeks of consuming the plant in either its dry or fresh form. Because the taste is not offensive to horses, some may be attracted to the weed and prefer it to other plants. Poison signs include impaired mastication, hypertonicity of facial muscles, frothing of the mouth and severe weight loss due to an inability to eat. Death is often the result of pneumonia or starvation.

  • Johnson grass

    Johnson Grass and Sudan Grass are grasses that can cause bloody urine, paralysis of the bladder and even hind limb paralysis. Sudan Grass is found throughout North America while Johnson Grass is generally in the south. These grasses can reach heights of six feet and are characterized by leaves that are veined and broad. Locoweed poisoning often does not occur until a horse has ingested large amounts of the plant for no less than two weeks. This is a plant that horses find palatable, which means that they will often return to it if it is present. It has short stems, leaves that grow on tufts, and when in bloom, there are flowers of white or purple. These plants may be found in the southwestern and western parts of the country. Depression, staggering, abnormal head bobbing and weight loss are signs of poisoning from this plant.

  • Tansy Ragwort

    Tansy Ragwort is a weed that is found in the coastal northwest United States. Horses that consume this plant will generally not display signs of poisoning until six months or longer have passed from the initial ingestion. When signs do appear, it may be in the form of appetite loss, weight loss, liver failure, jaundice, photo-sensitivity, and lethargy. People can recognize this weed by its multiple stems and flowers that are similar in appearance to daisies.

These are just a few of the plants that are toxic to horses. There are many more across the country that people must be aware of. In some instances plants may be native to one part of the U.S. and not found in other areas. Because some plants are potentially poisonous, horse owners and caregivers should recognize not only the signs that are specific to certain plants, but also the basic signs of poisoning. If a horse ever displays symptoms such as disorientation, difficulty swallowing, tremors, increased salivation, a noticeable change in urine color or muscle spasms, an equine veterinarian should be contacted immediately. Other potential warning signs that are associated with toxic plants and should be considered an emergency include changes in heart rate and respiratory rate, a horse that goes into a sudden frenzy, colic, and a horse that collapses.

Although horses are larger than smaller pets, they require the same care and consideration to keep them safe. Horse owners that are aware of the potential threat of poisonous plants can take the precautions necessary to protect their animals. Ensuring that horses have a healthy place to graze and learning to recognize dangerous vegetation are important steps to take.

For more information about plants that are poisonous to horses, click on any of the following links.

  • Published:
  • Updated: 3/15/2020: 7:01:38 PM ET
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