Poisonous Plants for Horses

December 1, 2021

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As a horse owner, I’ve always known to keep an eye out for potentially poisonous plants to horses because their GI system is incredibly sensitive. It’s also precisely why I nearly had a panic attack when Elvis recently snuck through an open gate and ate my 3’ by 3’ provencal lavender plant to the nub. 

As I frantically googled results for “is lavender poisonous to horses,” I realized it would probably save some panic in the future to keep a list of plants that are poisonous to horses on hand. So, this is the list I’ve been compiling, and I thought I would share it with you! 

As we’re building Fairway Stables, I’m also planning out the gardening and landscaping. While I definitely don’t plan on my horses being anywhere near my landscaping or garden, Elvis is proof that accidents do happen, and gates do get left open. That’s why it’s so important to know which plants are unsafe to have anywhere on your horse farm.

Note: Please remember that I, of course, am not a vet. If you have any questions about this list, please contact your own veterinarian. This article is in no way intended to be a substitute for medical or veterinarian advice and is provided for educational purposes only. 

Table of Contents:

Trees that are poisonous to horses:

Some trees on this list can cause illness if ingested; however, others are so dangerous that they have been known to cause death in horses.

Trees known to cause death in horses:

  • Plants in the cherry family. Black Cherry is linked to mare reproductive loss syndrome (MRLS), while chokecherry contains cyanide, which can be deadly. 
  • Japanese yew, oleander, or rhododendron can cause death.

Trees that can cause illness in horses: 

  • Boxelder trees are believed to produce a toxin that causes seasonal pasture myopathy when it comes into contact with horses.
  • Oak trees in the form of brand new buds and acorns contain tannins, which can lead to colic if ingested in large quantities. Speaking of colic, be sure to check out our article Identifying, Preventing, and Treating Colic in Horses for important tips.
    • Note: prior to moving to Fairway Stables, my horses have spent the last 25 years in a pasture full of oak trees. While they will nibble on other varieties of trees in the springtime, we’ve never had a single horse touch an oak tree or act interested in eating them.
  • Maple trees are more toxic when their leaves wilt (or dry). Vets have cited that ingestion of 1-3 pounds of wilted leaves can be toxic enough to cause death. When horses consume maple leaves, one of the first symptoms is red/brown urine. 
  • Black walnut trees have toxic bark, nuts, and roots. Some types of shavings can contain parts of this tree, resulting in lethargy, increased temperature, swelling, and depression in horses. It can also cause laminitis in horses.

Plants that are poisonous to horses

Knowing the plants that are toxic to horses is the most critical step in maintaining a horse farm. After all, trees are easy to spot and identify, but just about anything could be growing in your horse pastures. 

The bad news: many potentially poisonous plants appear as weeds in horse pastures. 

The good news: horses find most plants that cause illness unpalatable, and horses will typically avoid bitter plants. The other good news is that because horses are such large animals, they generally have to consume large amounts of the toxic plant to cause serious effects. (Of course, ponies are affected more quickly.)

One note about preparing your horse farm: it’s always wise to ask your vet for his or her opinion if you’re unsure about any plant. Another smart idea? Take samples to a local agricultural department. For example, aside from having my vet inspect the property with me, I also took samples to our local Oklahoma State University, which has a fantastic agriculture department. 

Here’s a list of the most poisonous plants for horses: 

  1. Johnsongrass/Sudan grass
    1. As we build out Fairway Stables, Johnsongrass is one of my chief concerns. The leaves and stems of these grasses contain cyanide. When the plant is injured, the cyanide can escape. If the grass is cured for hay, though, cyanide levels drop. However, if uncured, your horse may exhibit signs of cyanide poisoning (rapid breathing, tremors, frequent urination, and convulsions). Call your vet if you suspect cyanide poisoning. 
  2. Bracken Fern 
    1. Bracken fern contains thiaminase, which causes ill effects. But the good news is your horse would have to eat large quantities for them to get sick. The bad news? Unfortunately, this is one of the rare plants on the list that horses develop a liking for. Overdoses of bracken fern can cause blindness in horses. 
  3. Tansy Ragwort
    1. This can cause liver damage in horses when consumed in large amounts over time. 
  4. Oleander 
    1. Some people grow this as a potted plant, making it somewhat common. This plant can cause heart damage to horses, and unfortunately, it only takes 30-40 leaves to be deadly. So, call your vet right away if you suspect your horse has ingested this, as it can lead to colic, tremors, and heart issues. 
  5. Hemlock
    1. 4-5 pounds is considered lethal for an average horse (based on an appx. 1000lbs horse). Luckily, most horses will naturally avoid this plant, but it can be deadly.
  6. Yew
    1. Even a mouthful of this plant can be toxic to a horse, so call your vet immediately if you suspect your horse has ingested it. 
  7. Crazy weed (Locoweed) 
    1. This plant can cause neurological issues in horses. 
  8. Water hemlock
    1. Typically found in marshy areas, this is one of the most dangerous plants in the United States for any type of livestock. Symptoms include seizures, heart issues, respiratory paralysis, and can lead to death within hours. 
  9. Yellow Star
    1. This is a relatively common weed along roads and pastures. This weed causes a specific symptom in horses: a neurological effect that inhibits the nerves that control the horse’s chewing. But, the good news? A horse has to eat quite a bite (50% to 200% of their body weight) within a month to receive a toxic dose. 
  10. Buttercups
    1. This is another plant that most horses will avoid, but it can lead to colic, convulsions, and even death. 
  11. Morning glory 
    1. Morning glories can be poisonous to horses, dogs, and cats as a result of the GI distress and even liver failure the flower can cause.
  12. Foxglove 
    1. Another plant that’s also poisonous to cats and dogs, a horse only needs to eat .01% of their body weight to begin feeling symptoms (such as colic). This plant can lead to death. 

Other (potentially deadly) plants to avoid at all costs: 

  • Lily of the Valley
  • Leafy spurge
  • Rhubarb leaves and roots
  • Mustards
  • Jimsonweed
  • Azalea
  • Privet
  • Larkspur
  • Blue flax
  • Tomato or potato plants
  • Alsike clover
  • Horse Chestnut
  • Mountain Laurel
  • Pits of peaches, cherries, or avocados
  • Russian Knapweed or yellow star thistle (Centaurea spp)

Note: a complete list of poisonous plants for horses, compiled by the ASPCA, can be found here

In Conclusion…

I encourage you to keep this list on hand so you don’t find yourself in the same position I did when Elvis discovered my lavender plants. While, of course, he was never meant to wander into the garden, accidents do happen. 

In short, the best way to avoid accidents like that from becoming serious is to be aware of what trees and plants are poisonous to your horse and keep them as far away as possible!

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