Know your Plants, Protect your Horses

Looking at my backyard last night and all of the different species of plants that have cropped up over the last week or so made me thankful I only have a small space to manage, one where everything is free to grow! This would not be the case if I were managing a horse property.

Effective pasture management is essential to maintaining safe and healthy grazing areas for horses. This can be partially achieved through effective pasture management that includes rotational grazing. Bare patches of overgrazed grass present an ideal opportunity for more opportunistic plant species to grow, in many cases, ones that are toxic to horses. In Canada, the following 10 plants have been identified as being the most toxic to horses. Take some time to learn which are native to your area and how to identify them. Prompt removal of any toxic plants from paddocks or pastures the horses have access to is recommended.

Yew – ornamental, often used in hedges. Yew can be lethal if enough is ingested (0.5 pounds for a 1000 pound horse). Symptoms: muscular tremors, staggering, convulsions, difficulty breathing, collapse, heart failure. 

Water Hemlock – found in ditches and wet areas throughout Canada. Ingestion of just 1 root of Western Hemlock can lead to fatality in just a few hours. Symptoms: salivation, muscle spasms, violent convulsions, coma, asphyxiation. 

Poison Hemlock – found in ditches, streams, wet meadows and along roadsides throughout North America. Poison Hemlock can result in death from respiratory failure within 2 – 3 hours. Symptoms: frothing at the mouth, uneasiness, dilated pupils, weak, rapid pulse, convulsions, clamping of jaws, muscle tremors.

Oak – found throughout Canada. While not fatal, if regularly ingested, Oak can cause gastroenteritis and kidney damage. Symptoms: inappetence, constipation followed by bloody diarrhea, abdominal pain, depression, blood in the urine.

Rhododendron – ornamental plant. 2 pounds of Rhododendron per 1000 pound horse can be lethal within a few hours of ingestion. Symptoms: diarrhea, weakness, repeated swallowing, impaired vision, bradycardia, tachycardia, coma.

Cow Cockle – found in pastures, along roadsides, in cultivated fields and waste areas. Cow cockle seeds are toxic with a lethal dose of 2.45 pounds per 1000 pound horse. Symptoms: restlessness, grinding of teeth, salivation, colic, diarrhea, coma.

Cocklebur – found in farmyards, cultivated fields, streambanks and beaches. Cocklebur is not considered lethal but can result in extreme discomfort if ingested. Symptoms: weakness, unsteady gait, twisting of neck muscles, depression, nausea, laboured breathing, rapid/weak pulse.

Jimsonweed – found in cultivated fields and farmyards across most of southern Canada. Lethal at 1 pound per 1000 pound horse, Jimsomweed also has a narcotic effect that may be fatal to livestock. Symptoms: dilation of pupils, impaired vision, rapid/weak pulse, nausea, loss of muscular coordination, violent/aggressive behaviour, trembling. 

Nightshade – found in fencerows, shrubs and wood edges in southern Canada. Nighshade can be fatal at 1 pound per 1000 pound horse. Symptoms: abdominal pain, dilation of pupils, loss of appetite, diarrhea, loss of muscular coordination. 

Oleander – may be grown as an ornamental shrub in Canada

Other plants toxic to horses include:

  • Alsike Clover
  • Arrow Grass
  • Barnyard Grass
  • Bracken Fern
  • Burdock
  • Field Horsetail
  • Johnsongrass
  • Lupine
  • Mikweed
  • Puncture Vine
  • Spear-Leaved Goosefoot
  • Sneezeweed
  • Spurge
  • St. John’s Wort
  • Stinging Nettle
  • Stork’s Bill
  • Tall Buttercups
  • Tansy Ragwort
  • White Snakeroot
  • Witch Grass


Lawseth, A. undated. “Pasture Perils – Plants Toxic to Horses.” Horse Journals Retrieved from

Ontario Ministry of Agriculture, Food and Rural Affairs. “Common Weeds Poisonous to Grazing Livestock.” Retrieved from