White snakeroot causes scarring in the horse's heart and may decrease performance and cause severe heart failure. Horse owners should learn the list of plants, including many popular ornamentals, that can cause the biggest problems and even death in horses. A few castor beans or one little potted hydrangea can kill a horse. Larkspur, a plant sometimes found as an ornamental in the Midwest, can cause paralysis and respiratory failure. Wild black cherry, chokecherry, plum, and peach contain cyanide poison in the leaves, and especially in the seeds, which can be rapidly lethal.
Sorghum can also be a source of cyanide.
Socrates demonstrated the deadliness of poison hemlock, a plant that is ubiquitous in the Midwest. If a poisonous plant is consumed by your horse, another animal, or a human, Dr. Beasley recommends you take a sample of the plant, estimate the quantity eaten, note the time of onset of signs, and call a veterinarian or physician immediately. If you are not sure what certain poisonous plants look like, the University of Illinois Poisonous Plants Garden on St. Mary's Drive near Lincoln Avenue in Urbana, just northwest of the Veterinary Medicine Basic Sciences Building, is a good place to start. A quick stroll through the garden will show you what poisonous plants common to the Midwest look like, and you'll learn what effects they can have on your horse. There are also several good books available on horses and toxic plants.