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Hoary alyssum: the most common poisonous plant to horses in Minnesota

Quick facts

  • Don’t feed hay containing hoary alyssum to horses.

  • Hoary alyssum is common in disturbed areas, meadows, pastures and hay fields.

  • It’s toxic in fresh pasture or dried in hay and causes “stocking up,” fever and founder.

  • Remove horses from sources of hoary alyssum.

  • Hand pulling, mowing and herbicides can help control hoary alyssum.



  • Grayish-green

  • Hairy

  • 1 to 3 feet tall

  • Many branches near the top


  • Oblong-shaped

  • Grayish-green

  • Covered with rough hairs


  • White

  • Four deeply-divided petals

Seed pods

  • Hairy

  • Oblong shaped, swollen-like with a point on the end

Hoary alyssum flower
Hoary alyssum flower
Hoary alyssum in hay
Hoary alyssum in hay
Hoary alyssum seed head
Hoary alyssum seed head
Mature hoary alyssum plant
Mature hoary alyssum plant


Hoary alyssum is toxic when:

  • Horses graze the fresh plant in the pasture

  • Horses eat the dried plant in hay

Horses usually prefer other, more palatable forages over hoary alyssum. Cases of hoary alyssum poisoning still occur in pastured horses. Most hoary alyssum poisoning occurs when horses eat infested hay.



  • A healthy, dense stand of pasture forages can prevent the growth or spread of hoary alyssum.

  • Hand pulling or digging and mowing before flowering can control small infestations.

  • A few herbicides work on hoary alyssum. They may require you to apply it more than once. Apply the herbicide before the plant flowers. Handpull or mow flowering weeds before seed production.

    • Always follow grazing restrictions and pertinent information stated on the herbicide label.


Hoary alyssum can be an annual, winter annual, biennial or a short-lived perennial. It spreads rapidly due to the high amount of seeds it produces per plant.

Authors: Krishona Martinson, Extension equine specialist, Mike Murphy, DVM, former professor and Lynn Hovda, DVM, adjunct assistant professor, College of Veterinary Medicine, and Roger Becker, professor of agronomy, College of Food, Agriculture and Natural Resource Sciences.

Reviewed in 2021

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