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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.

white and yellow hoary alyssum plant growing in grass
Hoary alyssum. Photo: Dana Boyle

Hoary alyssum is common throughout Minnesota, the upper Midwest and Western states.

  • It is most abundant in disturbed sites.
  • It’s also found in meadows and pastures and is a common weed in hayfields.
  • Hoary alyssum is more adapted to dry conditions on sandy or gravely soils.
  • It prefers direct sunlight but tolerates shade.

Lifecycle

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

​​​​​Identifying hoary alyssum

Stems

  • Grayish-green

  • Hairy

  • 1 to 3 feet tall

  • Many branches near the top

Leaves

  • Oblong-shaped

  • Grayish-green

  • Covered with rough hairs

Flowers

  • 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

Toxicity

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.

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Control

  • 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.

    • You may need to apply it more than once.

    • Apply the herbicide before the plant flowers.

    • Hand pull or mow flowering weeds before seed production.

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

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.

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

Reviewed in 2021

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