Hoary Alyssum: Identification, toxicity and control

Krishona Martinson
University of Minnesota Extension
Hoary alyssum is a perennial, grayish-green weed that stands 1 to 3 feet tall.

During dry periods, many weeds more tolerant of drought invade pastures, seemingly appearing from nowhere and thriving among the dormant and struggling pasture grasses.

One such weed is hoary alyssum, a toxic weed commonly found in horse pastures and hay fields in the Midwest, especially on sandy, dry soils. Since hoary alyssum tends to be more drought tolerant than desirable pasture grasses, many horse owners have noticed it growing in their pastures, resulting in questions regarding proper identification, toxicity and control.    

Hoary alyssum is a perennial, grayish-green weed standing 1 to 3 feet tall. Its leaves are covered with rough hairs and flowers are petite, white and have four deeply-divided petals. Seed pods are oblong with a pointed end and become translucent, making the tiny black seeds visible as the weed matures. 

Hoary alyssum is toxic when horses graze the growing plant in pastures and when dried in hay. In pastures, horses usually prefer other, more palatable forages over hoary alyssum. But, hungry horses grazing on drought-stressed or dormant pastures without supplemental hay may ingest hoary alyssum.

However, most hoary alyssum poisoning occurs when horses accidentally ingest the weed in hay. Horse owners should carefully inspect hay and reject any bales containing hoary alyssum. Hoary alyssum is not toxic to other livestock.  

Along with the specific toxin, the amount of hoary alyssum a horse needs to ingest to cause toxicity is unknown and seems to vary between horses. In most cases, signs of toxicity occur 12 to 24 hours after ingestion. Most, but not all, horses ingesting hoary alyssum will have the following signs:

  • Swelling and fluid build-up in the lower legs or “stocking up”
  • A fever of 103 degrees or higher
  • Warm hooves
  • Pronounced digital pulse (laminitis)
  • Stiff joints
  • Not willing to move
  • A “camped out” stance

While death from ingesting hoary alyssum is rare, horses under stress or ones without prior exposure to hoary alyssum tend to be impacted more severely.

Controlling hoary alyssum in a grass pasture can be accomplished with a broadleaf herbicide; however, it may take several applications to completely rid the pasture of the weed.

Herbicides containing dicamba and/or 2,4-D can provide moderate control of hoary alyssum when applied in the fall (Aug. 1 to Sept. 15) in the Midwest. Fall is an ideal time to control perennial weeds, since they are translocating nutrients into the roots, helping to ensure the herbicide will also reach and kill the root.

If using a herbicide, ensure the product is labeled for a pasture (versus lawn) and follow all directions, including any listed grazing restrictions. Other options for control include mowing and strengthening the desirable pasture stand through over-seeding, fertilizing, avoiding over grazing and rotational grazing. However, these strategies may take years to provide adequate control.

Special consideration when controlling hoary alyssum and other weeds must be made during dry times. Although hoary alyssum may appear to be actively growing, it’s likely also impacted by the dry conditions currently found throughout Minnesota and the Midwest.

Herbicides work best on actively growing weeds, with poor control commonly reported during dry times. Therefore, owners should mow hoary alyssum found in pastures during the spring and summer months and plan for a fall herbicide application if rainfall returns and the weed and other plants appear to be actively growing.

If dry conditions continue, owners should remain vigilant, with mowing as the main strategy for controlling hoary alyssum and other pasture weeds.