Boundary Invasive Species Society
Regional Noxious. Annual to short-lived perennial. Hoary alyssum grows erect to 0.7 meters tall and has white flowers carried on long slender stalks. Introduced from Eurasia.
• White flowers with deeply notched petals.
• The entire plant is covered with star shaped hairs.
• Upper leaves are elliptic and clasp stem.
• Oval seedpods are 5-8 mm long, somewhat flattened and held close to the stem.
Hoary alyssum is toxic to horses, and can cause fever, edema, and laminitis. Sensitivity varies when small or single doses are ingested, and death has only been reported in horses that have consumed hay infested with a large proportion (30-70%) of hoary alyssum.
Small populations can be controlled by hand pulling or digging. Make sure to remove the top inch of the root. Easiest when soil is moist. Mowing can prevent seed production. If flowers and seed
are present bag the plants and dispose of in the landfill. Selective herbicides are effective.
Reproduction and Dispersal:
Hoary alyssum spreads rapidly through a long season of seed production. Each plant flowers and produces a large number of seeds between early summer and fall up to frost. Seeds disperse as a contaminant in hay and are spread by vehicles, equipment, footwear, wildlife, and birds.
Most common on sandy or gravely soils, hoary alyssum establishes in dry, disturbed habitats, such as roadsides and railway embankments. Hoary alyssum invades dry land, irrigated alfalfa fields, and rangelands, tending to increase in forage crops following periods of drought or winterkill, thus reducing hay quality and value.
Has small (5-8 mm in length), white flowers with deeply notched petals that are supported on slender stalks. Sepals are hairy and transient.
Leaves and Stems:
The whole plant is covered in star-shaped hairs that are rough to touch, with grey leaves that clasp closely to the stem.
Oval seedpods are chambered and held close to the stem with each chamber containing 5-7 black seeds.
Hoary alyssum is toxic to horses, causing swollen legs, laminitis and severe lameness. Consumption of large quantities can cause diarrhea, leading to dehydration, and can cause abortions in pregnant mares. The plant remains toxic after it is dried, and most poisonings are due to contaminated hay.
Check out our hoary alyssum identification video. You will need to turn up the volume to hear what is being said.