Hoary Alyssum

Hoary alyssum is an annual to short-lived perennial forb in the Brassicaceae (mustard) family. It grows from a taproot to form a basal rosette and later produces an erect stem that reaches 7 to 30 inches tall. Plants may produce a single stem or multiple stems from the base. Stems are frequently branched near the top. Basal leaves are oblanceolate (wider at the tip) and 1 to 2 inches long. Stem leaves occur alternately on the stem and angle upward. Leaves may appear folded at the midvein, and leaf margins are entire. The entire plant is covered in small, star-shaped hairs that give the plant a rought, sandpaper texture and a gray-green color. Hoary alyssum flowers from late spring to mid summer. Flowers occur in racemes at the end of branches. Racemes flower from the bottom to the tip, elongating through the season, and lower seed pods form and mature while upper flowers are in bloom or bud. Individual flowers are white and small (0.1 inch across) and have four petals. Seeds are formed in flattened, oval seedpods that contain up to 7 small, round, brown seeds.

Hoary alyssum is toxic to horses while both green and dried in hay.

Hoary alyssum is native to Europe and Asia and was likely introduced to North America as a contaminant in clover and alfalfa seed in the 19th century. It spreads solely by seeds. Hoary alyssum can be found in a variety of habitats including riparian areas, roadsides, disturbed sites, hay fields, rangeland, and forest systems.

Prevention and cultural control strategies should be utilized as much as possible.

There are not any biological controls for hoary alyssum currently available.

Because hoary alyssum has a taproot, it can be readily managed by hand pulling or digging out the root especially in the spring and early summer when the ground is wet. Mowing, when combined with extensive cultural control methods, can be effective in providing a competitive advantage for native and desirable species if well established on a site. Controlled grazing has not been successful in eliminating plants due to preferential grazing on other forages.

Larger infestations can be controlled with herbicides. Spring treatments of rosettes or early bolting plants are recommended.

• Biology, Ecology, and Management of Hoary Alyssum (Berteroa incana L.) (2017) Parkinson, H., Mangold, J., and Jacobs, J. Montana State University Extension. 
• Hoary Alyssum (Berteroa incana) (2019) Invasive Species Council of British Columbia. 
• Treatment Options: Berteroa incana Hoary alyssum (2013) DiTomaso, J.M., Kyser, G.B., et al. Weed Control In Natural Areas in the Western United States. University of California Weed Research and Information Center. 
• Hoary alyssum: the most common poisonous plant to horses in Minnesota (2021) Martinson, K., Murphy, M., Hovda, L., and Becker, R. University of Minnesota Extension.