Montana State University

Hoary alyssum--A weed to watch for on your property

August 19, 2010 -- Jane Mangold, MSU Assistant Professor and Extension Invasive Plant Specialist

Hoary alyssum plants that were mowed once, but re-grew and produced seed along a trail (photo courtesy of Hilary Parkinson).   High-Res Available

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Tel: (406) 994-4571
Have you noticed a small, white-flowered mustard growing prolifically in your neighborhood? It could be the noxious weed hoary alyssum (Berteroa incana). Hoary alyssum was added to the state noxious weed list in 2008, and may not be as familiar as other notables like spotted knapweed, Canada thistle or leafy spurge.

However, if you live in southwestern Montana, where this weed is most prevalent, you've probably seen it along a bike trail or road, in a waste area or pasture, or even in your yard. It flowers from spring through late fall, and is currently very noticeable as other vegetation begins to die back for the season.

Hoary alyssum is an annual to short-lived perennial that thrives in dry and disturbed ground such as along trails, roads, or waste areas. However, it is also problematic in pastures and lawns. Plants are typically 7-30 inches tall and densely branched. Flowers grow in clusters along the length of the stems, and have four small, white petals that are notched, giving petals a rabbit-ear shape. Leaves are grayish-green in color due to tiny star-shaped hairs that cover the surface. The weed produces small pods that contain numerous reddish-brown seeds.

Hoary alyssum is believed to have been transported to North America as a contaminant of clover and alfalfa seed. It was first found in Gallatin County in 1905, and is now most common in Gallatin, Jefferson, Madison, and Ravalli counties.

One of the most troublesome aspects of this weed is its toxicity to horses. While horses will avoid hoary alyssum in pastures where other forage is available, risk of toxicity is high if the pasture is dominated by hoary alyssum, or if hay contains more than 30 percent hoary alyssum.

Because this plant spreads by seeds only, it is extremely important to prevent seed production and dispersal to non-infested areas. Hand-pull small infestations when soil is moist, or use a shovel or hand trowel to remove the root crown. If plants are flowering, bag and seal them or burn them to prevent further seed spread.

Where possible, proper irrigation and fertilization can effectively suppress hoary alyssum. For lawns, fertilizing and watering to maintain healthy grass can reduce and ultimately eliminate hoary alyssum. For pastures, mowing to a six-inch stubble height, in combination with irrigation and nutrient management, can increase the vigor of desired plants and effectively suppress hoary alyssum. Where hoary alyssum is dense and desirable vegetation is sparse or absent, an herbicide application followed by reseeding is strongly recommended.

For more information on hoary alyssum download or order a printed copy of the MSU Extension bulletin "Biology, Ecology and Management of Hoary Alyssum" at

Jane Mangold (406) 994-5513 or