Invasive mustards: Hoary alyssum, perennial pepperweed and whitetop

While the unusually wet spring was beneficial to native and desirable grasses and wildflowers, many weed species took advantage of the excellent growing conditions as well. Annual mustards with yellow blossoms were abundant this spring and early summer. While "weedy" and too many "obnoxious", these plants are not regulated by the Montana Department of Agriculture, or "noxious". Three weed species that are regulated and required by law to control are perennial pepperweed, whitetop and hoary alyssum.

Montana Noxious weed status: All of these plants are invasive mustards (Brassicaceae family) native to Europe and/or Asia. Perennial pepperweed is a priority 2A noxious weed in Montana, while whitetop and hoary alyssum are priority 2B, meaning they are more abundant and widespread. Management criteria requires eradication or containment.

Identification: All three plants have small, white flowers with four petals. Key characteristics to differentiate these species from each other include leaves, fruits and height (See table to the right). Hoary alyssum can be distinguished from perennial pepperweed and whitetop by its tiny hairs that cover the leaves in a radiating pattern like that of a star, notched flower petals and differences in the fruits and stem. Whitetop has upper leaves that clasp the stem, whereas perennial pepperweed leaves have a short stem that attaches them to the main stem. Both whitetop and hoary alyssum have round or swollen fruits that hold the developing seeds, while perennial pepperweed has flattened fruits. Whitetop and hoary alyssum are generally not much taller than 2.5 feet, while perennial pepperweed can reach heights of six feet, especially in wetter areas. These species as well as other mustards are closely related and can be difficult to distinguish without flowers or fruit.

Management: Hoary alyssum reproduces by seed, therefore management should focus on preventing seed production through hand-pulling, mowing or herbicide applications. Whitetop and perennial pepperweed are rhizomatous, so management is generally more difficult and control techniques must reduce seed production and stress roots. Hand-pulling and mowing are not as effective on these two mustards. Herbicides that contain metsulfuron or chlorsulfuron are effective on all three species and application should occur rosette to bolting (hoary alyssum) or bud to bloom (whitetop, perennial pepperweed). Grazing is not recommended as all three species exhibit some degree of toxicity to livestock. No biological control agents are currently approved for these species, however agents are being developed for whitetop.

For more information contact your county weed coordinator.