A small population of an invasive weed, perennial pepperweed (Lepidium latifolium), was recently discovered along a roadside in Green Bay. This plant is very similar to Canada thistle as it can invade croplands and natural areas causing near monotypic stands. Perennial pepperweed has become a major problem in other states as it has invaded alfalfa fields, pastures, rights of ways, and natural areas causing significant economic and environmental damage. It has been found in other Midwestern states, but this is the first sighting in Wisconsin.
L. latifolium has numerous common names including perennial pepperweed, perennial peppercress, perennial peppergrass, broad-leaved peppergrass, peppergrass, slender perennial peppercress, tall whitetop or dittander. It is native to Eurasia, from North Africa north through Europe to Norway and east to the western Himalaya. It was discovered in the western US in the mid 1930’s, and has been reported in coastal, intermountain and mountainous areas in New England, in a few states along the eastern seaboard, in several midwestern states, and all the states west of the Rocky Mountains. It also occurs in Canada and Mexico. In its native range it grows in a wide variety of habitats, including fresh, brackish, and saltwater wetlands, in and around agricultural fields, in waste places, and even on stony slopes, from sea level to above 10,000 ft.
A member of the mustard family (Brassicaceae), it readily invades disturbed areas and bare soils. This extremely invasive weed spreads by seed and rhizomes. Rhizomes may grow 3-10 ft from the main plant and form new plants. New plants will grow from pieces of rootstock less than 1/3″ in diameter and less than 1″ long. Plants flower in early summer and thousands of seeds per plant mature by late summer or early fall. The small seeds are capable of being transported by wind, water, and possibly waterfowl. Seedlings grow rapidly and can produce flowering stems the first year. L. latifolium is a very competitive species that crowds out desirable vegetation and results in dense monocultures and a decrease in biodiversity. Large colonies replace native grasses, sedges and rushes, and infestations reduce the quality of pastures.
L. latifolium is an erect, branching herbaceous plant that grows one to three feet high, but may reach heights of eight feet in wet areas, with a heavy, sometimes woody, crown and a spreading underground root system. Stems and leaves are dull gray-green and waxy, sometimes with reddish spots. Toothed leaves are lance-shaped. The tiny white flowers are borne in dense clusters at the tops of the stems, and are followed by elongated seed pods that are rounded, flattened and slightly hairy. The plant has deep and spreading roots.
There likely are only a few small infestations currently in Wisconsin. It is important to rapidly respond to this pest and try to eradicate populations before they become well established. Fortunately the Brown County Invasive Species Team reacted quickly, finding all nearby infestations and conducting management to eradicate populations. Due to the perennial nature of this plant several years of monitoring will be required to claim victory, but this is feasible as long as populations are small.
The best management of perennial pepperweed is preventing its spread into non-infested areas. Early detection and treatment of perennial pepperweed is crucial for any land management program. If you think you have perennial pepperweed on your land or believe you have seen it elsewhere in Wisconsin, contact your county Extension agent or weed district officials.
– Susan Mahr, University of Wisconsin – Madison
All photos from The Global Invasive Species Initiative Team. ©The Nature Conservancy
- Invasive Plants of Wisconsin: Perennial Pepperweed – UW Extension publication A3832
- Perennial Pepperweed – Plant Conservation Alliance’s Alien Plant Working Group Least Wanted fact sheet
- Lepidium latifolium – on the USDA Forest Service website