by Alan Carpenter and Tom Murray
Other Common Names: broad-leaved peppergrass, tall whitetop, Virginia pepperweed
Description: Perennial pepperweed is a perennial forb of the Mustard (Brassicaceae) family. Mature plants are 1-3 feet tall. Under wet conditions plants can grow up to 8 feet tall. Leaves are lance-shaped, entire to toothed, bright-green to gray-green, and don’t have clasping bases (hoary cress leaves have clasping bases). The basal leaves are larger than the upper leaves. The white flowers are packed in dense clusters near the ends of branches. Fruits are nearly round, about 0.1 inch in diameter and usually sparsely hairy. The leaves and stems are covered with a waxy layer (Whitson et al. 1996).
Impacts: Perennnial pepperweed is an aggressive colonizer of pastures and particularly riparian and wetland habitats. It establishes rapidly and can eliminate competing vegetation (Anonymous 1998). Deep-seated rootstocks make this weed difficult to control. The leaves and stems are covered with a waxy layer that makes this plant difficult to control with herbicides.
Habitat: In Colorado, perennial pepperweed is found from 5,000 to 8,000 ft. Perennial pepperweed is most often found in open, unshaded areas on disturbed, and often saline soils. It is locally common in riparian areas, marshy floodplains, valley bottoms, and seasonally wet areas. Perennial pepperweed is found in big sagebrush (Artemisia tridentata) communities of the Piceance Basin of Colorado (Anonymous 1998). Plant associates in these communities include twisted moss (Tortula ruralis) and desert goosefoot (Chenopodium pratericola) (Anonymous 1998).
Stewardship Summary: Perennial pepperweed is an introduced forb of Eurasian origin that has established throughout the United States. It forms dense colonies by adventitious roots and deep-seated rhizomes. Perennial pepperweed is common in fields, wasteplaces, meadows, and along roadsides. It also invades irrigated pastures, cropland, and native meadows (Anonymous 1998).
Biology/Ecology: Perennial pepperweed reproduces mainly by spreading rhizomes and can be aggressive colonizers of disturbed areas (Anonymous 1998). Dense flower clusters appear in early summer and continue through August. Perennial pepperweed produces an abundance of highly germinable seeds that have no apparent dormancy. However, Young (1999) has never collected a tall whitetop seedling in dense stands. Seedling establishment is not a significant problem once established tall whitetop plants are suppressed. He speculated that seed dispersal is important in long-distance dispersal.
Control: The aerial portions of tall whitetop are easily killed by several herbicides, but it is very difficult to get enough herbicide to translocate to the roots and kill the numerous buds. (Young 1999). Tillage alone is ineffective because each small section of plant will produce new plants. However, a combination of mechanical control (e.g., mowing or pulling) and herbicide application can provide effective control of perennial pepperweed. For instance, tillage after herbicide applications may be an effective way to bring treated roots to the surface where they will dry out (Young 1999). Mowing followed by spraying the resprouts with chlorsulfuron or glyphosate is very effective at controlling perennial pepperweed. Mowing twice in a growing season followed by chlorsulfuron treatment controlled more than 99% of pepperweed in California. Chemical treatment with 2,4-D at the flower bud stage followed by treatment of resprouts with glyphosate achieved 87% control (Renz and DiTomaso 1999).
Plants should be cut or pulled during the flower bud stage. Herbicides should be applied to the recovering stems when they return to flower bud stage later the same year. Dicamba at 1 lb. active ingredient (ai) per acre or glyphosate at 1.5 lb. ai/acre will control perennial pepperweed. Other herbicides that proved to be effective include chlorsulfuron, metasulfuron and imazapyr. Of these, chlorsulfuron and metasulfuron applied in the fall are most effective in a study in Colorado (Beck 1999). However, these herbicides cannot be used for weed control in the San Luis Valley of Colorado. Periodic mowing and spring burning have reduced perennial pepperweed density in Utah (Anonymous 1998).
- Anonymous. 1998. Lepidium latifolium. Fire Effects Information System. U.S. Forest Service. Available 9/1/98. Internet: http://www.fs.fed.us/database/feis/plants/forb/leplat/all.html.
- Beck, K.G. 1999. Perennial pepperweed and hoary cress in Colorado. Proc. National Whitetop Symposium, June 9-10, 1999. Alamosa, CO. pp. 19-22.
- Renz, M.J. and J.M. DiTomaso. 1999. Seasonal carbohydrate translocation patterns of perennial pepperweed (Lepidium latifolium) and implications for control in California. Proc. National Whitetop Symposium, June 9-10, 1999. Alamosa, CO. pp. 31-36.
- Young, J.A. 1999. Lepidium latifolium L. biology and control. Proc. National Whitetop Symposium, June 9-10, 1999. Alamosa, CO. pp. 3-4.
- Whitson, T.D. Ed. 1996. Perennial Pepperweed. Weeds of the West. Western Society of Weed Science. Jackson: Pioneer of Jackson Hole. pg 230.