USGS Home Contact USGS Search USGS

Maps, Imagery, and Publications Hazards Newsroom Education Jobs Partnerships Library About USGS Podcasts/RSS

Pepperweed: A Growing Threat to Western Wildlife Habitat

The pepperweed flower is attractive and many unsuspecting people use cuttings as floral decorations in their homes. Photo by J. DiTomaso, University of California, Davis

Perennial pepperweed, also known as tall whitetop, is an invasive plant found throughout California and in all of the western United States. Native to portions of Europe and southwestern Asia, pepperweed was accidentally introduced into the United States around 1900, in a sugar-beet seed shipment, and has been rapidly spreading throughout the West.

Populations of pepperweed can establish and spread by seeds or by root fragments. This enables the species to expand into large, mostly one-species stands in a variety of environments, including wetland perimeters, rangelands, meadows, riparian areas, salt marshes/estuaries, roadsides, irrigation channels, and even irrigated alfalfa fields.

The competitive nature of perennial pepperweed poses a serious threat to many native undisturbed areas as well as to previously disturbed areas undergoing restoration. In these areas, perennial pepperweed can threaten environmentally sensitive habitats and displace threatened and endangered plant and animal species. Along riparian or streamside corridors, perennial pepperweed invasions can interfere with the regeneration of willows and cottonwood trees. Land managers have even observed invasions of pepperweed degrading waterfowl nesting habitat.

Currently, little is known about the biology and ecology of perennial pepperweed. To deal with the weed, land managers need more information about the potential spread of pepperweed and what impact this plant will have on the ecosystems it invades. While pepperweed appears to spread rapidly in some areas, it may not invade adjacent locations. This behavior suggests that environmental factors, such as moisture and competition, may determine the plant's establishment as well as how it spreads.

Scientists from the USGS Western Ecological Science Center and University of California, Davis, are collaborating to better understand environmental factors that influence the spread of perennial pepperweed. Experiments have been established throughout three bioregions of California-the Klamath region, the Central Valley region, and the San Francisco Bay region-evaluating the rate of spread and how species diversity, species composition, soil moisture, and salinity influence the spread of perennial pepperweed.