Perennial Pepperweed
(Lepidium latifolium):
Foreign invader in California's wildlands

What can be done to control perennial pepperweed?

This weed has proven to be very difficult to control, especially in sensitive wetland ecosystems. Maximum efforts should be made to prevent moving perennial pepperweed to uninfested sites and to eradicate small invasions before they become established. Sites with new perennial pepperweed infestations, or ones likely to be invaded should be monitored frequently. A visit to the site in early spring is recommended because pepperweed is typically one of the first plants to emerge from winter dormancy and the rosette stage is easy to recognize. Another visit in late summer is suggested to look for senesced stems, flowers, and for new rosettes produced by spreading roots.

When infestations are spotty and scattered, the best approach is to first kill the outlying populations to prevent spread and then work towards the center or origin of the invasion. Plants in outlying areas are younger and should be easier to kill. Control of invasive plants should use an integrated approach combining a variety of methods. The most effective control strategies for perennial pepperweed have been combinations of chemical and mechanical methods. Because perennial pepperweed has numerous relatives in California, including several rare species, no biological control agents have been developed.

Physical methods of weed control typically include burning, mowing, disking, and flooding. Burning is not effective for perennial pepperweed. Plants do not burn easily and the underground roots are unaffected and recover rapidly. Long-term flooding is effective, but only if the area is completely inundated for two growing seasons. Mowing or disking early in the season will reduce the leaf area of resprouts and decrease the number of seed set later in the season, but has limited long-term impact on perennial pepperweed. Disking can increase an infestation by cutting the roots into smaller pieces and spreading the fragments. Because perennial pepperweed often grows with native plants, mowing, disking, or the broadscale use of herbicides is difficult or impossible to do without causing damage to desirable plants.

Herbicides, especially when integrated with mechanical methods, are effective for controlling perennial pepperweed infestations. There are several herbicides available for use on perennial pepperweed, but there are restrictions on their use in specific environments. Since the legal requirements associated with herbicide use are important and these tend to change frequently, specific recommendations are not included in this document. The resources in Table 1 list the most current information on herbicides used to control perennial pepperweed. As a general guideline to herbicides it is important to use the recommended dosage of the herbicide at the optimum time of the year. Applying herbicide to resprouts is typically more effective than spraying mature plants. Care should be taken to avoid damage to non-target plants.

Resources for more detailed information on perennial pepperweed, especially in regard to recommended control practices, are available at the following web sites:

Weed Research & Information Center
California Exotic Pest Plant Council
The Nature Conservancy

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