Perennial Pepperweed in the Laguna

Publication Type  Web Article
Authors  Laguna Foundation Staff
Year  2008
Website  Laguna de Santa Rosa Foundation
Full Text  

Pepperweed is considered to be one of the worst weeds in California, because of its capacity for explosive spread and the difficulty of control. Native to Eurasia, this tall flowering plant is a member of the mustard family, believed to have been introduced to the United States in the 1930’s in a shipment of beet seeds. It has been grown commercially for the cut flower trade, but has little or no forage value, although it is sometimes used medicinally.
Pepperweed disperses readily by floodwaters - as seeds, rooting stems and root fragments; and seeds are also distributed in cut hay or in the gut of grazing animals. Perennial root masses can become very large, with long-lasting carbohydrate reserves that allow them to regenerate after long periods without top growth.
An Alarming Invader
Pepperweed is a particularly alarming invader for the Laguna ecosystem because it has the potential for transforming healthy habitats and undermining large, long-term restoration projects. This species favors riparian woodlands, valley oak savannah, and seasonal wetlands - spreading out into adjacent agricultural areas. Its growth pattern is often described as explosive, increasing rapidly across many acres. Researchers at Cosumnes Preserve have found that pepperweed can blanket vernal pools, displacing native wetland plants. Restoration and preservation of native species will have low success in invaded areas, because of pepperweed’s large root systems, choking growth pattern and ability to increase soil salinity.
2004 Pepperweed Distribution Survey
In 2004, staff from the Marin/Sonoma Weed Management Area and the Laguna Foundation surveyed the distribution of pepperweed along most of the Laguna’s main channel. Although it is scattered throughout the reach between Stony Point Road and Santa Rosa Creek, it has not yet reached high densities, and patches are relatively small. It is very important to act quickly to control this invasion, given its potential seriousness, and that it is still relatively limited in spatial scale (and therefore treatable). Pepperweed is a textbook case for the need to do collaborative management, and the need for early detection and rapid response. Failure by one landowner to control pepperweed sharply limits the ability to control the invasion on adjacent properties.
As pepperweed is a problem in other parts of California and many states, there has been considerable research on control methods. In general, strategies involve killing or removing both the above-ground plant parts, preventing seed production, and attempting to kill or remove the below-ground system of roots and rhizomes. These efforts have to be accompanied by long-term monitoring programs to identify and control sources of re-infestation, and restoration to re-establish native plants.
Herbicides are considered the most effective option for pepperweed management in wildland areas. However, there are substantial community concerns about the use of herbicides in floodplains and wetlands, and strong interest in further research on non-chemical control methods. As pepperweed is a rapidly-expanding problem throughout the western states, a number of research programs are already under way. Land managers in some preserves are experimenting with the use of tarps or sheep and goat grazing. Tarps allow managers to reduce or avoid the use of chemicals, but they are best suited to areas where pepperweed grows in small patches in relative monoculture, because tarps unavoidably kill surrounding plants and animals.
Sheep and goats provide some level of control, but care must be taken to avoid having seeds carried from infested to non-infested sites in wool or feces. The City of Sebastopol and a citizen volunteer group has initiated a grassroots effort to control pepperweed in the Laguna Wetlands Preserve without using chemicals. Taking the Adopt-a-plot approach, they are experimenting with a range of approaches from hand-pulling to tarping and cutting. Sustained, labor-intensive efforts have been reported to work on small infestations in other areas.
Three different herbicides are widely used for pepperweed control:
All are systemic herbicides, translocated by the plant to the root system. Each has advantages and disadvantages, relating to their effectiveness, their specificity, their persistence and their toxicity. The Pesticide Investigations Unit of the CDFG recommends treating plants in the spring, applied at bud stage. None of these treatments is 100% effective, and the control program must be accompanied with a sustained monitoring and spot-treatment to catch re-sprouts and new infestations. Where pepperweed is growing alongside sensitive native plants, land managers can use cut-and-paint methods for applying minimal amounts of herbicide directly to the exposed stem.
Key to Controlling Pepperweed
The key to controlling pepperweed is long-term vigilance, and careful monitoring, regardless of the control method used. As with Ludwigia, the pepperweed control program needs support and funding from public agencies and cooperation from private landowners. Weeds know no property boundaries. If funding is limited, control efforts should focus on the small, satellite patches colonizing new areas out from the main infestation site. Research has shown that these patches are most responsible for the spatial spread of the invasion, and as they are generally younger, they have smaller perennial root systems and are easier to kill.
Most of the information presented here was summarized from the following websites. For photographs, natural history, and more information on specific control methods see The Nature Conservancy’s Global Invasive Species Initiative Lepidium latifolium management summary (Global Invasive Species Initiative ), as well as (USDA Forest Services ). A full incidence and density mapping project was conducted in July and August of 2004.