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Project description

Integrated management of perennial pepperweed (Lepidium latifolium). (02CC014)
Program UC IPM competitive research grants program
Principal
investigators
R.G. Wilson, UCCE Lassen County
J.M. DiTomaso, Vegetable Crops, UC Davis
Host/habitat Unspecified
Pest Perennial Pepperweed Lepidium latifolium
Discipline Weed Science
Review
panel
Cultural Controls
Start year (duration)  2002 (Three Years)
Objectives Incorporate recently developed control options for perennial pepperweed into large-scale integrated management approaches and to provide sustainable suppression by re-establishing desirable, native vegetation in non-crop areas. Specific objectives include:

Determine if integrating non-chemical options, including burning, mowing, fall/winter grazing, or disking, with herbicides provides effective control of perennial pepperweed.

Evaluate the influence of litter on establishment of drill seeded native perennial grasses.

Examine the effect of integrating non-chemical and chemical control strategies on the ability to establish seeded native perennial grasses or re-establish resident vegetation.

Determine if the combination of control and native perennial grass establishment can suppress re-invasion of perennial pepperweed.

Final report Sites were located at the California Department of Fish and Game Honey Lake Wildlife Area and Mapes Ranch in Lassen County, California. Both sites were heavily infested with perennial pepperweed and had significant litter (thatch) accumulation from old stems. Perennial pepperweed cover was more than 50% and desirable vegetation cover was less than 5% at both sites in July 2002 before treatments were initiated. The experiment was a split-split plot with four replications. Whole-plot treatments evaluated the usefulness of winter burning, mowing, grazing, or fall disking for removing accumulated thatch to facilitate herbicide application and reseeding. Sub-plot treatments examined chlorsulfuron (Telar), 2,4-D ester, or glyphosate (Roundup) efficacy applied at the flower-bud stage. Sub-sub-plot treatments looked at the influence of no-till seeding native perennial grasses for preventing weed re-invasion. Burning and tillage were effective at removing accumulated thatch before herbicide treatment. All herbicides reduced perennial pepperweed cover compared to the control, but certain herbicide + whole-plot combinations provided better control than others. Averaged across sites, chlorsulfuron or 2,4-D applied alone, chlorsulfuron or 2,4-D in combination with burning, mowing, or grazing, and glyphosate in combination with mowing provided the best control reducing perennial pepperweed cover below 10% two years after treatment initiation. Disking before herbicide application decreased herbicide efficacy compared to using herbicides alone. Seeded perennial grasses successfully established in several treatments, and seeded grass cover was highest in burn + 2,4-D treatments. Chlorsulfuron injured perennial grass seedlings when applied before planting, but chlorsulfuron is safe on grasses applied after grass emergence. Seeded grass cover was less than 4% in plots that did not receive herbicide treatment due to excessive competition from perennial pepperweed. Winter burning in combination with yearly 2,4-D treatments gave the best combination of perennial pepperweed control and native grass establishment. No herbicides offered 100% control two years after treatment, suggesting multiple follow-up herbicide applications are needed for long-term weed suppression and vegetation restoration.

Third-year
progress
Sites are located at the CDFG Honey Lake Wildlife Area and Mapes Ranch in Lassen County. Both sites are heavily infested with perennial pepperweed and have significant litter (thatch) accumulation from old stems. Perennial pepperweed cover averaged 50% at Honey Lake Wildlife area and 71% at Mapes Ranch in July 2002 before treatments were initiated. Both sites had less than 5% desirable vegetation cover before treatments were initiated. In fall and winter 2002, selected plots were disked, grazed with cattle, or burned to remove old thatch accumulation and prepare the sites for herbicide treatment. Mowing and herbicide treatments were applied in June 2003 when perennial pepperweed reached the flower-bud stage. Mowed plots were treated with herbicides in August 2003 after plants re-sprouted and reach the flower-bud stage. Native perennial grasses were drill-seeded in spring 2004.

Preliminary results suggest burning, tillage, and mowing are effective at removing accumulated thatch before herbicide treatment. One year after herbicide application, the following treatments reduced perennial pepperweed cover by more than 80% compared to the control: winter grazing in combination with chlorsulfuron or glyphosate, mowing in combination with chlorsulfuron or glyphosate, winter burning in combination with glyphosate or 2,4-D, and chlorsulfuron, glyphosate, or 2,4-D applied alone. Due to drought conditions during spring 2004, perennial grass establishment was unacceptable at both sites. Treatments with the highest perennial grass cover by September 2004 included fall tillage in combination with 2,4-D or glyphosate and winter burning in combination with 2,4-D or glyphosate. Plots will be reseeded in March 2005.

Second-year
progress
Both sites had less than 5% desirable vegetation cover before treatments were initiated. In fall and winter 2002, selected plots were disked, grazed with cattle, or burned to remove old thatch accumulation and prepare the sites for herbicide treatment. Mowing and herbicide treatments were applied in June 2003 when perennial pepperweed reached the flower-bud stage. Mowed plots were treated with herbicides in August 2003 after plants re-sprouted and reach the flower-bud stage.

Preliminary results suggest burning, tillage, and mowing are effective at removing accumulated thatch before herbicide treatment. Mowing and tillage also significantly reduced perennial pepperweed cover compared to untreated, burned, and grazed plots the year of herbicide application. Chlorsulfuron or 2,4-D applied alone or in combination with burning, grazing, or mowing were the most effective treatments reducing perennial pepperweed cover by more than 93% the year of application. Plots will be seeded with native perennial grasses in March 2004. Preliminary results regarding re-vegetation and perennial pepperweed control one year after treatment will be available spring of 2004.

First-year
progress
Sites are located at California Department of Fish and Game Honey Lake Wildlife Area and Mapes Ranch in Lassen County California. Both sites are heavily infested with perennial pepperweed with significant litter accumulation. Perennial pepperweed cover averaged 50% at Honey Lake Wildlife area and 71% at Mapes Ranch in July 2002 before treatments were initiated. Perennial pepperweed formed near monoculture stands at both sites with other vegetation cover (primarily tall wheatgrass, salt grass, and summer annual broadleaf weeds) averaging less than 5 %. In October 2002, selected plots were disked with a 32 inch stubble disk. Two passes in opposite directions were needed to adequately incorporate perennial pepperweed litter into the soil. Plots will be burned and grazed in January 2003, and herbicide treatments will be applied in June of 2003. Preliminary results should be available spring of 2004.

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