Evaulation of perennial pepperweed (Lepidium latifolium) management in a seasonal wetland in San Francisco Estuary prior to restoration of tidal hydrology

Authors: WHITCRAFT, CHRISTINE - California State University; Grewell, Brenda

Submitted to: Wetlands Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2011
Publication Date: 12/16/2011
Citation: Whitcraft, C.R., Grewell, B.J. 2011. Evaulation of perennial pepperweed (Lepidium latifolium) management in a seasonal wetland in San Francisco Estuary prior to restoration of tidal hydrology. Wetlands Ecology and Management. 20:35-45.

Interpretive Summary: Wetland ecosystems are particularly vulnerable to invasion by perennial pepperweed. There has been a crucial need for continued development and application of effective eradication methodologies for perennial pepperweed that are tailored to the particular invaded ecosystem and to landowner concerns, because the outcome of management approach varies across wetland ecosystems and settings. This article provides results of an experimental evaluation of two approaches implemented by a land manger for control of perennial pepperweed in a seasonal wetland slated for restoration that is immediately upstream of a sensitive tidal marsh. We evaluated the efficacy of two herbicides applied in the wetland, evaluated the response of the associated plant community and its variation with microhabitat conditions. Two years of 2,4-D application were not effective in controlling L. latifolium (< 1% control) but had minimal non-target impacts on the native plant community. Imazapyr reduced L. latifolium cover by more than 90% after one year of treatment as compared to untreated controls although non-target impacts on the native plant community were severe and persistent. These results provide important information about the response of L. latifolium to management trials in a seasonal wetland and will be used to develop an integrated and adaptive management strategy for weed control as a component of a proposed tidal marsh restoration plan.

Technical Abstract: Herbicide applications have shown potential for control and management of invasive perennial pepperweed (Lepidium latifolium) in rangelands and tidal wetlands. However, reported efficacy of management methods varies widely, and the effects of more recently labeled aquatic herbicides on non-target vegetation in riparian corridors and seasonal wetlands are poorly understood. Weed control is often a necessary pre-cursor to tidal hydrologic restoration or revegetation. In a replicated field experiment, we applied two registered aquatic herbicides to control L. latifolium upstream of a sensitive tidal wetland reserve as a preliminary step towards hydrologic restoration to a degraded ephemeral creek channel and associated seasonal wetlands. Herbicide treatments (imazapyr, 2,4-D) were applied at flower bud stage in May 2007 and monitored at 1 year and 2 years following application. Two years of 2,4-D application were not effective in controlling L. latifolium (< 1% control) but had minimal non-target impacts on the native plant community. Imazapyr reduced L. latifolium cover by more than 90% after one year of treatment as compared to untreated controls although non-target impacts on the native plant community were severe and persistent. Both control and non-target impacts have been pervasive over the two years of observation. These results provide important information about the response of L. latifolium to management trials in a seasonal wetland and will be used to develop an integrated and adaptive management strategy for weed control as a component of a proposed tidal marsh restoration plan.