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

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 registered aquatic herbicides on non-target vegetation in riparian corridors and seasonal wetlands are poorly understood. 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 of 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 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 1 year of treatment as compared to untreated controls although non-target impacts on the native plant community were severe and persistent over the 2 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.