San Francisco Bay Joint Venture

Smooth cordgrass (Spartina alterniflora), a tall grass native to the east coast, has invaded Willapa Bay, Washington, and the San Francisco Bay, California. Management with glyphosate and imazapyr can be effective, but in the San Francisco populations, applications in several sites are confined to short periods in the fall in order to protect nesting habitats of Clapper rails (Rallus longirostris). Use of efficacious soil-active herbicides could mitigate this restriction. Acetic acid, a readily degraded natural product, has been shown to kill sediment-borne propagules of aquatic plants such as Hydrilla verticillata and Stuckenia pectinatus. Effects of acetic acid on sediment-free rhizomes of S. alterniflora were examined. Exposure of 0.1, 1.0, 1.5% vol/vol acetic acid for a few hours to several hours resulted in increased conductivity in distilled water compared to unexposed controls, indicating loss of cellular integrity and leakage of electrolytes. Regrowth from exposed rhizomes was significantly inhibited at higher (1.0% and 1.5%) concentrations applied for 2 or 4 hr. When rhizomes that had been directly exposed to 1.5% acetic acid were transferred to outdoor conditions in Albany, CA, both new shoot number and average plant height were reduced by over 90% at nine months post-treatment. The exposure to all concentrations of acetic acid for 4 hr also led to reduced frequency of inflorescence production, thus potentially diminishing the dispersal capacity of the treated plants. Field trials are needed to determine if judicious drenching of sediments with acetic acid (e.g., at low tide) may have utility as an alternative to foliar applied herbicides such as imazapyr and glyphosate.