Alliaria petiolata is an invasive herb impacting forests throughout the eastern United States. We studied the impacts of glyphosate, bare ground, and summer precipitation on A. petiolata cover and density in two forest stands of different ages at Hueston Woods State Park, Preble and Butler Cos., OH. Fifty 1 × 1 m plots were established in each stand and 25 plots per stand were treated with glyphosate each November 2000–2004. Cover and density of A. petiolata rosettes and adults were measured multiple times each year from 2000 through 2005. Percent bare ground was estimated using a point frame in May 2003.
Adult cover in May was significantly lower in sprayed vs. unsprayed plots in three of five years; in the other two years adult cover was very low in both sprayed and unsprayed plots. However, spray treatment did not significantly affect May rosette cover across years. ANCOVA revealed that May 2003 bare ground positively associated with density of A. petiolata rosettes in May 2003, but bare ground was not significantly associated with October 2003 rosette density. Variation across years in both October rosette density and May adult density of A. petiolata was significantly associated with precipitation the previous June; wetter Junes were followed by higher densities.
Despite sustained suppression of adult A. petiolata in plots sprayed with glyphosate, new rosettes appeared each spring at densities comparable to unsprayed plots, which we attribute to seed dispersal from outside of the plots. Bare ground had minimal effect on A. petiolata populations. Evidence that A. petiolata density was positively affected by June precipitation suggests that management efforts should be focused in years of wet summers.