The invasive-exotic Rhamnus cathartica has been growing in parks and natural areas of North America for over 100 years where it has replaced native vegetation. Chemical herbicides have limited success on R. cathartica and often require follow-up applications. This multiyear study is the first to investigate the efficacy of Chondrostereum purpureum, the active agent in Chontrol Peat Paste (CPP), as a biological herbicide for R. cathartica. The objective of this study was to determine the efficacy of CPP and Roundup on R. cathartica trees by comparing re-growth/mortality rates of mechanically wounded trees treated with either herbicide. Rhamnus cathartica trees were girdled or cut and received either CPP or Roundup applications in late-fall (LF), early-summer (ES), and late-summer (LS) at Assiniboine Park in Winnipeg, Canada. All trees were evaluated for mean re-growth, number, and condition of basal sprouts during spring following each application. It was expected that trees treated with CPP would show less re-growth than those that were solely mechanically wounded (controls). In LF, the most effective mechanical/herbicide combination for reducing overall stem re-growth was found to be the cut treatment followed by Roundup. In ES, however, the most effective treatment combination for suppressing re-growth was the CPP application to girdled trees as conditions were optimal for inoculation of trees. These results will allow herbicides to be effectively applied over a longer duration of the season and have implications for the development of future management protocol for R. cathartica in urban parks and natural areas.