DORMANT SEASON BASAL BARK SPRAYING OF GARLON 4 AS AN EFFICIENT METHOD FOR CONTROLLING BUCKTHORN (Rhamnus cathartica)

Matthew J. Peters, St. Croix Watershed Research Station
Shawn P.Schottler, St. Croix Watershed Research Station
Wiley Buck, Great River Greening

European Buckthorn (Rhamnus cathartica) is a common exotic invader of oak savanna and woodland in the upper Midwest that can dominate understory and herbaceous layers and cause reduced recruitment and richness in native plant communities. Common control methods, such as hand pull, cut/stack/stump treat, prescribed burn, and foliar spray, have limitations on the scale or type of site to which they can be applied. Dormant season basal bark spraying (DBBS) has the potential to be a highly efficient and broadly applicable control method, but has not been thoroughly tested on buckthorn. DBBS involves spraying a small band of herbicide on the lower section of the trunk of a standing live tree during the winter. This study tested application rate and timing of DBBS using Garlon 4 (61.6% triclopyr-ester) mixed with Diluent Blue basal oil (carrier) to determine optimum usage guidelines. The impact of buckthorn size on treatment efficacy was also examined. DBBS of buckthorn was highly effective at all tested rates, achieving 83, 89 and 99% mortality with 10, 20, and 40% solutions respectively. All rates performed well throughout the winter treatment period (Dec.-Apr.). All rates performed nearly equally against buckthorn one inch or less in diameter. As buckthorn diameter increased, 10 and 20% treatments became somewhat less effective, but efficacy of the 40% treatment was unaffected. Exotic Tartarian honeysuckle (Lonicera tatarica) was also sprayed in the course of the study and showed an overall mortality of 66%. Applicators estimated a fivefold increase in labor efficiency with DBBS over the common cut/ stack/ stump treat method, thus allowing more acreage to be treated.

We conclude that DBBS is an efficient, effective control method and that good control and minimal chemical use could be achieved by using 10% Garlon 4 and applying more to large individuals. Contrary to expectations there was localized non-target damage to herbaceous plants despite dormant season application. Further investigation is warranted to determine whether Garlon 4 or the Diluent Blue carrier is causing the damage. Due to potential non-target damage DBBS with Garlon 4 should be used with caution in areas of highly valued herbaceous vegetation.