Abstract: A plug may be releasably mounted on a tip of a nozzle of an injector, and the injector may be pushed toward a borehole in a tree trunk so as to introduce and mount the plug in the borehole using the injector. Then, the injector may be operated to inject liquid into the borehole by way of the plug. Then, the injector may be withdrawn from the tree trunk, and the plug may remain in the borehole, such as for reuse. The plug may at least seek to function as a barrier device for both restricting contact between the borehole and the injector's nozzle, and restricting any backflow of the liquid from the borehole through the plug. The plug may include an elastic tube that functions as a check valve for restricting backflow through the plug.