Abstract: An injection retention plug is insertable into a hole in the outer bark layer of a tree so that upon injection of fluid through the plug to an area between the xylem and inner bark layers, the plug will fill the hole to block leakage of the injected fluid outwardly through the outer bark. The plug is of a size and shape to fill the cross-sectional area of the preformed hole in the outer bark. It includes a core of resilient material which is expandable upon withdrawal of the fluid injection device to fill the hole. A method for injecting fluids into a tree includes forming a hole in the outer bark, inserting the plug into the hole, injecting fluid through the plug for retention between the xylem and inner bark layers and allowing the plug to block leakage of fluid outwardly through the injection hole.