Drilled injection holes can permanently damaging a tree’s ability to move nutrients and repeated drilling can seriously harm a tree’s long term health. Injections should be applied beneath the bark and into the tree’s water conducting system in the outermost ring. “Small, shallow holes, no deeper than two growth rings, cause the least amount of injury” according to “Modern Arboriculture,” Alex Shigo’s reference text that is recommended by the International Society of Arboriculture for arborists, landscape architects, urban foresters, fruit growers and students. Shigo was a leader in arboriculture research and study. For more information, click here.