The movement of trunk-injected chemicals occurs in outer three rings of the xylem (wood). Any portion of a chemical that is injected more deeply than the outer three rings will not move to where it is needed. Higher rates are needed when injection holes are drilled deep into the wood. Deep injections place much of the product into the inner rings where it stays. In these cases, higher rates are needed to replace the portion of the product that does not move. This is especially common in large trees, where slow diameter growth results in narrow rings and many rings are cut through by the drilled holes.