The Drip Line
This term does not refer to drip irrigation, but rather to the circle you'd draw on the soil around a tree or shrub directly beneath its outermost branch tips. Rainwater tends to drip from the leaves at this point. When watering young trees and shrubs, you can water up to and just beyond the drip line. As these plants mature, however, their roots usually grow farther out into the soil, eventually extending well beyond the canopy (roots grow wider in sandy soil). To water such plants adequately, you'll need to make a wider basin (for flood irrigation) or move sprinklers, soaker hoses, or drip emitters outward.
Using a Root Irrigator
The needlelike probe on the end of this tool injects water into the root zone of trees and large shrubs. Insert it into the ground slowly, holding it at an angle so the top layer of soil gets wet. Don't go deeper than 1 to 1½ feet or stay in one spot for more than a few minutes. Work in concentric circles around the trunk, gradually moving outward just past the drip line and using the injector about every 3 feet.