Rootspread of Landscape Trees in the Desert Controlled by Water

Q. If I have very mature shrubs (over 5 years) like  abelias etc. and keep it shaped to about 3'x3'. Will the "mature' root system stay within the canopy for watering purposes or extend out due to age and need more water?

A. The roots will stay where the water is and go not much further. You can contain the roots of trees and shrubs just with the placement of water provided the soil around it is dry.

This is the wetted pattern of a plant growing on a slope during the winter months. I took the picture to show you how applied water follows the laws of gravity. The wetted pattern only shows a portion of the soil actually wetted by the drip emitter. The actual wetted area the roots can use extends probably a foot beyond this surface wet spot in all directions, further on the down slope side. Always put drip emitters UPSLOPE of a shrub planted on a slope so the water moves downslope through the root system.

But you should try to water at least about half of the area under a plant's canopy. If plants are isolated from other water sources in the desert, roots will stay close to the water source and not go much further. The wetted surface of the soil gives you a much smaller idea of the true spread of the roots.