If you have mature trees and shrubs in your yard, you're in luck. You don't have to worry about watering them in most cases. New ones need water after you've planted them for about a year to become established. Rain does most of the work, but it helps to give even your mature trees and shrubs a good soaking now and then in the summer and for the first few years after planting.
The way your new trees and shrubs establish strong roots is by sending them out to where the water is. That's why you want to water deep. Shallow roots dry out and make the tree or bush unstable. Whether you use sprinklers, soakers, basins, furrows, or a drip system, the idea is to avoid runoff. Just stay under the drip line (the extent of the branches), water slowly over a long period of time, and let the water soak in.
Your shrubs need watering only when you plant them and for about a year afterwards. Their first summer is crucial, so water them well then. Once your shrubs are established, you only have to worry about watering them if you live in arid areas or during a drought.
In droughts, even mature trees can experience water stress. Signs to look for are wilting, leaf-color change, or a premature leaf fall. At those times, give your trees a good, long soaking.
Fruit trees are thirstier than other types. They typically need about two gallons of water per square feet of root space once a week. If you¿ve just planted your tree, figure on about two to four gallons a week. Of course, rainfall and soil type can affect your watering, so dig in a few inches of soil to see if you need to water again. If your tree is out in the lawn, give it an additional deep soaking twice in summer, over and above your normal lawn watering.