The goal is to give the tree’s roots a good soaking every week to ten days. The best time to water trees is early in the morning.
A general rule of thumb is to use approximately 10 gallons of water per inch of trunk diameter for each watering. Using a hose with medium pressure, you will typically generate about 2 gallons a minute, so for a tree with a 4 inch diameter trunk, water for about 20 minutes to provide 40 gallons each week.
You want to deeply soak the area in the tree’s dripline (the area under the tree canopy), and to water at a slow enough rate the water soaks in and doesn’t run off. You can also water by hand, using buckets—5 gallon utility buckets are readily and cheaply available.
One easy method is to use several buckets with 1/4 inch holes drilled in the bottom—space them around the tree’s dripline, fill them with water, and let the water soak in slowly. A long soaker hose can also be spiraled around the dripline.
To keep your tree roots from freezing, water before a hard freeze (20 degrees F or below). Trees planted in the past three years, trees that have recently gone through stress, or trees that have limited root space (in planters next to sidewalks) will need regular watering even in non-drought conditions.
During drought conditions, trees and landscape plants often show the effects of the hot, dry weather. To keep your trees alive and thriving, it is important to adequately water small (1-7 inches in trunk diameter) and medium trees (8-15 inches in trunk diameter). Healthy larger trees should be able to withstand a drought, although even they may start showing signs of stress.