Watering Trees

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

Proper watering is always important, but it becomes crucial when the temperature hits triple digits and stays there.  If plants are only provided with shallow waterings every day, they’re probably only getting a fraction of what they need.  Let’s look at the water needs of trees.  Large amounts of water evaporate through tree leaves.  This “pulls” water through the plant.  Water is the “vehicle” that nutrients use to enter the plant.  These nutrients are needed for plant functions and growth.  Water is also needed for vital physiological plant processes.  A lack of water means stress and disruption of plant functions.

While humans are supposed to drink at least eighty ounces of water a day and more in hot weather, trees need gallons of water.  A mature silver maple can lose up to 58 gallons of water per hour on a hot summer day!  This translates to over1000 gallons in a twenty four hour period.  Wow! Watering your tree for 10, 15 or 20 minutes a day just isn’t going to give that big old maple the water that it needs.  It’s like getting only a little glass of water after standing outside all day.

To water trees adequately, water should be applied over a longer period of time that will thoroughly moisten the soil to a depth of 18 inches or more.  Water should be applied slowly enough so that it soaks into the soil without running off down the driveway, sidewalk or street.  An excellent way to apply water slowly is with a soaker hose.  Soaker hoses are made of porous canvas, plastic, or rubber.  They allow water to seep out slowly and are useful in watering trees, flowers, vegetables, and shrub beds.

If you have a solid set irrigation system and numerous trees, soaker hoses might not be practical for you.  You’ll need to work with your system’s timer so that you can apply water for a longer period of time in the same location.  You may want to “pulse” the water, applying it several times with “resting” periods between applications to allow the water to soak in.  Check the soil several hours after you water to make sure that it’s moist to a depth of 18 to 24 inches.

It’s important to point out that the tree roots that absorb water are not right next to the trunk of established trees.  Water applied in the tree trunk area is wasted water.  The fine feeder roots that absorb most of the water for the tree are located at and beyond the “dripline.” The “dripline” is the outer edge of the branch spread.   Newly planted trees are an exception to this rule.  The water absorbing roots of recent transplants will generally be in the area of the root ball.  Care should be taken to keep that root ball moist and water may have to be applied close to the trunk to accomplish this task.