Designing drip irrigation for trees differs from designing a system for plants or grass. A tree's root system extends outward anywhere from 1 ½ to 4 times wider than the tree's canopy, depending on the tree species. The optimal design for tree drip irrigation is to wet at least 50 to 60 percent of the root zone around the tree. Instead of placing only one or two drip emitters near the tree as you would for other plants, you'll need to add enough emitters to cover more than half of the root area, using the tree's canopy as a guide.
Design the Drip Irrigation Layout
Lay a garden hose on the ground around the tree so that it forms a circle the same diameter as the tree's canopy. The hose should extend the same distance out from the trunk as the farthest-reaching branches on the tree.
Insert landscaping flags or small wooden stakes into the ground just inside the hose circle. These flags or stakes signify where the drip emitters are to placed. Space the flags or stakes 18 to 24 inches apart all the way around the hose circle.
Make another circle of flags or stakes, spaced the same way, 18 to 24 inches from the outer circle of flags or stakes. Continue moving inward until the innermost circle is less than 6 inches from the tree trunk.
Determine the Rate of Flow & Tree Water Usage
Calculate the tree's water use in number of gallons of water used daily to determine the proper flow rate and number of watering sessions per day for your tree. First, determine the plant area of the tree by measuring the diameter of the hose circle you laid around the tree.
Determine the "plant factor" for your tree, which is an adjustment that reflects the rate of water use for specific types of plants. If you have a mature shade tree, your plant factor is 0.80. Your plant factor will be 0.85 for apple, cherry or walnut trees, while pecan, peach, plum, pear, almond and apricot trees have a plant factor of 0.75.
Get the potential evapotranspiration (PET) figure from your local agricultural extension office. This is the number of inches of water that can potentially evaporate from the soil each day.
Multiply these factors together and by a 0.623 conversion factor to get the daily water usage for your tree, and then divide this figure by the drip irrigation system's efficiency, which is usually 85 percent (0.85). Use this formula: 0.623 conversion factor x canopy area diameter x canopy area diameter x plant factor x PET / drip irrigation system efficiency = daily water usage in gallons.
For example, if your hose circle's diameter is 5 feet, your tree is an apple tree and your local PET is 0.25 inches per day, your calculation would look like this: 0.623 x 5 feet x 5 feet x 0.85 plant factor x 0.25 PET / 0.85 system efficiency = 15.6 gallons per day water usage.