Knowing the drip line of a tree is important for three tasks in tree maintenance for every homeowner: mulching, fertilizing and watering. The approximate root zone where mulch is laid, fertilizer is applied and water is sprinkled is determined by the drip line of the widest part of the tree canopy. The drip line is defined as the point on the ground where water will drip off the widest-reaching branch. All you need to find it is a measuring tape and a few stakes.
Stand back from the tree and study its shape. Some are conical, some are columnar and some are pyramidal. Look for the widest part of the canopy and the widest-reaching branches on both sides.
Mentally draw a line from the tip of the widest branch on the left, straight down to the ground. Walk to that point and drive a stake into the spot. Do the same thing on the other side of the tree.
Place one end of a retractable measuring tape at the trunk of the tree. Feed out the tape to the stake. Do the same on the other side of the tree. Most trees are symmetrical in shape, so the numbers will be close. Take the larger number as the actual drip line of your tree in feet.
Use this measurement for spreading mulch, applying fertilizer and watering because it is the approximate size of the root zone of your tree.