Would the roots of a 25-foot plum tree go to a depth of 5 feet?
A plum tree does not have a notorious root system, one that heaves up sidewalks, for instance. The vast majority of roots of any tree species are found in the top 12 to 18 inches of soil. These feeder roots extend horizontally to the edge of the leaf canopy, or drip line, and beyond up to 1 1/2 times the height of the tree. A few anchoring "sinker" roots grow down several feet to stabilize the tree. It's likely that your plum tree has some roots that extend 5 feet deep.
Ellen Nibali, horticulture consultant, works at Maryland Cooperative Extension's Home and Garden Information Center, and David Clement is the center's director. The center offers Maryland residents free gardening information. Call 800-342-2507 or e-mail plant and pest questions at hgic.umd.edu.