Mature trees have expansive root systems that extend from 2 to 3 times the size of the leaf canopy. A major portion of actively growing roots is located outside the tree’s drip line. It is important to understand this fact when applying fertilizer to trees as well as turf. Many “off the shelf” lawn fertilizers contain weed and feed formulations that may be harmful to trees. For instance, when applying a broadleaf herbicide to turf, it is important to remember that tree roots coexist with turf roots. The same herbicide that kills broadleaf weeds in your lawn is picked up by tree roots and can harm or kill your broadleaf trees if applied incorrectly. Understanding the actual size and extent of a tree’s root system before fertilizing is necessary to determine how much, what type, and where to best apply the fertilizer.