Weed and feed sounds like a dream product: kill weeds and fertilize you lawn with one quick application. However, even though the product is intended to kill only weeds, the herbicides used in the weed and feed can also harm other plants, including nearby trees. The spacing necessary between trees and weed and feed products varies by tree. Careful and precise application of weed and feed is the only way to guarantee healthy and safe trees in your landscape.
Danger Area -- The Root Zone
As a general rule, a grower or landscaper should never apply any type of chemical herbicide, including weed killer, within the root zone of a tree. The root zone is the area in which the roots spread, and from which the tree sucks its nutrients and water. Chemicals in that area will be absorbed with the water and nutrients, and the results of such exposure would be the equivalent of you mixing rat poison into your morning cup of coffee. Though it is not intended to kill humans, it can still do serious damage, just like weed killers can do your tree.
Root Zone Sizes
The size of a tree's root zone varies by the tree. Younger trees have thinner and shorter roots, but as the tree grows, roots can extend outwards from the tree for up to 30 or 40 feet in all directions. This is especially tree of thick, long-rooted trees such as maples, willows and poplars. Tree roots generally only sit 8 inches or less below the surface, so if you apply and chemical to the surface grass, it will inevitably sink down with water to the roots of the tree.
When in doubt, don't apply weed and feed fertilizer anywhere under the branches of your tree. Some root systems can reach as far as the largest branches, so this is a safe judgment for spacing. However, to be more accurate about your specific tree, consult a local extension service, professional landscaper or garden center for details regarding your species of tree. Such professionals will be able to estimate the size of the root zone based on the species and the age of the tree.
Never use any weed and feed product that advertises that it kills broadleaf weeds. Most deciduous trees, including maple, birch and hickory, share a number of characteristics with broadleaf plants and will easily be damaged by these chemicals. In general, most pre-emergent herbicides, which are designed to prevent weeds from sprouting, are safe to use near trees since the dose of chemical is not as strong.