Keep Grass Away From Tree Trunks
There are two reasons why you should not let grass grow right next to tree trunks. First, keeping a mulched circle free of grass and weeds will significantly increase the growth rate of young trees. Second, mowers and trimmers can do considerable damage to tree bark, which can stunt or even kill trees.
Grass growing right over tree roots is very competitive for water and nutrients. Grass gets most of the fertilizer before it reaches the tree roots. Grass also produces chemicals which inhibit the growth of other plants growing in their root zone. The net effect is a significant reduction in tree growth rate.
There are several ways to prevent weeds from growing in this area. A 2 inch layer of bark dust will reduce weed sprouting by about 90%. Any weeds which sprout can be sprayed with a weed killer such as Roundup. Roundup will not damage tree bark but should be kept off leaves. Organic weed killers are also available.
A weed preventer such as Casoron will prevent new weed growth. Organic weed preventers such as Corn Gluten are not quite as effective as Roundup.
Weed barrier fabric mulch is also effective in preventing weed growth. It is not only a physical barrier but excludes light which is necessary for weed growth. It is porous so air, water and nutrients can flow through it. Do not use poly plastic around trees and shrubs.
The inner bark contains the tubes which transport food made by the leaves to the roots. If some of these tubes are damaged, root growth is reduced and top growth becomes stunted. If the bark is cut through all the way around the tree it will die. Mature trees may be able to withstand some trimmer damage because their bark is thicker.