Planting a tree is an investment. It can take years for a tree to reach maturity and exhibit an impressive crown of growth. When you plant a sapling, give it the healthiest start that you can. One thing that can stunt a tree's growth is the presence of grass at the base of a tree. Aggressive grasses such as Bermuda grass gain a foothold before a sapling can produce enough shade to prevent their growth. These grasses can steal moisture and nutrients from the tree's roots. When you see aggressive grasses growing around a tree's roots, your best bet is to kill them before they can harm your tree.
Treat grass around the base of your tree with a grass-specific herbicide that contains the active ingredient fusillade. Grass-specific herbicides such as this will kill grass beneath your sapling without harming the tree.
Place a barrier of mulch around the base of the tree and in a ring spreading out past the dripline. Mulch will help hold moisture into the roots of the tree while choking out grass seed. Do not mound mulch up around the trunk of the tree. Doing so can cause the spread of fungus around the base of the sapling.
Cover the grass and ground around the tree with a light barrier such as black plastic or newspaper. Light blocking barriers will kill grass by smothering it. To use newspaper to kill grass, first mow around the tree to cut the grass short and allow newspaper to lay closer to the grass. Then lay down a layer of wet newspaper. Cover with landscaping fabric or mulch.
Kill grass by covering it with clear plastic. Temperatures beneath clear plastic will heat up and cook grass beneath it. This will not only kill the grass, but also grass seed.