Keep your trees healthy and reduce your workload by creating a mulch bed around your trees.
Consider forgoing chemicals and use a more eco-friendly method to kill existing grass. First edge the bed to disconnect the grass and weeds from surrounding plants that can serve as their life support.
Now cut the grass as short as possible to speed up its decline. Cover the area to be mulched with newspaper or cardboard. This creates a temporary but extra layer of mulch to help kill the grass. The paper will eventually break down and improving the soil below.
Cover the cardboard with shredded bark or woodchips. A 2 to 3 inch layer is all you need. Keep the mulch away from the trunk of the tree to avoid insect, disease and rodent problems.
The wider the bed - the better for the tree. Plus a bigger bed means less mowing and trimming for you.
A bit more information: Consider planting shade and drought tolerant groundcovers under trees if a large mulch bed does not fit your landscape design. These plants are better suited to the environment and less competitive than grass is to young trees. Deadnettle (Lamium), ginger (Asarum), coral bells, and foam flower (Tiarella) are just a few to consider. Do not add soil over the roots of the trees. Plant the groundcovers in the existing soil and mulch to suppress weeds and conserve moisture.