If you can, it is best to eliminate as much grass as you can and replace it with mulch. By mulching out to the very tips of the branches, you are recreating the trees natural condition in a forest. If you cannot get rid of all the grass growing under your tree, consider at least mulching a few feet from the trunk. It looks great and keeps the lawn mower and weed eater away from the tree. The best mulches for trees are organic mulches like shredded bark, compost or wood chips.
Wood chips are available from local nurseries.
You can also check with your local parks department, landfill, tree care companies and electric utilities to see if they have any wood chips available. These organic mulches, applied three to four inches deep, will preserve soil moisture, improve the soils air circulation, keep soil temperatures more moderate and reduce weed problems. Organic mulches will also provide your tree with nutrients as it decomposes. Fertilize Give your tree a little help this winter by applying a slow release form of nitrogen at the rate of about one pound of nitrogen per 1000 feet of open area under the tree. But, the best time to avoid drought stress is when you pick the type of tree to plant, Giedraitis said.