Good mulches for the base of a tree include wood chips, pine needles, and shredded bark. You can use grass clippings, too, but be aware of a few things. If you put down fresh grass clippings in a layer more than a couple of inches deep, the clippings can form a mat that discourages air from penetrating the soil. This mat will eventually rot and smell bad. Make certain that the grass from which the clippings were cut has not been treated with any herbicides that might be harmful to trees. Grass clippings and other organic mulches won't directly harm trees, but they can harbor rodents in winter that feed on the bark. To reduce the chances of rodent feasting, as winter approaches pull mulch away from the trunk. Wrap young trees with hardware cloth or tree wrap.