We love answering this question because it is so easy: mulch under them!  Applying an organic mulch layer really is the single most effective preventative care that you can provide for your trees.  It is more effective than trimming, fertilizing or spraying.  It separates lawn from trees preventing damage from lawn mowers and weed whips; it moderates soil temperatures; it decreases soil compaction; it increases water holding capacity; and it increases roots, and and that’s just part of the story!  Best of all, it is one of the least expensive treatments you can do. (ref: Mulching Abstract)