The specimen Japanese Maple at my Rockville home has lacked vigor coming out of the last two winters. It does not recover fully during the season and some smaller branches are completely defoliated. Can I trim it, or start over? Should I fertilize? Thanks for your consideration of this matter.
Japanese maples are subject to many environmental stressors such as poor drainage, too much moisture, drought, temperature extremes, too much mulch, planting too deeply, soil compaction, etc. Also, with Japanese maples, the bark can freeze and kill the cambium which can cause dieback. Any combination of these factors can result in dieback like you see in your tree.
At this point, you can prune out the dead branches. Make sure your mulch is no deeper than 3" and keep mulch away from the base of the trunk. Keep your tree well watered during dry periods. There is not mulch else you can do.
Here is an article about Japanese maples written by our plant pathologist. It addresses some of the common problems that occur in these trees. https://marylandgrows.umd.edu/2018/08/24/japanese-maples-in-maryland-landscapes-plant-location-care-...