Ask Mr. Smarty Plants

You'd better first check and see who that strip of greenbelt belongs to. If it is city or utility easement, they would be the ones responsible for trimming the tree. You could probably get permission to trim it on your own, but it would be safer to check.

The other thing that we're concerned about is the possibility of introducing disease into the tree if you trim it now. You didn't say what species of oak you have, but in Texas we are constantly on guard against the spread of oak wilt, a devastating disease of certain oaks spread by a small beetle which is active in warm weather. This USDA Forest Service Pest Alert discusses a recently discovered pathogen for oaks found, at the present, only in California and Oregon. The point being, many of these oak problems can be spread if there is an open wound on an oak that attracts insects that are the carriers of these diseases. The best time to do anything about trimming an oak, or just about any other woody plant, is in winter, when cool temperatures render most insects inactive and the plant is more dormant.

Because this is a more localized problem, we would suggest you go to the home page of the University of California Cooperative Extension office for El Dorado County. We noticed that on that home page there is a link to information on a Master Gardeners class on summer pruning, scheduled for July 29. It would seem that would be the best place to get good information on how, when and whether to prune that tree now. If you can't attend the class, there are contact links on the home page to reach personnel in the office who should be able to help you.