FAQ - Answer
Generally, in Orange County, there's a problem with powdery mildew on coast live oak. Consequently, it's recommended to prune in the summer, after new foliage has matured. Therefore, you can still prune your tree, but it is recommended that you identify specific reasons and/or goals for pruning and then select the appropriate pruning practices that will accomplish your goals. Tree maintenance guidelines, developed with the help of UC input, require that all pruning follow current ANSI and ISA guidelines. For oaks, avoid pruning unless necessary for safety, clearance, or the health of the tree. In Orange County, powdery mildew is generally not a problem, but summertime cankers are not uncommon. If you have concerns about canker-causing pathogens, then summer pruning would be best. If this is not a problem, then winter or summer will work. If you haven't pruned this year, then I'd wait until summer and just do safety, cleaning, and clearance pruning. All “trimming” should be light selective cuts. The “best” time is when the trees are still dormant (at least in Northern California). In Orange County, you may be seeing bud break already, if that’s the case I would trim very lightly for now and wait until next winter for more extensive, but still selective trimming. If you want more info on this subject, see Chapter 5 of "Oaks in the Urban Landscape,” UC ANR Publication 3518.