Each type of plant will require specific trimming. Typically most zone 7 shrubs will require twice a year pruning once after Spring growth and once after Fall growth. Certain flowers will require pruning after the first fall while other plants will require pruning near the end of winter.
One of the biggest oops is when customers call us to prune Crape Myrtle trees. Many times they will call for these to be pruned well after Spring time has begun or even later. Crape Myrtles should be prune as close to the last front of winter so they don't miss the time they need to grow and flower.
Shrubs can some time be kept up with by home owners then a few misses can cause a huge problem. When a few years go by and shrubs have not been trimmed like they have been they may become to large to deal with. This might be that home owner don't have the type of equipment needed or they cant get them pruned down low enough with out looking dead. Some times we may have to remove them or take a season of hard pruning to get them back to a desired size.