It's not uncommon for a residential tree to suffer a pruning mishap, either from unintentional neglect or due to a DIY error in judgment. Fortunately, trees can be quite resilient and they will likely recover — especially if you bring in a professional tree service to help with the remedy.
1. Missed Trims
Missed trims are most detrimental to young trees, as they depend on training to develop a strong mature form. For mature trees, a missed trim just may result in excess dead or weak wood in the crown. It may be necessary to thin out excess branches quite heavily in order to regain a good form. As too much pruning at once can weaken a tree, your tree service will usually opt to do this heavy pruning gradually over a period of a few years.
2. Wrong Timing
Pruning a tree at the wrong time is rarely fatal, but it can lead to issues such as poor budding and flowering, increased sap production, and pest issues. Late winter is the ideal time for pruning many trees, although some late-flowering trees are better served by a trim right after the blossoms drop. Most trees will recover within a year if they are pruned in the wrong season, although extra wound care may be needed to reduce sap production and prevent pathogens and pests from entering the pruning wound.
3. Topped Crowns
Topping is when the crown of the tree is cut flat on top. This is often done in a misguided attempt to control the height of a large tree, such as when it is growing near power lines. Topping will require several years to fully remedy, as the recovery trimming must be done slowly to avoid shock to the tree. First, your tree service will choose a new branch in the crown to train as the upright leader. Then the remaining branches will be trimmed lightly to bring back the rounded natural tree shape.
4. Over Pruning
Cutting out too much wood can stress a tree by removing too many leaves, as well as lead to unattractive bare patches in the crown. Unfortunately, over-pruning can't be easily repaired -- time is simply needed to allow for possible regrowth and recovery. Your tree service will recommend a strict water and fertilizer schedule to help encourage new growth. Going forward, careful light pruning must be practiced to avoid further damage.
Contact a professional tree trimming service if your trees need help recovering from a pruning mistake.