How to Prune Lilacs
Lilacs grow so vigorously in the Pacific Northwest that they eventually become trees instead of shrubs. June is the time to prune lilacs. If you would like to turn your lilac back into a shrub, this is how to do it. There are 3 ways to accomplish it.
The drastic one year approach
Prune all major branches down to within one foot of the ground. Small branches coming directly out of the ground which are less than an inch in diameter can remain. Leave all sprouts which are in the immediate vicinity of the shrub base. Sprouts outside this area can be removed. Any remaining branches more than 2 feet tall should be pruned by half. One month after the original pruning shorten new growth by about half. This will cause branching so that you get a full shrub at the base. After 2 months, any growth which reaches more than 4 feet in height should be shortened again to approximately 4 feet. Do not shorten all branches to the exact same height. Some variation looks more natural. You will not get any flowers the year following this drastic pruning.
The 3 year approach
Prune 1/3 of major branches to within a foot of the ground. If you have 5 or 6 major branches, prune 2 on different sides. In June of 2014 you will again prune 1/3 of major branches. The last remaining major branches will be pruned in June 2015. Leave all sprouts which are in the immediate vicinity of the shrub base. Sprouts outside this area can be removed. Shorten all major branches which are not removed to a height of 4 to 5 feet. All rapidly growing branches which reach more than 6 feet should be shortened to 6 feet during the summer. You will have fewer flowers next spring, but they will be at a height where you can enjoy them more.
The long term approach
Prune a single major branch to within one foot of the ground. Do this every year in June until all major branches have been removed. Then follow the instructions for the 3 year approach. You will have a more normal amount of flowers next spring but they will be better placed.
Prune lilacs every year in June to keep them as compact shrubs
Yearly pruning is necessary to keep lilacs at a height of about 6 feet. Shorten all branches which reach a height of more than 6 feet. Shorten side branches in the upper part of the shrub so the plant is tapered in toward the top.