Lilacs have long been a favorite in American landscapes, and with good reason. Lilacs are fairly disease and problem free and bloom heavily year after year if they continue to receive lots of direct sunlight.
Lilacs are usually successful because they tolerate most soil types oas long as there is good drainage and plenty of sunlight. Poor soil drainage, lack of sunlight and old age can all cause lilacs to struggle.
When the problem is old age, and some big stems have become unsightly or show some die-back, it is often possible to rehab the lilacs with pruning.
There are two basic ways to do rejuvenation pruning of lilacs and neither are particularly easy. Most pruning manuals recommend removing 20% to 30% of the lilac stems each year on an old lilac, beginning with the biggest and oldest stems. Continue for about 3 or 4 years each year removing until all the old stems are gone, and only young vigorous new shoots remains. This method requires a determine and consistent 4 year effort, and will give a good result without losing all the height of a lilac cluster.
The second way to rehab an old lilac cluster r hedge is brutal, but effective, and a lot quicker. Use a chain saw to cut the entire overgrown mess down to a ehgith fo 6 to 10 inches.
When you get done removing all the brush created by giving your old lilac a severe crew cut you will be looking at stubs/stumps. Do this kind of attack dog pruning only after the leaves have fallen off in late fall, and or definitely before the buds swell in early spring. By doing this when the lilacs are dormant, all the stored energy in the root system will push an explosion of new sprouts once the ground thaws.
The first year you should see sprouts of one to two feet. After three years, you ought to be enjoying a nice dense stand of all young and vigorous stems of four to six feet tall. All your work has been done over ne winter, with no need to go back and re-visit pruning year after year.
Chose the lilac rejuvenation method that best fits you. Gradual and incremental, or the shock and awe of cutting it all down at once. Both methods can give you good results!