Question
I have had a Japanese Lilac for about 15 years, grown from a sapling. Over the past year or two, it has grown "leggy", with foliage mostly at the top and gaps in the middle. Can you help determine what the probelm is and how I might restore it to a bushier appearance? Thanks.

Answer
Hi Dave

To address your problem I would recommend pruning your lilacs to achieve the look you抮e going for.

Pruning Japanese lilac (Syringa reticulata) or Japanese tree lilac, as it抯 sometimes called, is relatively straight forward. The lilac genus (Syringa) responds well to pruning whether it抯 drastic or gradual. Whether you抮e dealing with Common lilac (Syringa vulgaris) or Japanese lilac (Syringa reticulata) the methods used to prune them is relatively the same. The two options you have is a staggered 3 year pruning plan or the 搊ne-chop?rejuvenation method. In my experience I抳e personally implemented both and had equal success.  Both methods have their advantages and disadvantages, some of which I抣l discuss.

The three year method is designed to spread out the pruning over multiple years in order to slowly ease the plant into the new shape you抮e looking for without sacrificing. The three year method is as follows:

Year One:
If it has multiple stems remove about one-third of the largest branches, cutting back to ground level. If grown as a small tree, remove about one-third of the biggest horizontal branches, evenly removing branches throughout the canopy.

Year Two:
Plan on pruning out another third of the oldest branches in the same way you did in year one. Head back any new shoots that may appear, making them various lengths and removing 30-50% of the shoot.

Year Three:
Remove all the remaining older branches in the third year, using the same methods you used for the last two years. Cut back any of the new shoots again to about 30-50% of its height.
The only drawback to this method is it takes time (3 years) and thoughtful pruning to achieve the desired shape.

揙ne-Chop?Rejuvenation Method

In this method you simply cut the entire plant back to ground level in the early spring. As a result the plant will respond by sending up lots of new shoots. These new shoots will have to be thinned out in subsequent years to ensure you get the look you抮e going for. The drawback to this method is that you will lose at least one year of flowering.

Here抯 a couple links for some more information on pruning methods and Japanese Lilac:
http://cmg.colostate.edu/gardennotes/619.pdf
http://hort.ufl.edu/database/documents/pdf/tree_fact_sheets/syrreta.pdf

Hope you found this information helpful.
James