Pacific Northwest Gardening: Laurie's Garden Tour 2, 0 by HoosierGreen

HoosierGreen wrote:
If I could add my two cents worth about pruning lilacs: There are two general ways of maintaining French lilacs. The first is to remove about a third of the largest stems (trunks) every few years, leaving the newer growths to mature. That way, you always have older growth that is of blooming age and fresh growth coming on. This method will result in a full, rounded shrub appearance. (Be sure to first establish the overall arrangement of branches to allow for good air circulation, lessening the danger of mildew)

The second method is to leave three to five main trunks, and remove all smaller branches and suckers as they appear each year. The trunks can grow to quite thick diameters and the resultant appearance is more of a tree than a shrub. If anyone has ever been to Mackinac Island, Michigan, they have HUGE, gnarled, hundred-year-old lilacs that are just magnificent.

I personally favor the second method, which is less work after the initial pruning. However, if you are in an area that has borer problems, the first method will be more successful and healthy for the lilac.