How and when do I prune lilacs?
Unless the shrub’s in need of rejuvenation, cutting off bouquets of flowers is all the pruning you’ll want to do. I never prune them here in my Zone 5B climate after July 4, and like to finish up earlier.
Most lilacs don’t need much pruning beyond the “musts”, the 3-D’s (top of page). But bydoing a little “pruning” (read: cutting bouquets of flowers to enjoy) you do the plant a favor, and prevent the ugly aftermath of lilac-blooming season, those dried-up trusses that persist forever, or so it seems.
Always cut out dead, damaged or diseased wood as it occurs on any shrub or tree, and likewise with suckers that sprout from the base (and may in fact be growing from the rootstock if it’s a grafted plant).
Sometimes an older lilac needs reshaping. Conventional gardening wisdom says any shrubs can be “rejuvenated” over three years by cutting one-third of its oldest stems to the base each year, but I ask this: Look at your lilac (or any other shrubs) carefully. Sometimes you don’t want to reinvent (a.k.a. “rejuvenate”) the thing but just to tweak it, so look and think, and look some more before the saw comes out. I like the way Jeff Jabco of the Scott Arboretum of Swarthmore College explains the various stages of the pruning process—whether the yearly flower-harvesting kind or the more ambitious undertakings.