EXCEPT IN THE warmest zones, where they are basically disinclined to bloom, every garden should include some common lilacs (Syringa vulgaris) or their various beautiful cousins, old-fashioned shrubs that couldn’t be easier to grow. Given full sun, good air circulation (to lessen the incidence of powdery mildew) and proper pruning, they produce profuse numbers of fragrant flowers. If lilacs fail to bloom, it usually means they have been deprived of sufficient sunshine—or pruned at the wrong time.
Like other spring-blooming woody plants, the lilac produces its flower buds from late summer through fall for the following year’s display. Prune after, say, July 4th in the north and you risk reducing next year’s bloom. Prune in fall or early spring, and you guarantee that disappointment.
A lilac is happiest if you cut bouquets from it each spring—essentially you are just deadheading it. Though not essential to its health or survival, the lilac isn’t really asking much from you but paying heftily, since the trusses make extravagant indoor arrangements. You work a little, you win. (A tip: before bringing the cuts inside, hammer the bottom few inches of the stem ends on a hard surface like stone to crush them, so they can drink up the water in a vase, or the flowers will wilt almost at once.) Bonus: By harvesting flowers you avoid the unsightly issue of all those large, dried-up flowerheads that hang on tenaciously all year.
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 a 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.