The lilacs have been beautiful this year and the flower show continues with some of the later blooming types such as 'Miss Kim' Korean lilac. Now is the time to do a little lilac care.
One question I often get is when and how to prune lilacs. When is easy. You have about a 6 week window to prune your lilacs after flowering before they set flower buds for next spring. So, by early July you should be finished pruning. If you're just wanting to maintain a nice shape and size, prune back the old flowers to side branches. Do this annually and you'll keep your lilac full and a manageable size.
If your lilac has gotten out of control, cut it back severely to rejuvenate it. There are two ways to do this. Prune the whole bush down to about 2 feet tall. You'll see suckers start growing for the trunk and ground. Leave about 4 or so suckers to grow into your smaller shrub. Your lilacs may not flower for a few years. Another method is more gradual. Each year cut 1/3rd of the oldest branches to 2 feet tall. After three years you'll have a shorter lilac with new shoots and the oldest shoots should be flowering by then.
If your lilac didn't flower make sure it gets at least 6 hours of sun. If it doesn’t, you may have to move it to a sunnier location or limb up nearby trees that are shading the bush. The rejuvenation pruning also will stimulate better flowering. Add compost in spring and mulch around the base to reduce weed competition.