When and how to prune lilacs is relatively easy.
If the shrub is looking good and you are happy with its size, then pruning is as easy as cutting a few nice bouquets. Yup, bringing those beautiful blossoms indoors or gifting to coworkers is good for the lilac.
Lilacs are a super easy shrub to care for. They don’t need fancy fertilizers or extra care. In fact, if you don’t prune the worst that can happen is that after seven or eight years it might get leggy and have fewer blossoms. But even that is an “if.” I’m sure you’ve seen gigantic, gorgeous lilac bushes ignored but flourishing at the edge of a farm field.
If your shrub isn’t producing as many blossoms and is more than six feet tall, then you may want a more thoughtful approach to pruning than just picking bouquets. Schedule to prune early in the spring before it leafs out. Remove about one-third of the oldest branches by lopping them off at the base. The oldest branches will probably be more than two inches in diameter. If you remove a third three years in a row you will totally rejuvenate the entire plant. Of course you lose the huge profusion of flowers for a few years but in the long run you will have a healthier shrub and even more blossoms.
Every year I remove the suckers, these are the little “babies” that are sprouting around the base. I also remove any twiggy and messy looking growth and any diseased or sickly looking branches. This effort is more about helping the plant remain vigorous than true pruning.
One year my mother took a chainsaw to one of her overgrown, leggy, aged lilac. I was crushed thinking she was getting rid of it. Nope, not at all… she was just using a more drastic pruning method. If you don’t have the patience for the one-third over three years plan. It is just fine to cut the entire thing down to about six or eight inches.
My one word of caution is that some lilacs are grafted. The base of the shrub is a hardy but less attractive lilac and a beautiful and maybe more exotic lilac has been grafted to it. Run your hand down the branch if you can feel a raised circle of bark a few inches above the base on all your branches, then there is a good chance this makes pruning to the ground a problem. Honestly I personally have not had good luck pruning above the graft and getting the youth, shape and vigor I want for my lilacs. So I recommend not buying grafted shrubs and if you have inherited one with your new house proceed with care.
I’ve read conflicting articles and advice on deadheading. For me it’s just impossible. There are too many blossoms on too many shrubs and many of the blossoms are out of my reach. However, if you feel compelled to deadhead, then it must be done immediately after the flowers are done and cut right next to the blossom.
More on pruning lilacs from some experts