Now that the final lilac flowers are done, it’s time to prune the bushes. This is the ideal time to deadhead lilacs- the buds for next year’s flowers are forming right now. If you do it later, you may end up cutting them off and having fewer flowers next year. It’s also the time to do harder pruning or renovation.
Deadheading is the simplest cutting back to do on a lilac. We do this because if we keep the bush from putting energy into forming seeds, it can put more energy into growing and into making more buds for next year. While you might not notice much difference in a mature lilac, it’s vital with a young one. To do this, cut directly below the spent flower. You will see new stems coming out at about 45 degree angles right below the flower; cut above these. Those new stems will carry next year’s flowers. This is the only pruning (other than removing dead or damaged wood) that a lilac will need for the first 5 years of its life.
For real pruning, start with taking out anything dead. Next, take out anything crossing and rubbing. When you do this, remember to think about the form of the lilac bush- which branch will result in the nicer looking shape?
When pruning, do not leave stubs; prune to right above a node/place where branches or leaves are coming out. This holds true for any plant, not just lilacs!
Left on its own, most lilacs will grow to 12’ to even 20’ tall. If that’s what you want, that’s fine, but most people prefer a shorter shrub. A lilac that’s grown too tall shades it’s lower branches and no leaves or flowers grow out of them, which means bare gray stems for the first few feet. The reflex action is to just prune back the ends of the branches to the desired height, but this doesn’t work. If you prune 2’ off the ends of the branches, that 2’ (or 3’!) will grow back the next year. You need to go further down- right down the bottom.
What you want to do is get your lilac onto a three year plan. Every year, you’ll remove about 1/3 of the older, larger stems right to the ground. Basically, any branch more than 2” in diameter should come out. These are the oldest, tallest, and least productive of flowers. Next year, take out another 1/3, always removing the oldest wood. And so on. After three years, you’ll have an entirely new shrub, and if you keep on this schedule you’ll always have a youthful, leafy, heavy flowering lilac. And the branches will never get old enough to be taller than your house.
Tempted to remove more than 1/3 to get it over with and have a shorter shrub? You can- but it’s a terrible shock to the plant and you’ll have no bloom for at least 3 years. This action shouldn’t be taken more than once every 15 years on any shrub- it just takes too much energy for the shrub to recover.
Suckers are a problem with most lilacs. These days, most lilacs are not grafted, so suckers will be the same variety as the main bush. You can cut these off below soil level, cut them off and root them in another spot for more bushes, or let a few come up and become a sort of lilac thicket or hedge. Don’t let them get so dense that they keep light from getting to the center of the bush, or so dense that you can’t get in to cut out the older stems.
You can also feed your lilacs now. They want a fertilizer that is around 5-10-5 to promote bloom. They also like an alkaline soil, so if you have acid soil putting some lime around the shrubs will give you more bloom.