It seems lilacs, more than any other shrub (honorable mention: privet), tend to overgrow their space. Without regular pruning they get lanky, droopy, produce less flowers and generally start looking like a mess. Lilacs can grow pretty fast and before you know it you can have a small lilac forest. Fortunately they’re not too fussy about when they are pruned, but there are two times per year in particular that are best. Of course, some light shaping or removal of dead branches can be done at any time of year.
Prune Lilacs in Winter
Winter (December to early March) is a good time to prune your lilacs if they are seriously overgrown and need to just be whacked back. There is a technique called “rejuvenating” that can be used in winter to give your overgrown lilacs a fresh start. Basically, rejuvenating involves cutting the stems down to between a foot to three feet off the ground. I pick a height and cut them all at the same level. Any dead stems or stems over 2″ should be cut right down to the ground as well as stems that are too close to eachother or rubbing together.
You want to clean up the stems that are left and make sure there are no suckers or shoots, cut everything right back to the main stems. Also be sure to make a nice clean final cut.
In the spring the stems will push out a flush of new growth and by mid summer you’ll have nice, neat lilac bushes again. This technique should only be done when the shrubs are out of control- not every year otherwise you they may not have the energy stores to send out new growth. One thing to consider with winter rejuvenating is that you will not have flowers the following spring, because flowers develop on year-old wood and not on new growth.
Prune Lilacs Just After Flowering
The other time to prune lilacs is just after they have finished flowering in late spring (early June usually). Pruning at this time assures you will have flowers the next year. There are a few different ways you can prune at this time.
The first technique is to rejuvenate the whole plant, as described above. This is a risky thing to do though because it’s very stressful for the plant and there’s a risk it may not leaf out again. It’s stressful because the energy reserve in the roots is depleted from the spring growth and flowering, and the plant hasn’t had time to build up energy reserves again. Only do this if you can live with the possibility that your lilacs might die. In my experience lilacs in full sun have a better chance of coming back from this method.
The better way to go about pruning at this time is to rejuvenate just 1/3 of the plant. I prefer to cut out the largest stems and leave the smaller ones. Anything over about an inch and a half gets cut down to about a foot. Anything over 2″ to 2.5″ gets cut right down to the ground. This is a great thing to do every year- it will keep your lilacs looking their best and producing consistent flowers, and keep them to a manageable size.
The other way to prune your lilacs at this time is to prune as you would any other shrub or small tree. This technique is best for newly planted bushes or those that don’t need major work. You can thin and shape as you please- they won’t really mind.
When rejuvenation pruning lilacs, it is a good idea to supply them with some fertilizer, preferably the summer before you plan on rejuvenating them. This will help them bounce back quickly and ensure maximum flowering.
Although we don't do pruning services, we can certainly help with fertilizing your trees and shrubs!