Several different lilac species are dubbed "dwarf," including widely-used Syringa meyeri and S. chinensis hybrids. At 8-15' they are small compared to the common lilacs (15-20'; perfect to sniff from a second floor window), but not tiny enough to be cast alongside Snow White. S. meyeri grows to about 8', S. chinensis to nearly 15'.
So if you want a dwarf lilac to remain less than 8' or a common lilac to be less than 20', plan to prune it regularly.
This article is for you if you have an overgrown lilac of any kind:
• First, to bring it down to manageable size,
• Then to keep it smaller than it would if left alone.
A dwarf lilac (Syringa meyeri) is at the right side of this foundation bed (below). It's been pruned regularly so it's not as tall as it could be. However, it has bulked up and also increased its height beyond what's good for this bed. In addition, a survey of the branch tips shows it has not produced many flowering stems. That's probably because the former gardeners kept up with the shrub by shearing it repeatedly through the summer. By late summer, those shearing cuts were removing flower buds.
We'll show you how to prune it just once a year to keep it small and blooming well.
Cut out all the oldest stems after the shrub blooms. That reduces the height and the bulk.
New, shorter canes (suckers) grow from the cut-back stubs.
Meanwhile, the remaining original canes produce flower buds for the next year.
At the beginning of year two, we had everything we'd aimed for:
The remaining, too-tall canes were set to bloom, plus
the rest formed a neat mound of new growth (below, orange dashed line) which will be old enough to bloom next year.
The shrub's owners cut plenty of lilac flowers. After the bloom was done, they cut out all of the too-tall stems that remained.
Then they thinned the thicket of short new stuff coming up from the base (below).
(More detail on these a-b-c steps in A Lilac's Annual Cut.)
Every existing cane now adds about 6 inches in height. At the tips will be flower buds for Year Three.
Meanwhile, more brand new suckers develop. In this, their first summer, they grow to about 30" tall.
To keep a dwarf lilac small, start out by accepting that it has the potential to be as big as those at the very top of this page and that you must prune it regularly. Then, make these cuts once a year -- it takes about 15 minutes:
Spring: Each spring the shrub will begin growth with two-year-old canes ready to bloom, plus one-year-old canes ready to grow a bit taller and set the next year's flower buds. Enjoy the flowers.
Late spring: Cut the tallest canes to the ground. Thin out excess suckers. Shorten any remaining cane to allow it room to grow un-cut the rest of the summer.
The choice is always yours. Shear a shrub, do more selective pruning or simply let it grow -- all of these are "right" if the effort is tolerable and the result is pleasing to you.