Question: I have a large lilac that blooms erratically. Would pruning help? When should lilacs be pruned?
Published in the Contra Costa Times on June 7, 2008
Question: I have a large lilac that blooms erratically. Would pruning help? When should lilacs be pruned?
Answer: Lilacs (Syringa vulgaris) should be pruned immediately following bloom, so this is the correct time of year.
Lilacs are large, deciduous suckering shrubs, native to southern Europe, which can reach to 20 feet in height if not pruned.
As the lower foliage becomes shaded out, few flowers are produced, so maintaining shrubs at a 8-10 foot height allows for light penetration, easier maintenance and more blooms.
If your lilac is large and rangy, you may need to take several years to get it into the shape you desire. Lilac shoots take about three years to produce flower buds, so you will want to be careful not to prune out more than a third of the growth if you want flowers.
This is called renovation pruning. Choose the oldest, woodiest stalks and cut a third of them down to the ground. Next year repeat the process, removing another third of the growth. New shoots will emerge, and within three years you will have a new shrub.
If the plant is really overgrown, you can cut all the old growth down to the ground, leaving only new growth. Use sharpened loppers for clean cuts. You won't have blooms for a few years, but it is a bit simpler. Head back new shoots to the height you want, cutting just above a bud.
If you do serious renovation pruning, be sure to add finished compost around the roots to provide nourishment for the new growth, and soak the roots once a month in the dry months.
Lilacs tolerate our high pH soils, so no serious amending is needed. Lilacs also tend to be alternate bloomers, flowering heavily one year, then resting the next. If you remove the flower heads immediately following bloom, this tendency is reduced.
This is a full-sun shrub, and if it has become shaded by nearby large trees, bloom will be sparse, regardless of your pruning technique.
The lack of blooms also may be related to the variety. Lilacs are hardy shrubs, but many varieties require substantial winter chill to bloom, and our relatively warm winters may not provide enough cold. The plants survive, but flowers fail to form.
"Lavender Lady" is a warm-tolerant hybrid that has been around for a while; however, new varieties of low-chill lilacs are becoming available that can set flower buds even in warm Southern California. Check with your nursery for recommended varieties for your area.
Bethallyn Black is director of the Contra Costa Master Gardeners, a program associated with the University of California Cooperative Extension and supported by Contra Costa County. Reach her at bblack@ucdavis.edu, or 925-646-6586. General questions for the Master Gardener should be sent using our Question Form