Fragrant, spring-blooming lilacs are a sentimental garden favorite. According to Ross Penhaligon, horticulturist with the Oregon State University Extension Service, lilacs bloom best on young, vigorous growth rather than on the older stems. A little pruning each year will keep lilac shrubs blooming beautifully. Even an old, overgrown lilac can be rejuvenated.
Cut any diseased or misshapen branches to the ground; thin out stems to open up the plant and encourage vigorous growth. Any stems pruned away should be removed at soil level or slightly below. Remove a few of the oldest branches on an established lilac every few years.
Remove some of the new shoots that appear every year from the plant's roots to help keep the shrub under control. Allowing a few inches between each stem keeps them from becoming overcrowded.
Prune small, twiggy growth because it is not productive. Jeff Jabco, director of grounds at Scott Arboretum in Swarthmore, Pennsylvania, recommends keeping shoots that are pencil thin all the way to the top because these produce the most flowers.
Perform annual pruning immediately following bloom. Pruning can also be performed in late winter, but expect to lose some blooms as a result.
If you have a dwarf lilac such as the Palibin or Miss Kim varieties, prune lightly and only on older plants. Because dwarf lilacs grow less rampantly than the large varieties, they rarely require any more pruning than the removal of the oldest stems every few years.
Perform rejuvenation pruning on large, neglected lilacs that are blooming poorly. Cut all the stems six to eight inches from the ground in late winter. This will encourage new shoots to grow over the summer.
In late winter of the following year, choose several strong healthy shoots to keep and trim away the rest at ground level. Cut back the remaining shoots to just above a bud; this will encourage them to branch.
Alternatively, prune an old lilac over a three-year period by removing a third of the oldest stems in late winter each year, along with thinning out some of the new shoots. Lilac wood requires about three years to reach blooming stage, so this method should yield some flowers even as you rejuvenate the shrub.
Follow up rejuvenation pruning with a general fertilizer in the spring to encourage new growth.
Perform deadheading (removal of finished flowers) on all lilac varieties in June or as soon as possible after blooming so that the shrub can put all its energy into new growth and flower formation.
When deadheading, cut off the flower cluster at its base, right above the new shoots below it. This is the spot where next year's buds will form over the summer.
If a stem is still vigorous but getting too tall, cut not only its flower cluster but farther down, to a side shoot. This will encourage branching and allows you to control the plant's height.
About this Author
Gwen Bruno has been a full-time freelance writer since 2009, with her gardening-related articles appearing on DavesGarden. She is a former teacher and librarian, and she holds a bachelor's degree in education from Augustana College and master's degrees in education and library science from North Park University and the University of Wisconsin.