Why and How Should I Prune My Lilac Bush?

Many lilac shrubs that are allowed to grow and spread without pruning may reach a height of well over 20 feet. The lower branches will likely be bare, and the blooms will be out of sight and reach. This is particularly true for “old-fashioned” varieties. Cultivars such as Korean lilacs, which generally have a more compact habit, may need an approach to pruning different from what I describe here.

Lilacs bloom on previous season’s wood, but, if you don’t prune your shrub, you’ll end up with a woody plant that is unattractive and doesn’t produce a nice spring bloom show.  Ideally, a lilac bush needs a mix of young new shoots and older stems.  The newer stems won't bloom for a couple of years, but to keep the lilac bush blooming, these young shoots are essential.


The large old branches need to systematically be removed.  A rule of thumb is that when a branch is more than two inches in diameter it should be removed.  With regular annual pruning you can maintain your lilac bushes at a height no greater than eight feet tall with a mix of stem diameters and with foliage and blooms dispersed throughout the bush.


Prune your lilac bushes just after the blooms have wilted and dropped off.  Lilacs form next spring’s buds soon after the spring blooms are gone.  Late pruning will result in hacking away next spring’s blooms.   


To keep your lilac bush developing new stems while the older stems are blooming, do not remove more than one-third of the branches each year.  The idea is to prune the lilac bush to have a mix of stems that range from pencil thickness whips to branches that are 1 to 2 inches in diameter.  Shape the bush as you are pruning.


Small branches that do not conform to the natural shape should be removed.  Removal of the oldest branches can be achieved by making thinning and heading back cuts.  


Heading Back:  The selective cutting of terminal shoots or branches back to a lateral bud is called heading back.  The purpose is to develop vigorous new shoots and denser foliage.  Head back the branch to 1/4 in (0.6 cm) above a bud that is facing the direction you want the new shoot to grow.  Heading back branches to several different heights will produce a well balanced fuller natural looking bush and eliminate dense outer foliage that shades out lower growth.


Thinning:  This procedure refers to the total removal of a stem and should be used for deadwood and for shaping the bush.  


Renewal Pruning:  This is a more severe form of thinning that may be applied to older lilac bushes to bring them under control by cutting back about half of the stems to six to eight inches from ground level.  This will allow the light to reach the lower levels of the bush.  New shoots will develop from the stumps and create a good mix of new growth within the older growth. 


Rejuvenation:  If you are trying to prune a large older totally out-of-control lilac bush, you may need to resort to more drastic measures.  The best remedy may be to simply cut the entire bush to about 6 to 8 inches from the ground level.  This should be done in the early spring.  New whips will sprout from the stumps.  By the next spring, prune the weaker whips out and begin to shape the bush keeping the better growth.  To further promote branching, prune the best stems just above a bud.  


This renewed bush will still have a strong root system.  Be forewarned, though, that it will be several years before it comes back into bloom.  However, when it does, you will have a restored, well-shaped lilac.


Conclusion:  To remain at their best, lilacs need a bit of maintenance pruning each spring, but few flowering shrubs contribute more beauty and fragrance to our lives. 


For more information on home, lawn, indoor, or outdoor garden care and tips, as well as other garden topics, visit ohioline.com on the web or call the OSU Extension, Butler County, at 887-3722, or in Middletown at 424-5351, ext. #3722.


News Release provided by Richard Sunberg, Butler County Master Gardener

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