I answered the Garden Helpline in the Extension Master Gardener office recently and spoke to a very upset individual: “Why on earth, after all my hard work and attention, did my beautiful lilacs (Syringa vulgaris) not bloom this year?” I asked about growing conditions—full sun is needed for bloom—and watering and fertilizing, receiving all the correct answers. Then we talked about pruning: “It looked very scraggly and overgrown and I wanted to make it look nice for the spring, so I pruned it in February.” There was the answer to the question!
Pruning time matters
Timing is critical, especially when pruning spring-blooming shrubs that develop their flower buds during the summer and fall of the previous year. This is often called “blooming on old wood.”
Plants, such as flowering quince (Chaenomeles speciosa), forsythia (Forsythia x intermedia), oakleaf hydrangea (Hydrangea quercifolia), bigleaf hydrangea (Hydrangea macrophylla), mountain laurel (Kalmia latifolia), azaleas and rhododendrons (Rhododendron spp.,), doublefile viburnum (Viburnum plicatum var. tomentosum), as well as lilac, are in this category.
While June 15 is often mentioned as a cut-off date for flowering time for “spring-blooming shrubs,” we gardeners know that Mother Nature does not always adhere to clear-cut time frames. What’s important is to correctly identify the plants and prune these shrubs soon after flowering. Pruning these shrubs in late summer, fall, or early spring will remove the flower buds—so pruning your lilac in February will remove the flower buds set last year!
Pruning soon after flowering means pruning no more than six weeks after this years’ blooms die. So even if your work or vacation time prevents you from getting to the task immediately, you still have time to prune and not adversely affect next years’ blooms.
How to prune
The object of pruning is to open up the top of the plant to permit light and air to reach the interior. It can promote new plant growth, maintain plant size, encourage flowering, remove diseased or dead limbs, and help control insect and disease problems.
When thinking about pruning, consider the “one-third” rules: remove about one-third of the oldest wood at the ground level and cut back one-third of the younger, newer canes about one-third of their height per season.
The two basic types of pruning cuts are heading cuts and thinning cuts. Heading cuts remove part of the branch back to a bud. The direction in which the top remaining bud is pointed will determine the direction of the new growth. Selective heading cuts reduce the shrub’s height and retain its natural form.
Thinning cuts remove an entire limb to where it originates—in the case of shrubs, this could be at ground level. Thinning cuts remove the thickest, oldest stems with the fewest flowers. This opens up the canopy and increases light penetration and air circulation. To rejuvenate overgrown specimens with a suckering growth habit, such as forsythia and lilac, periodically removing the biggest, oldest stems at ground level allows new stems to replace the older ones.
An exception
If you have overgrown or unsightly spring-flowering shrubs, you may need to sacrifice one year’s bloom to get your garden looking good again. Renewal pruning is called for if you need to start over! In this case, prune the entire shrub back to 6 to 12 inches before your shrubs leaf out in the spring—either in late fall or early winter. Prune new growth lightly during the summer to get your shrub into shape for the future!
Article written by Bob Wardwell, Extension Master GardenerSM Volunteer.
More information:
Pruning Flowering Shrubs
by Elaine Fogerty, Agricultural Assistant
Rutgers Cooperative Extension
(Diagrams show different types of pruning cuts.)
Pruning Flowering Shrubs
Penn State Extension