Spring flowering Shrub Pruning

I see it every year – an enterprising weekend garden warrior steps outside, looks at some vigorously growing shrubs, and heads for the garage to pick up the gas powered hedging shears. Our industrious gardener pulls the cord and creates perfect little balls, leaving a nicely sheared green sheep. There are just a couple of problems with this quick and easy method; the biggest problem being it is not going to make for a beautiful shrub in the long run, especially if the goal is lots of spring flowers with minimal effort.

In the fall, spring flowering shrubs such as forsythia, lilac and some viburnums (shrubs flowering around or before June 15th) set flower buds which will be ready to explode in the spring, before the shrub begins putting on new shoot growth. Many homeowners and landscapers make the mistake of hedging plants that do not respond well to hedging.  A hedger cuts indiscriminately, making some cuts too close to new buds and some too far away. Improper cuts lead to buds drying out or to dead branch tips. One can also cut into much older wood, which will not respond with new growth, creating dead zones within the shrub canopy. Shrubs such as a yew or boxwood respond well to shearing because they possess very short internodes.

The standard advice given regarding spring flowering shrubs is to prune them shortly after they are finished flowering. However, I recommend a slightly different tactic. If one prunes too soon after flowering, the pruning will only encourage more growth. If one is attempting control the size of the shrub, I encourage you to wait up to a month or more for shrub growth to decrease. By then, most spring flowering shrubs begin to slow shoot growth and prepare for winter. Pruning at this time will decrease the amount of growth pruning would stimulate as day length begins to tell the plant to slow down naturally, yet there is still enough time for the shrub to develop flower buds. As a rule of thumb, do not prune much later than late July.

Horticulturists pruning, say, a forsythia, may use a combination of what are called thinning and heading back  cuts rather than shearing.

spring shrub prune diagram
Diagram courtesy of Cleveland Botanic Gardens

As the diagram shows, shearing leads to leggy, top heavy, ugly shrubs with few flower buds. Removing a combination of some tip growth and deeper, thinning cuts creates a denser, more robust flowering shrub. Thinning cuts are made by reaching below the dense canopy of leaves to remove larger limbs. This stimulates denser growth, letting more sunlight into the shrub’s interior and shortens its overall height at the same time.

Following these simple steps will bring you happy shrubs and much joy next spring.

Josh Steffen
Horticulture Manager

Leave a Reply