Ask a Master Gardener: Pruning spring-blooming shrubs

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Knowing when to prune flowering shrubs, like forsythia, is important.

(Posted April 8, 2021)

By Jane Kutzley, Madison County Master Gardener

Question: My forsythia shrub looks big and healthy, but it had very few blooms this spring. It was gorgeous last year. What happened? 

If your shrub appears to be healthy, other than the lack of blooms, then it was probably pruned at the wrong time of year. Knowing when to prune flowering shrubs is important. If done at the wrong time, the blooms will be eliminated or greatly reduced in number.

Some shrubs bloom on old wood. Forsythia is one example. This means that the shrub sets its blooms for the next year shortly after blooming this year. So, up to nine or 10 months before it blooms, even though you can’t actually see it, a forsythia has started to develop blooms for the next spring. By the time it actually blooms, the branches are nearly a year old and are considered old wood.

When pruning shrubs that bloom on old wood, it is critical not to prune between the time the buds are set and the time it actually flowers. That means that there is only a small window of time shortly after the shrub blooms when it can be pruned without worry about reducing next year’s blooms.

Other shrubs bloom on new wood. Butterfly bush and button bush are two examples. These set their buds shortly before they bloom, so the branches are younger and the time from bud set to bloom is much shorter. New wood shrubs can be pruned much later in the season without reducing the quantity of blooms.

A general rule of thumb is that spring-flowering shrubs bloom on old wood while summer flowering shrubs bloom on new wood. There are exceptions, of course, and just to make things even more interesting, some families of shrubs have members that bloom on old wood and others that bloom on new wood. Hydrangea macrophyllum (bigleaf hydrangea) blooms on old wood but Hydrangea arborescens (smooth hydrangea) and Hydrangea paniculata (panicled hydrangea) bloom on new wood. Spirea is another example. Spring-blooming spirea blooms on old wood. Summer-blooming spirea blooms on new wood.

A quick Internet search will tell you whether your shrub blooms on old or new wood. If you don’t know what kind of shrub you have and you want to have as many blooms as possible, then limit your pruning to the month after your shrub finishes its annual bloom cycle.  Then there’s no need to worry about old or new wood. However, you must resist the urge to “neaten it up” as the season progresses.

One more interesting twist is that some of the newer shrub cultivars bloom on both old and new wood and are called rebloomers. This means that whenever you prune, you will be eliminating some blooms. Current advice is to prune a reblooming shrub as infrequently as possible and to do any pruning at the end of a particularly heavy bloom cycle.

Watch for upcoming details about the Madison County Master Gardeners’ new Ask A Master Gardener Help Line, coming soon.

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