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Hydrangeas are one of the most popular landscape shrubs in the US.  They are known for their exquisite flowering display, with many offering a season of beautiful blooms that remain attractive into winter.  Beyond flowers, these amazing shrubs offer additional ornamental beauty from neat and interesting leaves to ornate, peeling bark, making them quite versatile in the landscape.

Each year, we receive quite a few questions about hydrangea care, with the most common question being, “Why isn’t my hydrangea flowering?”  

Since hydrangeas are so versatile, there is a hydrangea species for most any landscape setting from full sun to complete shade.  So, my first question back to an inquiring homeowner is usually aimed at making sure their hydrangea has an adequate amount of sunlight.  Hydrangeas that don’t receive enough light will not produce the flower buds needed for a full display of booms. 

If we rule out lack of sunlight, then the next most likely reason for poor flowering may be due to pruning.  I find that hydrangea pruning is one of the most misunderstood cultural practices among gardeners.  Similar to assessing light requirements, each species has specific pruning needs and it’s all based on how flower buds occur on the plant. 

Some hydrangeas flower on ‘new wood’, whereas some flower on ‘old wood’.  By using the term ‘new wood’, I mean that these plants produce flower buds on new growth, or stems that grow early in spring and produce flowers in summertime.  Hydrangeas that flower on ‘old wood’ produce blooms on stems that grew last year which also grew flower buds in the previous year.

It is commonly recommend that woody plants be pruned during winter dormancy, which is the ideal time for pruning with respect to plant health.  However, if you prune a hydrangea that flowers on old wood in the winter time, you will remove flower buds it produced last year.  With each snip of the hand pruners, you are taking away buds that would otherwise bloom later in the year, which will greatly reduce, or possibly eliminate, their flowering display.  These plants should be pruned shortly after flowing in summertime to allow for bud growth and development during the remaining portion of the growing season.

On the other side of the coin, hydrangeas that bloom on new wood can be pruned in winter or late spring since they have not put on their flower buds for the year yet. These plants will initiate new growth later in spring and flower buds will develop on new stems.  Winter pruning has no impact on the flowering display that can be expected in the coming growing season.

So, how can you tell if your hydrangea flowers on new wood or old wood if you don’t know the species?  By observing the flowering time of your plant, you can pretty easily unravel the mystery. 

Hydrangeas that flower on old wood, are commonly referred to as spring-flowering shrubs.  Their blooms occur in the spring time, or before the end of June.  This always makes sense to me because these plants flower on buds they grew last year (on ‘old wood’), which allows them to flower early in the growing season since their flower buds were already there from the previous year’s growth. Spring-flowering hydrangea species include: oakleaf hydrangea (Hydrangea quercifolia) and bigleaf hydrangea (Hydrangea macrophylla).

Hydrangeas that flower on new wood are referred to as summer-flowering shrubs since their bloom time occurs in July or later.  This also makes sense when you consider their flower bud development.  These plants must develop flower buds in the same year they will bloom, so it takes them a little more time to put on the new growth and their flowers appear a little later in the year as a result. Summer-flowering hydrangea species include: smooth hydrangea (Hydrangea arborescens) and panicle hydrangea (Hydrangea paniculata).

As with any plant question, there are always exceptions.  In this case, growers have developed newer cultivars that flower on both new and old wood. The most common varieties are bred from bigleaf hydrangea, with ‘Endless Summer’ being one of the most popular.  They are beautiful plants, but they won’t fit in either category listed above.  However, their pruning needs are easily met if pruned during winter dormancy.  Since they host flower buds on new and old growth alike, you won’t sacrifice too much of their flowering display with your pruning cuts, while preserving plant health by pruning during their dormant period.

To learn more about hydrangea care, including details on how to identify and prune your hydrangea species, check out this new Illinois Extension Factsheet on Hydrangeas at: go.illinois.edu/PruneHydrangea.

For a live and interactive discussion of hydrangea care, join me and the other co-hosts on LIVE with the Horticulturists at noon on Wednesday, April 22 on Facebook Live.  The show will feature audience questions and a discussion about everything you need to know to keep your hydrangeas happy and healthy in 2021.  Access the live show on April 22 at: https://www.facebook.com/UIExtension.  Or, view the archive of our past shows at: go.illinois.edu/FacebookLive.