The question I get asked most often regardless of where I am or what I am doing is “Why doesn’t my hydrangea bloom?” In fact, if I had a penny for each time I have been asked that – well, you can finish this sentence!
For now, let’s talk about big leaf hydrangeas (Hydrangea macrophylla), the ones we all swoon over.
You first have to know if it blooms on old or new wood since the troublemakers are those that bloom on old wood. If it’s the mature stems from last year which produce the flowers, then it’s an old wood bloomer. BIG HINT: If your plant pre-dates 2004, then it’s definitely an old wood plant. However, if your plant does indeed flower on this season’s new growth, then it’s a new wood bloomer.
There are lots of reasons old wood hydrangeas don’t bloom, many of which we as gardeners can control. (See chart below)
The most common issue is that they frequently have been pruned after August 1 and before their next “summer” bloom cycle. Here’s the science of old wood blooming hydrangeas. They set their flower buds for the next year on short day length, i.e., after June 21 AND when night temperatures drop below 60 degrees. Therefore, consistently cool August nights signal the time to hang a “Do Not Prune” sign on your hydrangeas as that is when both criteria are usually met.
What about rebloomers, you ask? Can’t I cut them to my heart’s content when prune other plants?
The answer to that is yes and no. You see, rebloomers also bloom on old wood and in fact those early flowers you get from them are those that have been set on those short days and cool nights from last season. Their new wood flowers on the other hand are stimulated by other factors in their genes, like warmer temperatures and the cutting of flowers throughout the season. You see, new wood flowers come from stems that don’t need that “chilling period” that old wood flowers require.
Simply stated, if you do any pruning of your reblooming hydrangeas after about August 1, you still risk losing your early season flowers for next year.
Here’s my suggestion: plant all your hydrangeas where they can comfortably grow to their mature size, eliminating any need to cut them back. That way you’ll have a vigorous, healthy plant and one less chore.
A second cause of bloom shyness is too much nitrogen, a strong possibility when the shrub is adjacent/downhill of a well-fed lawn or sits too close to the septic system. That produces lush green foliage at the expense of colorful flowers.
The same thing happens when the plant gets overwatered, either by an irrigation system or in a wet summer. Hydrangeas like to grow leaves and will readily do so before pumping out flowers. Fertilizers that are formulated for roses are what you want for your hydrangeas to promote more flower production and provide the right level of other nutrients for a healthy plant without going overboard.
And lastly, although we think of hydrangeas as shade lovers, they do need sun and in fact can take more sun than people think. As long as they have enough moisture, their sun quotient can be quite high with one or two exceptions. That mid afternoon droop that catapults most of us into a watering frenzy is a temporary condition that the plant experiences after sunbathing for a few hours. Once the sun is off them, hydrangeas usually fully recover without supplemental water to face another day with their garden buddies. If they don’t, only then do you need to rehydrate them.
What we can’t control is weather. Frost sensitivity is a notorious issue with this plant, more so than with forsythia or rhododendron, for example, both of which are early season bloomers. We already have experiences with cold winter temperatures causing “bud blast,” freezing the flower buds off the tips of our hydrangeas, denying us our early season show. Past springs have brought late season ice storms that freeze newly opened buds and leaves, again blasting the flowers that we would hope to see. If that wasn’t enough, some winters we get arctic cold and Polar Vortex assaults.
Success With Hydrangeas delves deeper into managing these challenges for big leaf hydrangeas as well as other hydrangea species. With nearly 150 photos and illustrations, this new book provides an overview of the science of rebloomers and details about rebloomers, winter protection, plant selection, how pruning the right plant at the right time can protect and/or induce better bloom cycles. etc. Success With Hydrangeas will answer the question, “Why doesn’t my hydrangea bloom?” and so much more!
Guest Blogger Bio
Lorraine Ballato is a highly regarded Connecticut horticulturist, garden writer and speaker. Her obsession with plants comes through in her lectures, social media writings, magazine articles and photographs which you can find in Connecticut Gardener, Edible Nutmeg, and elsewhere. She is an instructor at the New York Botanical Garden and in the Connecticut Master Gardener Program as well as for the Federated Garden Clubs of Connecticut. In her packed lectures she covers a broad array of topics from container gardening to roses to organic gardening to growing vegetables as well as her beloved hydrangeas. Her previous book is Successful Self Watering Containers: Converting Your Container to a Self Waterer. Click here to learn more.