Earlier this year we discussed how to prune Hydrangea paniculata, the woody, sun-loving white hydrangea. Today we are going to discuss how to prune its semi-woody cousin the mophead hydrangea, Hydrangea macrophylla. These are the hydrangeas with glossy green leaves and blooms that are blue or pink and can change color depending on the soil pH.
This summer we sold blue hydrangea floral arrangements at a local farmers market. There was one topic was brought up by customer after customer. It was a question asked so many times, it prompted us to write this article.
Their complaint was that their hydrangea macrophylla never blooms or only has a couple blooms on it every year.
Many people had even removed and replaced their plant after being disappointed by its failure to bloom. If you happen to find yourself in this situation, don’t worry. Today we are going to learn how to get your hydrangea blooming.
First, if you are new to pruning we would encourage you to read a portion of the grapevine pruning article we wrote a while back. If you don’t know how to prune something properly, taking the time to observe what kind of wood the blooms are on will tell you exactly how and when how to prune your plant. This holds true for the plants we are pruning today though mophead hydrangeas can be a little trickier to decipher than some plants.
Hydrangeas can be confusing because some of the newer ‘re-blooming’ varieties bloom on both new and old wood. In that case, pruning time doesn’t matter. However, most of the hydrangeas planted in people’s yards bloom on old wood. So, you should make the assumption that your hydrangeas bloom on old wood (unless they were recently planted) and prune accordingly.
This means that if you shear your shrubs every fall or winter, you are removing all of your flower buds every year.
Many homeowners and landscaping companies like to shear every single plant in the winter to create a tidy appearance and to prepare for spring. However, plants like Hydrangea macrophylla or Forsythia that bloom on old wood develop their flower buds in late summer/early fall. These flower buds sit dormant over the winter and are ready to open in the spring. If you shear your plants during the winter you remove the flower buds and they will not be replaced until next summer resulting in the sacrifice of this years flowers. This results in a plant that seemingly never flowers even though it might be lush and healthy looking.
To maximize your blooms, your Hydrangea macrophylla should be summer pruned.
Your plant will start making its new flower buds sometime in August or September, so the best time to prune the plant is towards the end of July. Prune immediately after it has finished blooming. If your hydrangea is not too large for the space it is growing in, minimal shaping pruning is sufficient. These plants do not need a lot of pruning to keep them looking their best.
Start by removing any dead or damaged stems back to the base of the plant. Then, thin the plant by removing about 1/3 of the oldest stems. You can trim back the top of the plant as needed to maintain its size and a desirable shape. Trim any shoots you cut back to just above a leaf node. That’s it!
With this type of hydrangea you want to encourage it to send up new shoots from the base frequently. These are often the healthiest shoots, and the ones that bloom the best. Thinning out shoots this summer will encourage it to send up new shoots in the spring. Those shoots will be the ones that bloom the following year.
We would also like to note that the hydrangeas featured in the pictures of this article were pruned very harshly because they had significantly outgrown their space (see first picture). It is not necessary or even ideal to prune your hydrangeas this hard regularly. It is better to maintain them to a manageable size with less harsh yearly summer pruning.
Bonus information about your Hydrangea macrophylla.
Are the blossoms on your plant pink? Do you covet those rich blue blooms you see in stores and on TV? If so, the solution lies in your soil pH. Hydrangea macrophylla blooms are pink in alkaline soil, and blue in acidic soil. To modify flower color all you have to do is modify your soil pH.
Adding sulfur to your soil will produce blue blooms though it might take a couple years for the plant to fully change color. We prefer to do this using elemental sulfur rather than aluminum sulfate as aluminum can become toxic to plants if applied to the soil year after year. If you want bright pink blooms you can add lime to your soil to raise the pH. An intermediate pH will result in the purplish blooms you see sometimes.
Pruning and achieving the desired color of your hydrangea macrophylla is as simple as that! Now that we have unlocked the mystery of how to get that stubborn plant to bloom we hope you have a plethora of blue blossoms next summer! If you would like to learn more about caring for plants in your garden, don’t forget to sign up for our newsletter!
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