Greetings, Sandy! Thank you for your question to AskExtension/ Ask a Master Gardener.

Your Hydrangea paniculata are beautiful! I can see, though, that you would like your foliage to be denser and the stems to be thicker and more supportive of those big, beautiful blossoms. From the pictures you sent, your hydrangeas appear to be very healthy. You are pruning them at the appropriate time for panicle hydrangeas, also called PeeGee or PG.

There are two things that I believe may be causing your hydrangeas to have weak stems. You might be over fertilizing them which would explain large flowers (inflorescence) and weak stems. Or they may be receiving too much or too little sunlight. I have included additional information and links to reliable research-based websites to help you determine.

Soil: Hydrangeas prefer loamy, consistently moist but not wet soil with an average to slightly acidic soil. If you are interested in having a soil test (which is highly recommended) you can find directions here. A soil test is recommended to be completed every few years.

Fertilization: If your soil test indicates the need for nutrients, a slow-release fertilizer applied in April and June is recommended. “Avoid fertilizing panicle hydrangeas excessively, as this can lead to weak stems. Be particularly aware of incidental fertilizing, such as that applied to a nearby lawn or flower bed, as these tend to be high in nitrogen and more likely to push soft, rapid growth that causes weak stems.” From Proven Winners Ultimate Guide to Panicle Hydrangeas

Light: Although panicle hydrangea can withstand more direct sun than some other hydrangea species, such as bigleaf hydrangea, it is still a woodland plant. Full sun can cause lack of vigor, including weak stems. Likewise, this variety is not suited for dense shade; 4-6 hours of direct sunlight a day with morning sun and afternoon shade is ideal. From your photos, it looks like there may be a tree that provides shade throughout some of the day, but I am unable to determine direction or time of day from the photo.

Variety: You don’t mention the variety of your hydrageas. Some varieties are developed to cascade, others are grown to have a more upright habit.

Pruning: There is some controversy as to whether it is necessary to prune panicle hydrangeas. The Chicago Botanical Garden says: “Accounts vary on whether annual pruning improves stem strength or enhances inflorescence size.” If you choose to prune, it should be in fall, late winter or early spring, just as you have been. Cut above a “fat” bud. Panicle hydrangea can withstand “severe” pruning, meaning you can remove one-third to one-half of the stem. That said, pruning is an art and knowing where and when to prune takes study and experience. This article from the University of Minnesota has some drawings that might be helpful. There are several articles in the “Pruning” section below that you might find interesting and informative.

When answering your question, I referred to several research-based websites from trusted sources to be certain I was sharing accurate and updated information with you. I include the best of those sites below. Please feel free to respond to this post if you need clarification or more information about this topic. I would be happy to hear from you anytime.

Best of luck with your beautiful hydrangea plants!

--- Websites for you ---

Pruning Hydrangeas

How to Prune Hydrangea – University of Illinois Extension https://extension.illinois.edu/sites/default/files/hydrangea_v032021.pdf

When to Prune Hydrangeas for Best Bloom – University of Minnesota https://extension.umn.edu/yard-and-garden-news/when-prune-hydrangeas-best-bloom

Pruning Hydrangeas – Iowa State University Extension https://hortnews.extension.iastate.edu/pruning-hydrangeas

How to properly prune hydrangeas – University of Illinois Extension https://extension.illinois.edu/blogs/good-growing/2022-03-10-how-properly-prune-hydrangeas

General Knowledge about Hydreangeas

Ultimate Guide to Panicle Hydrangea https://www.provenwinners.com/panicle-hydrangeas

Hydrangeas University of Alabama A&M https://www.aces.edu/blog/topics/lawn-garden/hydrangeas/

Plant Evaluation – Chicago Botanical Garden https://www.chicagobotanic.org/sites/default/files/pdf/plantevaluation/no47_hydrangea.pdf

Selecting Hydrangea for the Home Landscape – Ohio State University https://ohioline.osu.edu/factsheet/hyg-1263

Hydrangea paniculate – North Carolina State Extension https://plants.ces.ncsu.edu/plants/hydrangea-paniculata/

Soil Testing

https://ohioline.osu.edu/factsheet/hyg-1132