Hydrangea question

Question: Greetings Ask Extension colleagues.  Since Christa has forwarded me a lot of questions recently :), I wanted to turn the tables and ask my own question.

I have a large, 30+ year old stand of hydrangeas.  Aside from being able to tell they are macrophylla, I have no identifying info about them.
They had performed well for many years, but starting a couple of years ago they would leaf out in the spring but never flower.  Frustrated by this, last fall I had the crew that mows my lawn cut down all the dead stalks in the fall.  Voila, as shown in the photo, this year they flowered beautifully.  But then all the flowers and foliage turned brown and fell off--they never did that before.  In the last few weeks, they have started to green up again.  As shown in the photos, the greening is mostly near the bottom but in some places mid-way up on otherwise dead-looking stalks.

Question:  should I cut back the stalks again?  If so, where should I cut them, just above emerging leaves, or lower down, a foot or so above the ground?  Or should I leave everything be and only cut back stalks next spring once I am sure nothing is growing on them?  Do you have any idea why the plants lost all their foliage after flowering?  Are they perhaps near the end of their useful life?

Thanks, Bob

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied September 10, 2024, 12:44 PM EDT
response: Hi Bob,  We're not certain why this extensive and comparatively sudden dieback occurred, though Hydrangea can contract Botryosphaeria canker if stressed. Were the shrubs watered periodically this summer, during the drought (which is still ongoing for much of Montgomery County)? If not, the dry weather combined with high heat and location next to the brick wall likely contributed to the issue. If they were watered enough, then another stem infection might be responsible, though it's hard to tell since there are no distinctive features pointing towards a specific pathogen. We presume no herbicide was applied near this planting in the past few weeks or months? We ask because some of the regrowth looks odd, as if the foliage were manifesting mild phytotoxicity. (It might just be an artifact of stress or the original cause of dieback, though.)  Any dead growth on a shrub should always be removed promptly so it doesn't crowd the live wood or harbor pests or diseases. (The stems that were cut last fall might not have been actually dead, just dormant if they grew normally and then had shed leaves by that time. Interestingly, that action would typically have removed lots of flower buds, but apparently enough remained to still bloom this year. If the stems were indeed dead when they were pruned, then that pruning would not have directly impacted flowering since it would have been removing growth that was already non-living and therefore incapable of developing buds later.) Trim the stems wherever you need to to reach live wood in order to remove the dead tissues.  If the pictured stems suffered a loss of foliage, blackened and shriveling stems, and withered (and still attached) flower clusters, that all suggests that the stems in question are fully dead, not just temporarily leafless, and so there is no benefit to keeping them since they can't recover. For the stems with dead tips but live lower halves, just trim off the dead portion to just above a node where leaves are emerging. Getting the dead growth out of the way of the new growth should not only help to maintain air circulation and light through the shrub, to promote general health, but also should remove any potentially infectious spores. This autumn/winter, once the current foliage has died off after a freeze or two, that can also be raked-out and disposed of so the debris doesn't risk harboring a pathogen for next year.  We don't have a resource that states how long Bigleaf Hydrangea shrubs can live, but several decades is not uncommon, so they are not necessarily at the end of their useful lifespan at this point.  Miri

responder: The Question Asker Replied September 10, 2024, 2:44 PM EDT
response: Miri, thanks so much for your very prompt and comprehensive response.  I will prune selectively as you advise. On reflection, lack of water may indeed have been the problem this year.  I have never watered these plants in all these years, but in light of the extreme temperatures experienced this summer, coupled with the plants' location next to a brick wall that gets strong, late afternoon sun, that likely was a mistake.  I'll be attentive to watering rest of this growing season and next year.Thanks so much again.BobOn Tue, Sep 10, 2024 at 12:44 PM Ask Extension <<personal data hidden>> wrote:

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied September 10, 2024, 2:53 PM EDT
response: You're welcome!