When you don't deadhead hydrangeas, your plant shifts energy from new growth into making seeds inside those faded flowers. This matters most on reblooming types because you lose the chance for a second flush. On once-blooming types, skipping has little downside. The dried heads even give you frost protection for the buds below.
I ran my own test with two Endless Summer bigleaf hydrangeas planted three feet apart. On one plant I snipped every flower within a week of it fading. The other I left alone to dry on the stem. The clipped plant pushed out a full second round of blooms in late August. The untouched plant made zero new flowers. Same bed, same water, same sun. The only change was whether I removed the old blooms or not.
My mother-in-law ran the same test the next year after I told her about my results. She got the exact same outcome with her pair of Endless Summer plants. The deadheaded one rebloomed in September. The other one stayed quiet the rest of the season. Two tests in two gardens confirmed what I suspected about these reblooming types.
The science is simple. Once a flower finishes its show, your plant starts making seeds inside the faded cluster. RHS notes that seed production drains energy your plant could use for roots and stems. On reblooming types, this energy drain stops the second flowering cycle from starting. Snip the old flower before seeds form and your plant puts that energy back into growth.
If you skip deadheading hydrangeas on once-blooming types, you won't miss extra flowers. These plants bloom once per season no matter what you do. The seeds will form and your plant will spend some energy on them. But a healthy hydrangea has plenty of reserves to handle it. Hydrangea spent flowers also look nice through winter and protect bud nodes from ice.
You can spot hydrangea spent flowers by their color and feel. Fresh blooms are plump with rich color. Spent ones turn papery and fade to brown or tan at the edges. When you see this change on your reblooming types, that's your signal to act. UMD Extension notes that deadheading can boost the size of each flower cluster. You trade total cluster count for bigger, showier blooms.
Here's your plan. For reblooming types like Endless Summer, walk through your garden every week during bloom season. Clip any faded flowers on the spot. For standard once-blooming types, leave the spent flowers on through winter. Remove them in late February before new growth starts. This saves you time and gives your garden texture during the cold months.
Match your effort to your plant type and you'll get the best results. Your reblooming types reward quick clipping with bonus flowers. Once-bloomers do just fine if you leave them alone all season.
Read the full article: How to Prune Hydrangeas by Type
