From the plant's perspective, the primary purpose of a flower is reproduction. Once a hydrangea bloom (which is actually a cluster of small flowers, called an inflorescence) has been successfully pollinated or has simply aged past its prime, its role shifts. The plant may begin to divert energy away from maintaining the showy, sterile florets and towards developing seeds within the fertile florets. The old, fading bloom, now called a spent bloom or "old wood," can be seen as a sink for the plant's finite resources. By remaining on the stem, it continues to consume water and nutrients that could be redirected to more critical functions, such as root development, new vegetative growth, and the formation of next year's flower buds.
Deadheading is the horticultural practice of removing these spent blooms. For the hydrangea, this act is an intervention that mimics natural processes. In the wild, petals would eventually fall, and the seed head would disperse. When you make a clean cut just below the flower head but above the first set of large, healthy leaves, you are effectively signaling to the plant that its reproductive effort on that stem has concluded. This has two main physiological benefits. First, it prevents the plant from expending energy on seed production, which is an energy-intensive process. Second, by removing a potential site for disease or pest infestation (as decaying flowers can attract fungi and insects), you help the plant maintain its overall health and vigor. The energy saved can then be invested in strengthening the root system and producing robust new growth.
This is the most crucial point from the plant's perspective, and the answer to "should you cut off old blooms?" depends entirely on it. Hydrangeas are not a monolithic group; they bloom on different types of growth, commonly referred to as "old wood" or "new wood."
Old Wood Bloomers (e.g., Bigleaf Hydrangeas - Hydrangea macrophylla, Oakleaf Hydrangeas - Hydrangea quercifolia): For these varieties, the plant sets its flower buds for the next summer on the stems that grew the previous year (the "old wood"). These buds develop in late summer and fall and remain dormant over the winter. If you deadhead these hydrangeas too late in the season (typically after late summer) or incorrectly by cutting too far down the stem, you risk removing the nascent flower buds that are forming just below the current year's bloom. From the plant's viewpoint, a severe cut at the wrong time would mean it has lost its entire reproductive potential for the following year.
New Wood Bloomers (e.g., Panicle Hydrangeas - Hydrangea paniculata, Smooth Hydrangeas - Hydrangea arborescens): These hydrangeas are more forgiving. They form their flower buds on the new growth that emerges in the current spring. Therefore, you can deadhead them at almost any time without affecting next year's blooms. The plant will simply channel its energy into producing new stems from the base or from lower nodes, which will then bear flowers later in the same season.
To support the plant's natural cycle, the timing and method are key. The safest approach for all hydrangeas, especially if you are unsure of the type, is to deadhead shortly after the blooms fade in the summer. Locate the first set of large, healthy leaves beneath the flower head. Using clean, sharp pruners, make a clean cut just above this set of leaves. This method removes the spent bloom cleanly without disturbing the buds that may be developing below for the next year on old-wood bloomers. For new-wood bloomers, you can be less precise and even cut the stems back more severely in late winter or early spring to encourage vigorous new growth, which is the wood on which they will flower.
