From the plant's perspective, the act of deadheading—removing spent flowers—is a question of resource management. A hydrangea's primary goals are survival, growth, and reproduction. Once a flower fades and begins to senesce, it may start to form seeds. This seed production is an energy-intensive process, diverting sugars, nutrients, and hormones away from other critical functions like root development, new vegetative growth, and storing energy for the winter. By deadheading, you intercept this process, effectively signaling to the plant to cease investing in seed production for that particular bloom. This can redirect the plant's energy toward strengthening its overall structure and root system, which is particularly beneficial for younger plants.
For certain types of hydrangeas, primarily those that bloom on new wood (the current season's growth), deadheading can encourage a second, albeit smaller, flush of flowers. Varieties like panicle (Hydrangea paniculata) and smooth hydrangeas (Hydrangea arborescens) initiate flower buds on growth that emerges in the spring. By removing the spent blossoms, the plant may be stimulated to produce new lateral shoots that can bear additional blooms later in the summer. This is not a guaranteed outcome but is a possible pro from the plant's viewpoint, as it extends its reproductive window. Furthermore, the energy not spent on seeds can fuel the development of stronger, more numerous stems for the following season.
The most significant con from the plant's perspective applies to hydrangeas that bloom on old wood (last season's growth). This group includes bigleaf (Hydrangea macrophylla), oakleaf (Hydrangea quercifolia), and mountain hydrangeas. These species form their flower buds in late summer and fall, which then overwinter on the stems and bloom the following spring. If you deadhead these types too late in the season—typically after late summer—you run the very high risk of accidentally removing these nascent buds, along with the spent flower. This would eliminate the plant's primary floral display for the next year, a major setback for its reproductive cycle. For these varieties, the plant's energy is best spent protecting and nourishing those existing buds rather than being forced to produce new, vulnerable growth that will not survive winter.
The correct method for deadheading aligns with the plant's natural growth habits. The key is to remove only the spent flower head itself, leaving the dormant buds on the stem untouched. You should make your cut just above the first set of large, healthy leaves below the flower head. For older, woodier stems, you will need to look for a pair of healthy buds and cut just above them. The optimal timing is soon after the flowers fade, which allows the plant to quickly heal the wound and redirect its energy efficiently. For old-wood bloomers, it is safest to deadhead immediately after the flowers fade in mid-summer. For new-wood bloomers, you have a longer window, often into early fall, though it is still best practice to do it promptly after blooming.
