From a botanical perspective, the defining characteristic of reblooming hydrangeas like the Endless Summer series is a genetic mutation that alters their flowering cycle. Most traditional bigleaf hydrangeas (Hydrangea macrophylla) are what horticulturists call "short-day plants." They initiate flower buds in late summer or early autumn, triggered by the lengthening nights. These buds then overwinter on the previous year's growth (old wood) and bloom the following summer. The primary vulnerability of this system is that a harsh winter or a late spring frost can kill these tender buds, resulting in a complete loss of blooms for the year.
The reblooming trait arises from a mutation that allows the plant to initiate flower buds on both old wood and the new, current season's growth (new wood). This genetic change effectively decouples the plant's flowering from a strict photoperiod response and makes it less dependent on the survival of overwintering buds. The Endless Summer cultivar, for instance, was discovered as a spontaneous mutation on a traditional hydrangea plant. This single genetic alteration is the core physiological reason these plants can flower repeatedly throughout the growing season.
The physiology of a reblooming hydrangea involves a continuous, sequential process of bud formation and development. After the initial flush of flowers from the old wood buds in early summer, the plant does not enter a period of rest concerning reproduction. Instead, it immediately begins to produce new flower buds at the tips of the stems that have just grown. As the plant continues to generate new vegetative growth, new apical meristems (the growing tips) are formed. These meristems receive hormonal signals that prompt them to develop into flower buds rather than just more leaves.
This process is energy-intensive and requires consistent resources. The plant must simultaneously support the existing blooms, develop new flower buds, and sustain overall vegetative growth. This is why factors like adequate sunlight, consistent moisture, and proper nutrition are even more critical for reblooming varieties than for traditional ones. The plant's internal resources are being allocated to a nearly continuous reproductive effort, demanding a high level of metabolic activity throughout the season.
While the genetic propensity to rebloom is inherent, its expression is heavily influenced by environmental factors and horticultural practices. Light is a primary driver. For optimal reblooming, the plant requires sufficient sunlight to photosynthesize and produce the carbohydrates needed to fuel the constant production of flowers. A hydrangea in deep shade will struggle to rebloom effectively because it lacks the energy reserves.
Pruning practices also directly interact with the plant's physiology. Since reblooming hydrangeas flower on new wood, they are more forgiving of pruning than traditional varieties. However, the timing of pruning is crucial. If a plant is pruned heavily in late summer or fall, the new growth that is stimulated may not have enough time to harden off before winter, making it susceptible to cold damage. Conversely, if pruned too late in spring, the first set of flower buds on old wood may be removed. The most supportive practice is deadheading – removing spent flowers just below the bloom head. This signals the plant to redirect energy from seed production back into vegetative growth and the formation of new flower buds further down the stem.
