The "Endless Summer" hydrangea (Hydrangea macrophylla) and similar reblooming cultivars represent a significant horticultural advancement rooted in plant genetics. Unlike traditional bigleaf hydrangeas, which form their flower buds exclusively on "old wood" (stems from the previous season), reblooming types possess a genetic mutation that allows them to initiate flower buds on both "old wood" and the current season's new growth, known as "new wood." This mutation occurred naturally and was identified, propagated, and introduced to the market. This genetic trait is the core physiological reason these plants can flower repeatedly throughout the same growing season.
The flowering process for these plants is a continuous cycle of energy allocation and meristem differentiation. After the initial flush of spring flowers from buds on the old wood, the plant does not enter a singular reproductive phase and then stop. Instead, it continues to photosynthesize and divert energy to its apical and lateral meristems (growth tissues). These meristems receive hormonal signals to differentiate into new flower buds rather than just vegetative leaf buds. This ability to initiate and develop floral primordia on new stems within a single season is the defining physiological characteristic that sets reblooming hydrangeas apart from their traditional counterparts.
From the plant's perspective, this repeated flowering is an energetically expensive process. It requires consistent resources—water, nutrients, and sunlight—to support both vigorous vegetative growth and prolific blooming. The primary environmental trigger for flowering is photoperiod (day length), similar to many other plants. However, the reblooming trait allows the plant to respond to these cues multiple times. Stressors like extreme heat or drought can cause the plant to abort the energy-intensive flowering process to conserve resources for survival, prioritizing root and leaf maintenance over bloom production.
The dual-flowering wood characteristic changes the plant's relationship with pruning and winter injury. For a traditional hydrangea, a late frost or improper pruning that removes the old wood results in a complete loss of blooms for that year, as those flower buds are the plant's only chance to reproduce. A reblooming hydrangea has a built-in reproductive redundancy. If the old wood buds are killed, the plant can still produce flowers on new wood, ensuring it fulfills its biological imperative to flower and set seed. This makes the plant more resilient and adaptable to the variable winter climates found across much of the USA.
The flowering performance of these hydrangeas is also deeply tied to soil chemistry, specifically pH and aluminum availability. The iconic blue or pink color of Hydrangea macrophylla blooms is a direct response to soil aluminum ions, which the plant's roots uptake in acidic soil conditions (pH below 5.5). In alkaline soils, aluminum is locked away, resulting in pink blooms. For a reblooming variety, which is constantly producing new stems and flower buds, the demand for nutrients and the ongoing interaction with soil pH is a continuous process. The plant must effectively gather and shuttle these ions to the developing sepals (the colorful part of the flower) throughout the entire growing season to maintain its vibrant and variable coloration.
