From my perspective as a hydrangea, light is my primary energy source. If I am planted in deep shade, my photosynthetic machinery cannot produce enough carbohydrates to fuel the development of large, showy blooms. The energy I do generate must be allocated to essential survival functions—leaf production, root growth, and basic metabolism—leivering little surplus for expansive flowers. Conversely, if I am subjected to intense, all-day sun, especially in hot climates, I become stressed. To prevent excessive water loss, I may close my stomata, which in turn limits photosynthesis. This stress can result in smaller flowers as I struggle to conserve resources.
My ability to produce large inflorescences is heavily dependent on the availability of essential nutrients in the soil. A lack of phosphorus, a key element for energy transfer and flowering, can directly inhibit the development of my bloom size. However, an excess of nitrogen is equally problematic. If the soil is too rich in nitrogen, I am signaled to channel my energy into vigorous vegetative growth—producing an abundance of large, green leaves at the expense of flower formation. The result is often a lush, leafy bush with disappointingly small blooms.
Pruning at the wrong time of year is a common reason for my small flowers. My flowering strategy varies by species. For bigleaf and oakleaf hydrangeas (Hydrangea macrophylla and H. quercifolia), I set my flower buds on what you call "old wood"—the stems that grew the previous season. If you prune me in late fall, winter, or early spring, you are likely removing these developed buds. When the growing season arrives, I must then use my energy to produce new stems and a new set of buds, a process that often results in fewer, and typically smaller, flowers that bloom later in the season.
Water is fundamental to my every function. My cells need it to maintain turgor pressure and to transport nutrients. A consistent water imbalance causes significant stress. During drought or periods of inconsistent watering, my root system cannot supply enough water to support the demanding process of flower expansion. The developing flower buds are a low priority compared to keeping my leaves alive; therefore, they are often stunted. Chronic water stress forces me to abort some buds entirely and undersize others to ensure my overall survival.
It is also important to consider my age and natural life cycle. If I am a very young, newly planted hydrangea, my primary biological imperative is not to flower but to establish a strong root system and structural framework. In my first few seasons, I may produce flowers, but they will often be smaller as I allocate limited resources to ensure long-term survival. Furthermore, as a mature shrub, if I become overly crowded with old, woody stems, the competition for resources among my many branches can lead to a general reduction in flower size across the entire plant.
