You must understand my fundamental need for light. I am a photosynthetic organism, and sunlight is my primary energy source for creating the food that fuels all my processes, including bloom production. However, my tolerance for direct solar radiation varies by type. Bigleaf and Smooth hydrangeas (like me, Hydrangea macrophylla and H. arborescens) prefer bright, dappled light or morning sun with afternoon shade. Too much intense, direct light stresses my leaves, causing them to scorch and reducing my energy reserves. Too little light, and I cannot produce enough sugars to support the massive energy expenditure of flowering. Panicle hydrangeas (H. paniculata) are more sun-tolerant and can handle, and even crave, several hours of direct light to produce their best floral show.
My nutritional needs are specific, especially regarding phosphorus, the nutrient that directly supports the development of my flowers, roots, and seeds. A fertilizer with a higher middle number (e.g., 10-30-20) applied in early spring as I break dormancy gives me the right signal and resources to produce blooms. However, an excess of nitrogen (the first number) is disastrous. It encourages a frenzy of weak, leafy green growth at the absolute expense of flowers. You will get a lush, beautiful bush with no blooms. It is a cruel trick. Feed me once in early spring and perhaps again as my flowers begin to form, but then stop. Late-season feeding pushes new, tender growth that will not harden off before winter and can be killed by frost.
This is the most common reason I am denied my blossoms. You must know whether I bloom on "old wood" or "new wood." As a Bigleaf hydrangea, my flower buds are formed on the stems from the previous growing season ("old wood"). If you prune me in the fall or early spring, you are ruthlessly cutting off my already-formed potential blooms. If you must tidy me up, only deadhead the spent flowers and do any shaping immediately after I finish blooming in summer, giving me ample time to develop new buds for next year. Conversely, Panicle and Smooth hydrangeas bloom on "new wood" (growth produced in the current season). You can prune them in late winter or early spring, and they will still flower for you that year.
For those of us who form our buds on old wood, winter is a perilous time. Desiccating winds and hard freezes can kill my delicate flower buds, leaving you with a healthy green bush but no flowers come summer. After I go dormant in late fall, a thick layer of mulch (like bark chips or leaves) over my root zone helps regulate soil temperature. For extra protection, especially in colder zones, consider wrapping my branches in burlap or using a cage stuffed with leaves to shield my bud-laden stems from the harsh winter elements. This ensures the survival of the precious buds I worked so hard to create.
While not directly about blooms, my root system's health is everything. I require consistent moisture to transport nutrients and maintain turgor pressure in my cells. Periods of drought, especially during the bud formation and blooming period, will cause me to abort flowers to conserve resources for survival. Well-draining yet moisture-retentive soil is ideal. A layer of organic mulch helps immensely by keeping my roots cool and moist. Furthermore, for Bigleaf hydrangeas, soil pH influences the availability of aluminum, which affects my bloom color, but it does not inhibit the quantity of blooms if my other core needs are met.
