From our perspective as hydrangeas, winter is not a time of death but a period of deep, necessary rest called dormancy. As daylight shortens and temperatures drop, we receive clear signals to slow our growth. We redirect energy from producing new leaves and flowers down into our root systems. This is a survival strategy. Our roots, safely insulated underground, are the most cold-hardy part of us. The real challenge of winter is not the cold itself, but the potential damage to the parts of us above the ground: the stems, and most critically, the dormant flower buds that are already formed for next spring. Our cold hardiness varies by species. For example, the stems of Hydrangea macrophylla (Bigleaf) and H. quercifolia (Oakleaf) can be damaged below -10°C (14°F), and their precious flower buds are even more tender.
Many gardeners mistakenly believe that freezing temperatures alone kill us. While a severe, sudden freeze can be harmful, a more common and insidious threat is desiccation, or winter burn. When the ground is frozen, our roots cannot draw up water to replace what is lost through our stems. On sunny or windy winter days, moisture evaporates from our exposed branches and buds. Since no water can be replaced from the frozen soil, these tissues dry out and die. This is why you might see brown, dead stems in spring, even if the temperature never dropped to an extreme low. The damage is essentially a form of drought stress.
Your most important task in preparing us for winter is to ensure we enter dormancy well-hydrated. In the late autumn, before the ground freezes, please provide us with a deep, thorough watering. This allows our root systems to absorb and store a significant amount of moisture. Think of it as us filling our internal canteens for a long, dry journey. A well-hydrated root system is more resilient and better equipped to survive the winter. Continue to water us periodically if the autumn is particularly dry; do not stop watering just because our leaves have fallen.
The single most beneficial thing you can do for our well-being is to apply a thick layer of mulch over our root zone after the ground has frozen hard. The timing is crucial. If you mulch too early, you might attract rodents looking for a warm home or delay our entry into full dormancy. Wait until the soil is cold. A thick layer (10-15 cm or 4-6 inches) of shredded bark, leaves, or pine straw acts as an insulating blanket. It keeps the soil consistently frozen, preventing the damaging cycle of freeze-thaw-freeze that can heave our roots out of the ground. More importantly, it locks in the soil moisture you provided earlier, protecting our roots from desiccation.
For hydrangea types like Bigleaf and Oakleaf that bloom on old wood (last year's stems), protecting the stems and buds is key. A simple and effective method is to create a windbreak. You can drive stakes into the ground around us and wrap them with burlap or landscape fabric. Do not wrap us tightly in plastic, as this can cause overheating on sunny days. Alternatively, you can construct a cage around us and fill it with loose, insulating material like oak leaves or straw. The goal is to break the force of the drying wind and to shade our stems from the intense winter sun, thereby drastically reducing moisture loss.
