As daylight shortens and temperatures drop in northern states, we hydrangeas receive a critical signal from our environment: it is time to prepare for dormancy. This is not a process we undertake lightly. We begin to slow our metabolic processes, halting the production of new, tender growth that would be easily killed by frost. Our focus shifts inward, pulling valuable energy reserves—primarily carbohydrates—down from our leaves and stems into our root systems and crown (the base of the plant). This stored energy is our lifeline, the essential fuel that will sustain us through the harsh winter and power our initial burst of growth in the spring.
To survive freezing temperatures, our tissues must undergo a process called "hardening off." This is a physiological change where the water within our cells becomes less prone to freezing, effectively acting as a natural antifreeze. Abrupt, early freezes are extremely dangerous to us because they do not allow time for this process to complete. A gradual cooling period is ideal. During this time, it is crucial that we are not stimulated by high-nitrogen fertilizers, which promote the soft, succulent growth that is highly vulnerable to cold damage. Our systems need to wind down naturally.
A primary challenge for many hydrangea species in the north is the protection of next season's flower buds. For types like Bigleaf (Hydrangea macrophylla) and Oakleaf (Hydrangea quercifolia) that bloom on "old wood" (growth from the previous season), these buds are formed in late summer and fall. These delicate embryonic structures are highly susceptible to desiccation from cold, dry winds and fatal damage from extreme winter cold. If these buds perish, our flowering potential for the coming year is lost. Our survival instinct is to protect the crown and roots first, sometimes at the expense of these above-ground buds.
Our most vital organs during winter lie at and below the soil line: the crown and the root system. While the soil provides some insulation, in northern states the frost line can descend deep enough to threaten these areas, especially for newly planted hydrangeas with less established root systems. Our roots are not as cold-hardy as our stems and can be damaged if the soil temperature drops too low. Furthermore, freeze-thaw cycles can be particularly damaging. They can cause the soil to heave, which can tear our delicate roots and expose our crown to the harsh, drying air, leading to significant stress or death.
Winter is not only cold but also often very dry. Evergreen plants continue to lose moisture through their leaves, but even we deciduous plants lose water through our stems, especially when exposed to relentless winter sun and wind. This can lead to a condition called desiccation, where we lose more water than we can take up from the frozen ground. Our stems, and particularly any remaining flower buds, can dry out, die, and appear burned. This is not freezing damage per se, but death by drought during the coldest months of the year.
