As a hydrangea plant, my survival through winter is a complex biological process. I am not truly dormant; I am in a state of quiescence, patiently waiting for the signal of spring warmth. My primary vulnerabilities are my delicate root system, which is sensitive to frozen soil, and my precious flower buds, which are formed on old wood (last year's stems) in many popular varieties like the mophead and lacecap types (Hydrangea macrophylla). A hard frost can easily kill these buds, meaning I will not be able to produce those beautiful blooms you cherish so much come summer.
Your actions in late summer and early autumn are crucial for my internal preparation. Please stop applying nitrogen-rich fertilizers by mid-August. This discourages me from producing tender new growth that would be instantly killed by the first frost, wasting my stored energy. Instead, allow me to naturally slow down and begin to harden off. This process involves moving water out of my cells and into the spaces between them, and converting starches into sugars which act as a natural antifreeze. This protects my cellular structures from freezing temperatures. Withholding water slightly in the fall can encourage this process, but do not let me become severely drought-stressed.
My roots are my lifeline. While my stems may die back, if my roots survive, I will regenerate. The best protection you can offer is a thick, fluffy layer of mulch applied after the ground has frozen hard for the first time, typically after a few hard frosts. This timing is key; applying it too early can attract rodents seeking a warm winter home and delay my natural dormancy. Pile 6-8 inches of loose, airy material like pine bark, straw, or shredded leaves over my root zone, extending out to my drip line. Do not pack it down. This layer acts as a blanket, keeping the soil consistently frozen and preventing the damaging cycle of freeze-thaw-freeze that can heave my roots right out of the ground.
For those of us who bloom on old wood, protecting the stems and the crown (the base of the plant where stems meet roots) is essential. After the first killing frost, you can build a simple cage around me using chicken wire or stakes and burlap. Fill this cage with more loose insulating material like oak leaves or straw. This protects the buds on the lower stems from harsh, drying winds and extreme cold. An alternative method is to loosely tie my stems together and then wrap them with burlap or frost cloth, again creating a protected air pocket. Avoid using plastic sheeting, as it can trap solar heat and cook my stems on a sunny day.
My roots in a container are exponentially more exposed to the cold than those in the ground. The entire pot can freeze solid. The best strategy is to move my container into an unheated garage, shed, or cold frame for the winter. I do not need light while dormant, but I do need occasional slight moisture; check the soil monthly and water lightly if it is bone dry. If I must stay outside, cluster my pot with others, mulch heavily on top of the soil, and wrap the entire pot in insulating bubble wrap or burlap, placing it in a sheltered location out of the wind.
