Winterizing a hydrangea involves preparing the shrub to endure freezing temperatures and winter damage, primarily by protecting the flower buds and the root crown. This preparation ensures the plant’s survival and, for some varieties, guarantees the return of blooms the following season. The goal is to encourage a deep, healthy dormancy while insulating the most tender parts of the plant from extreme cold and drying winter winds.
Preparing for Cold Weather
Preparation for winter begins in late summer to early fall, well before the first hard frost arrives. The first step involves ceasing all nitrogen-heavy fertilization, typically by late July or early August. This stops stimulating new, tender growth that is highly susceptible to freezing damage. Hydrangeas must be encouraged to “harden off” and direct their energy toward dormancy.
Deep watering is an important pre-winter action, especially in dry autumns, as well-hydrated stems and roots tolerate cold better. Water the plant slowly and deeply until the ground freezes, ensuring the root system has sufficient moisture reserves to combat the desiccating effects of winter wind. Do not prune the stems or remove spent flower heads in the fall, particularly on varieties that bloom on previous year’s growth. The existing structure provides a layer of natural insulation and protects the dormant buds.
Methods for Physical Protection
Physical protection methods are employed in late fall, often after the ground has frozen, to insulate the plant’s vulnerable parts. Root protection is achieved by mounding a thick layer of mulch around the base of the plant, extending out to the drip line. This crown-mounding should be six to twelve inches deep, using materials like shredded bark, straw, or pine needles. This stabilizes soil temperature and prevents the heaving caused by freeze-thaw cycles.
To protect the stems and buds above ground, caging and filling the shrub with insulating material is an effective technique. A cylindrical cage, often constructed from chicken wire or hardware cloth, is placed around the plant and secured to the ground with stakes. This enclosure is then loosely filled with dry, organic material such as shredded leaves or straw. This material acts as a thermal buffer against harsh temperatures and wind.
For smaller shrubs or newly planted specimens, wrapping the plant in breathable fabric offers additional defense. Burlap or specialized frost cloth can be loosely draped over the cage or directly around the stems, secured with twine. This reduces damage from icy winds and sun scald. Avoid using plastic, which traps moisture and can encourage rot or disease beneath the cover.
Adjusting Care Based on Hydrangea Type
The specific winterizing strategy depends on whether a hydrangea blooms on “old wood” or “new wood,” which determines where the flower buds are located during winter. Bigleaf hydrangeas (Hydrangea macrophylla), including mophead and lacecap varieties, typically bloom on buds set on the previous year’s growth (old wood). This makes them the most vulnerable to winterkill. For these hydrangeas, full stem protection is necessary, requiring complete wrapping or caging to ensure the survival of the tender terminal buds.
In contrast, Panicle hydrangeas (Hydrangea paniculata) and Smooth hydrangeas (Hydrangea arborescens) bloom on new wood, forming flower buds on the stems that grow in the current spring. These varieties are naturally hardier, and their lack of exposed flower buds means they require minimal intervention. Winterizing these types is typically limited to applying a deep layer of root-insulating mulch around the base of the plant.
Even though Panicle and Smooth hydrangeas are hardier, they can still experience branch die-back in extremely cold conditions. For younger plants or in extreme cold, adding a lighter layer of stem protection, like an open A-frame or light wrapping, can prevent structural damage. The distinction between old wood and new wood varieties informs the scale of physical protection needed, prioritizing full enclosure for Bigleaf types to save their dormant flower buds.
Transitioning Out of Winter Dormancy
The removal of winter protection must be timed carefully to avoid shocking the plant or exposing new growth to a late frost. Wait until the threat of the last hard frost has completely passed and consistent springtime temperatures have settled in, often in mid to late spring. Removing insulation too early can expose dormant buds to freezing temperatures. Removing it too late risks encouraging premature, tender growth in the sheltered, warm environment.
Begin by gradually removing the protective layers, such as the burlap wrap and the insulating fill, over a few days. This helps the plant acclimate to the brighter light and cooler air. Once the insulation is gone, gently rake back the deep layer of mounded mulch from the crown of the plant, leaving a two- to three-inch layer to retain moisture and suppress weeds. Assess the stems for winter damage, looking for wood that is cracked, brittle, or completely brown. Prune this dead wood back to a healthy bud or to the ground. Initial spring watering should be deep and thorough to rehydrate the roots as the plant prepares to break dormancy.