Hydrangeas generally do not die in the winter, but instead enter a biological state known as dormancy. This process is a survival mechanism that allows the plant to withstand cold temperatures and reduced sunlight by slowing its metabolic functions to a near halt. The plant sheds its leaves and the stems become woody, giving the shrub the appearance of being lifeless. The continued survival of the plant relies heavily on the specific hydrangea type and the severity of the local climate.
Understanding Dormancy
Dormancy is a natural, annual cycle for woody perennial plants and is fundamentally different from a plant dying. The plant’s above-ground growth stops all visible activity, and the plant focuses its resources on protecting the root system and the buds that will produce next year’s growth. This period of required cold, sometimes referred to as vernalization, must occur for the plant to properly flower in the spring.
During this time, the plant is conserving energy, which is why the stems look like dry, brown sticks. The hardiness of the root crown, the area where the stems meet the soil, determines the plant’s overall survival. Even if the stems above the ground are damaged by extreme cold, the plant will often regrow from the crown if it remains healthy. The chilling requirement for the flower buds of some hydrangeas, like Hydrangea macrophylla, is approximately 6 to 8 weeks at temperatures just above freezing (34 to 36 degrees Fahrenheit).
Hydrangea Types and Cold Tolerance
The level of winter protection a hydrangea needs depends on its species and whether it sets flower buds on “old wood” or “new wood.” The Bigleaf Hydrangea (Hydrangea macrophylla), which includes Mophead and Lacecap types, is the least reliably hardy in cold climates (down to USDA Zone 5). This type blooms on old wood, meaning it forms flower buds on the previous season’s stems, making these buds vulnerable to deep winter cold or late spring frosts.
The Panicle Hydrangea (Hydrangea paniculata), such as ‘Limelight’, and the Smooth Hydrangea (Hydrangea arborescens), like ‘Annabelle’, are significantly more cold-tolerant, thriving reliably down to USDA Zone 3. These varieties bloom on new wood, producing flower buds on the growth that emerges in the current spring. Because the new wood does not exist in winter, the plant can die back to the ground without compromising the next season’s bloom, making them the most dependable bloomers after a harsh winter.
Fall Preparation for Winter Survival
Preparing a hydrangea for winter begins in the fall, focusing on conditioning the plant for dormancy. A crucial step is to stop applying nitrogen-rich fertilizers by late summer or early fall. Continuing to fertilize late encourages the plant to produce tender, new growth that will not have time to harden before the first frost, making it susceptible to cold damage.
Proper hydration before the ground freezes is also important. Hydrangeas should receive a deep watering in late fall, especially if rainfall has been scarce. Adequate moisture helps the plant’s root system retain hydration throughout the cold, dry winter months. Major pruning should be avoided in the fall, especially for old wood bloomers, as this removes the flower buds set for the following year.
Physical Protection Methods
Once the plant has entered dormancy and the ground begins to freeze, active measures can be taken to physically protect the most vulnerable parts of the shrub. For Bigleaf Hydrangas, which require protection to save their old wood flower buds, deep mulching around the base is the minimum requirement. Applying a 4-to-12-inch layer of organic material, such as shredded leaves, straw, or wood chips, insulates the root crown and regulates soil temperature, preventing the cycle of freezing and thawing that can damage roots.
More comprehensive protection for old wood varieties involves creating a physical barrier around the entire shrub. This can be achieved by constructing a cage using wire fencing or wooden stakes and then wrapping the structure in burlap. The inside of this enclosure is often filled with insulating material like dry oak leaves or straw, which helps to shield the dormant buds on the stems from harsh winter winds and the lowest temperatures. This deep insulation is not necessary for the hardier Panicle and Smooth hydrangeas, whose survival is guaranteed by the new wood blooming habit.
Assessing Plant Health in Spring
After the threat of frost has passed, gardeners must assess the plant to determine which parts survived the winter. The most reliable method for checking stem health is the “scratch test.” By lightly scratching the bark, a gardener can look for the color of the tissue underneath.
If the exposed tissue is bright green, the stem is alive and should be left intact, as it holds potential flower buds. If the tissue is brown or gray, the stem is dead and can be pruned back to a point where green tissue is visible or cut back to the ground. It is important to be patient, as hydrangeas can be slow to emerge from dormancy, and new growth may first appear low on the stems or directly from the crown. Pruning should be limited to dead, diseased, or broken wood until new growth is clearly visible.