The question of whether hydrangeas are hardy depends entirely on their species and your location. In gardening, “hardiness” refers to a plant’s capacity to survive adverse climatic conditions, particularly winter temperatures, without significant damage. Hydrangeas, as a diverse genus, exhibit a wide spectrum of cold tolerance, ranging from those that thrive in frigid northern climates to those requiring mild winters for successful blooming. Understanding the specific temperature thresholds for each variety is necessary before selecting a plant.
Understanding Hardiness Zones
Gardeners rely on the USDA Plant Hardiness Zone Map to determine which perennial plants are likely to survive the winter in a specific area. This map divides the United States and other regions into 13 zones based on the average annual extreme minimum winter temperature. Each zone represents a 10-degree Fahrenheit band, split into ‘a’ and ‘b’ subzones for finer detail.
The map’s data provides a reliable guide for predicting a plant’s winter survival. When a plant is labeled as “hardy to Zone 5,” it means it can tolerate the minimum temperatures recorded for that zone and all warmer zones. Matching a hydrangea’s listed hardiness range to your local zone is the foundational step in ensuring its long-term success.
Hardiness Based on Hydrangea Species
The greatest difference in cold tolerance is found among hydrangea types, particularly concerning where they form their flower buds. The most resilient species are the Panicle Hydrangea (Hydrangea paniculata) and the Smooth Hydrangea (Hydrangea arborescens), which thrive in USDA Zone 3. These “new wood” bloomers form flower buds on the growth produced during the current season. Because the buds do not have to survive the winter, these hydrangeas reliably produce flowers even if the previous season’s stems freeze back.
The Bigleaf Hydrangea (Hydrangea macrophylla), including mophead and lacecap types, is less cold-tolerant, generally hardy only from Zone 5 or 6 to Zone 9. Many traditional Bigleaf varieties are “old wood” bloomers, forming buds on woody stems that survive the winter. If a cold snap kills these over-wintering buds, the plant survives but will not flower that season. Newer cultivars, called rebloomers, produce buds on both old and new wood, ensuring blooms even after a harsh winter.
The Oakleaf Hydrangea (Hydrangea quercifolia) falls in the middle, typically hardy in Zones 5 to 9. This species also blooms on old wood, but its buds are naturally more robust and winter-hardy than the Bigleaf types. While stems may suffer dieback in the coldest parts of its range, the plant often retains enough flower buds to bloom reliably.
Protecting Borderline Hardy Varieties
Gardeners in colder zones, especially near the northern edge of a hydrangea’s range, can take steps to increase survival and flowering potential. The primary goal of winter protection is to insulate the dormant flower buds and root crown from extreme cold and damaging freeze-thaw cycles. Applying a thick layer of mulch is a highly effective first line of defense.
After the ground freezes in late fall, apply a 6- to 8-inch layer of straw, shredded leaves, or marsh hay around the base of Bigleaf Hydrangeas to insulate the root zone. For above-ground protection, construct a barrier around the shrub using chicken wire or stakes and burlap. The space within this cylinder can be loosely filled with insulating materials, such as dried leaves or straw, to shield the stems and buds from winter winds.
Choosing an appropriate planting location can significantly reduce winter damage. Hydrangeas planted in a spot sheltered from harsh winter winds, such as near a wall or fence, are less likely to suffer bud dieback. Avoiding southern exposures that receive intense winter sun is also advisable, as this can cause the plant to warm prematurely during the day and suffer damage from rapid refreezing at night. These measures are most beneficial for sensitive Bigleaf Hydrangeas, helping them conserve the buds necessary for the following summer’s floral display.