The need to cover hydrangeas for winter depends entirely on your specific climate and the variety you are cultivating. Winterizing these shrubs primarily protects the flower buds that will produce the next season’s blooms. For many varieties, the challenge is ensuring these already-formed buds survive the colder temperatures and fluctuating conditions of winter. The necessity of physical covering is a variable decision determined by your garden’s location and the plant’s unique biological needs.
Deciding Factors: Plant Type and Hardiness Zone
The most significant factor determining the need for winter cover is whether your hydrangea blooms on “old wood” or “new wood.” Hydrangeas that bloom on old wood, such as Bigleaf (Hydrangea macrophylla) and Oakleaf (H. quercifolia), form their flower buds on the previous year’s growth during late summer and fall. These delicate, fully formed buds must survive the entire winter on the exposed stems to produce flowers the following summer. If the buds freeze and die, the plant will survive but will likely not bloom, leading to a “bloomless” year.
For old wood bloomers, winter protection is necessary, particularly in USDA Hardiness Zones 6 and colder, or in areas with unpredictable temperature swings. The buds are susceptible to damage when temperatures consistently drop below 0°F to 10°F. Conversely, new wood bloomers, including Smooth (H. arborescens) and Panicle (H. paniculata) hydrangeas, are hardier because they produce flower buds on growth that emerges in the current spring. These varieties rarely require physical covering, as their flower production is not threatened by cold weather, though a heavy layer of mulch is beneficial for root insulation.
Pre-Dormancy Preparation
Every hydrangea benefits from simple pre-dormancy practices that prepare it for the winter rest period. The primary step is to cease all nitrogen-heavy fertilization by late summer, typically no later than the end of August. Applying fertilizer too late encourages a flush of tender, new growth that is highly susceptible to damage from the first hard frost, compromising the plant’s ability to enter dormancy.
Maintain adequate soil moisture until the ground completely freezes solid. Hydrangeas require water even when dormant, and deep watering in late fall helps ensure their roots are hydrated, protecting the plant cells from desiccation during deep freezes. Also, remove any fallen leaves or plant debris from the crown and base of the shrub. This minimizes the risk of fungal diseases or pest infestation developing over winter.
Physical Protection Techniques
For old wood hydrangeas in colder climates, physical protection is necessary to insulate the vulnerable flower buds and the crown of the plant. One common method is mounding, which involves piling organic material like shredded leaves, straw, or wood chips directly around the base of the plant. This mound should be built up to a height of about 12 inches to protect the graft union and the lower, most insulated stems. Mounding is most effective at protecting the roots and the crown, where new shoots will emerge, but it may not save the higher flower buds.
To protect the entire shrub and its stems, a caging method is often employed, using stakes or wire fencing to create a cylinder around the plant. This structure is then loosely filled with dry, insulating materials such as oak leaves or pine straw, which maintain air pockets for insulation and do not compact easily. The goal is to create an air-filled, insulated chamber around the stems. The outer cage is often wrapped in burlap to contain the insulation and provide a windbreak.
A third technique, burlap wrapping, is suitable for smaller shrubs or individual stems, where a breathable fabric is loosely wrapped and secured with twine. Burlap provides a barrier against wind desiccation and sun scald, both of which can damage buds even in mild winter temperatures.
Seasonal Timing: Covering and Uncovering
Covers should not be applied until the hydrangea is fully dormant, which typically occurs after the plant has experienced two or three hard frosts and the leaves have dropped. Placing covers on the plant too early can trap moisture and heat from warmer daytime temperatures, creating an environment conducive to mold, rot, or premature bud break.
The insulation should remain in place throughout the coldest part of winter and be removed gradually in the spring. Removal should be delayed until the persistent danger of hard freezes has passed, often coinciding with mid-to-late spring when temperatures are consistently above freezing. Uncovering the plant too late can cause overheating, while uncovering too early exposes the tender, protected buds to late-season frosts.