The need to cover hydrangeas for frost is a common concern for gardeners, particularly during the unpredictable temperature swings of early spring. Frost protection is not always necessary and depends heavily on the specific type of hydrangea you are growing and its current stage of seasonal growth. A “frost” in the context of plant damage refers to the formation of ice crystals on plant tissue when the air temperature drops to or below 32°F (0°C). Protecting the plant’s developing buds and tender new shoots from this rapid freezing is the primary goal of covering.
Understanding Hydrangea Vulnerability to Cold
Hydrangeas vary significantly in their susceptibility to cold temperatures. Varieties like Bigleaf (Hydrangea macrophylla) and Oakleaf (Hydrangea quercifolia) bloom on “old wood,” meaning their flower buds for the coming season are set on the previous year’s stems. These dormant buds are relatively hardy during the deep cold of winter, but they become vulnerable to damage when warm spring temperatures encourage them to swell and begin growth. A late spring frost can easily kill these newly awakened buds, resulting in a season with few or no flowers.
In contrast, Smooth (Hydrangea arborescens) and Panicle (Hydrangea paniculata) hydrangeas bloom on “new wood.” These types are far more cold-tolerant because the flower buds have not yet formed when the late frosts occur. Even if the new foliage is damaged by a freeze, the plant will produce new stems and set new flower buds later in the season, ensuring a reliable summer bloom. Therefore, temporary frost protection efforts should be focused almost entirely on the old wood varieties when new growth has emerged.
Critical Temperatures and Timing for Covering
The most critical temperature threshold is 32°F (0°C) when new leaves or flower buds have begun to emerge after the plants have broken dormancy. Tender, new spring growth contains a high water content, making its cell structure easily ruptured by the expansion of freezing water. Even a light frost can cause significant damage to the plant’s upper portions and destroy the developing flower buds.
The need for covering is highest during late spring freezes, especially after a period of unseasonably warm weather has stimulated the plant’s growth. Monitoring local forecasts is important, particularly when temperatures are predicted to dip into the high 20s Fahrenheit (-3 to -1°C). The duration of the cold snap also affects the level of damage; a brief dip below freezing is less harmful than sustained temperatures well below 32°F. While temporary protection is focused on the spring, a sudden, sharp freeze in early fall before the plant has fully hardened off for winter can also cause tip damage to late-season growth.
Step-by-Step Guide for Temporary Frost Protection
Temporary frost protection is a necessary, short-term measure designed to shield tender growth from overnight temperature drops. The chosen material must be breathable to allow air circulation and prevent moisture buildup, which can exacerbate cold damage. Suitable materials include frost blankets, horticultural fleece, burlap, or old bed sheets and towels. Plastic sheeting is not recommended for direct contact with the foliage, as it offers little insulation and can trap moisture, leading to tissue damage.
To apply the cover, drape the material completely over the hydrangea, ensuring it reaches the ground. Tenting the material so it does not rest directly on the new leaves and buds creates a small insulating air pocket around the plant. This trapped air, combined with residual warmth radiating from the soil, helps keep the air temperature directly surrounding the plant a few degrees warmer than the outside air. Secure the edges of the cover to the ground with rocks, bricks, or stakes to prevent wind from blowing it off and maximize heat retention.
Prompt removal of the covering the following morning is important once temperatures have risen above freezing. Leaving the material on for too long can cause the plant to overheat quickly in direct sunlight, especially if the material is dark. Excess heat and moisture under the cover can cause more damage to the tender growth than the frost itself. This temporary covering method should only be used for a few nights at a time during unexpected cold snaps.
Assessing and Treating Frost Damage
Once the threat of frost has passed, wait to assess the full extent of the damage. Visual signs of frost injury include new leaves and shoots that appear blackened, wilted, or collapsed. The stems themselves may appear discolored, turning pale or dark brown at the tips. Wait several weeks after the last predicted frost date before pruning to allow the plant to clearly delineate the dead tissue from the healthy wood.
Pruning too early can stimulate new, vulnerable growth that may be damaged by subsequent cold snaps. Use a fingernail to gently scratch the bark of the affected stems, starting from the tip and moving down. If the tissue underneath is green, the cane is still viable; if it is brown or dried out, the cane is dead and should be removed. Cut the damaged canes back to a healthy green bud or a set of leaves.
Be patient with hydrangeas that bloom on old wood, as the entire plant may look ragged, but some flower buds lower on the stem may still be intact. Provide adequate water and a light feeding of a balanced fertilizer to help the plant recover its foliage. The loss of the primary flower buds means the flowering potential for the current season is likely compromised.