Hydrangeas are popular garden shrubs, but their ability to withstand cold temperatures and still produce flowers is not uniform across all types. The specific species determines its vulnerability to frost, particularly a late spring frost that occurs after the plant has begun seasonal growth. While the root system is generally resilient, a sudden drop in temperature can easily compromise delicate flower buds, leading to a year without blooms. Understanding how various hydrangeas form their flower buds is key to predicting survival and potential for flowering.
The Key Differentiator: Species and Bud Location
A hydrangea’s vulnerability to frost damage depends on the location and timing of its flower bud formation. Some types, including Bigleaf and Oakleaf hydrangeas, are known as “old wood” bloomers. They develop their flower buds on the previous year’s growth, typically in late summer or fall. These fully formed buds remain exposed on the stems throughout winter and early spring, making them highly susceptible to damage from an unseasonable freeze. If a late frost damages these buds after growth has initiated, the plant will survive and leaf out, but the season’s flower production will be lost.
Conversely, types such as Panicle and Smooth hydrangeas are classified as “new wood” bloomers. They produce their flower buds on the stems that grow in the current season. These plants are less vulnerable to spring frosts because the flower buds do not exist when the cold snap occurs. Even if the initial new foliage is damaged by a freeze, the plant will generate new stems and buds that will still flower later in the summer. This difference in timing gives new wood bloomers an advantage in regions with unpredictable spring weather.
Protecting Vulnerable Plants During Unexpected Frost
When a late spring frost is predicted after hydrangeas have begun to actively sprout, action is needed to protect vulnerable buds. One effective short-term measure involves deeply watering the soil around the plant a day or two before the freeze is expected. Moist soil retains heat better than dry soil, radiating warmth upward throughout the night to create a slightly warmer microclimate near the plant’s base.
Physical insulation protects above-ground stems and buds. Gardeners should cover the entire shrub with breathable materials like old bedsheets, burlap, or commercial frost cloth, draping the cover all the way to the ground. Use stakes or a frame to prevent the covering from resting directly on tender new growth, which can cause damage if the material becomes heavy with ice. Applying a thick layer of organic mulch, such as shredded bark or straw, around the base of the plant further insulates the root crown from cold temperatures.
Assessing and Managing Frost Damage
Once the frost has passed, the first sign of damage will be foliage that appears blackened, wilted, or mushy. While the top growth may look severely compromised, the root system is typically unharmed, meaning the plant will likely recover. Resist the urge to immediately prune away the damaged material, as this can expose healthy underlying tissue to a subsequent freeze.
Gardeners should wait several weeks until the threat of frost has ended before pruning. At that time, use a fingernail to scratch the stems. If the tissue beneath the bark is green, the stem is alive; if it is brown, the stem is dead and should be removed. Pruning back to the first set of healthy, viable buds will help the plant redirect its energy toward new growth. Even if the flower buds were lost on old wood varieties, the plant will focus on developing a strong structure for the following year’s bloom.