Protecting bigleaf hydrangeas from winter
By David Sprague
OSU Extension master gardener volunteer
Mention hydrangea and what most people picture is the bigleaf hydrangea (Hydrangea macrophylla). This is a large rounded plant with masses of huge snowball-shaped flowers of either pink or blue and large, soft-green leaves. These hydrangeas are stunning garden plants grown either as single specimens or in masses. Most of the hydrangeas people buy from garden centers are macrophylla. However, these impressive plants can be damaged or even killed by our low winter temperatures. Bigleaf hydrangeas trace their origins back to costal areas of Honshu Island, Japan. Winter temperatures in coastal Japan can go below freezing, but they never experience below- zero weather. Bigleaf hydrangeas have never developed a high tolerance to very low temperatures.
The usual problem people notice with their bigleaf hydrangeas after a cold winter is that there are few or no blooms the following summer. Bigleaf hydrangeas form their flower buds in late summer on old wood for next year’s blooms. Cold temperatures can freeze and kill these buds, thus eliminating the blooms that would form next year. This is also the reason you do not want to prune bigleaf hydrangeas after mid-summer, because you may cut off next year’s blooms.
Since we can’t do anything about our cold winters, we need to take steps to protect our bigleaf hydrangeas. One way is to grow them in containers. They make excellent container plants, and the containers can be moved to a protected area for the winter. If planted in the ground, the hydrangeas will need to be covered for the winter. One of the best ways to protect them is to make a cylinder of wire fencing around the plant and fill the cylinder with leaves in the fall. I’ve found that shredded leaves work better than whole leaves because they fill in around the plant better. Try to cover the entire plant with the leaves. For larger hydrangeas you will need a cylinder at least 4 feet high supported by stakes. Next spring take the cylinder down and remove the leaves, but don’t remove the protection too early because any hard freezes can still damage the buds.
As impressive as the bigleaf hydrangeas are, other species are more cold-hardy and are also beautiful plants. Oakleaf hydrangeas are native to North America and tolerate our winter weather. Smooth hydrangea (Annabelle and Incredibelle Æ are popular cultivars) and climbing hydrangea are good choices for our area. Perhaps the hardiest hydrangea is the Panicle Hydrangea (Hydrangea paniculata). Some panicle cultivars are Limelight, Tardiva and Little Lime. In the last several years, new varieties of the bigleaf hydrangea have been introduced. Called Remontant or ever-blooming, these hydrangeas bloom on new wood as well as old wood. Field tests for cold hardiness with these hydrangeas have been mixed, and to ensure successful blooms for next year, they should be protected for the winter. Spring fertilization will assist with plant rejuvenation when plants are not protected in winter.