In order to winterize perennial hydrangeas, choose a cover, apply the correct amount of cover after the first few frosts and then gradually remove the cover in the spring. Hydrangeas are a less hardy plant, so if they freeze during the winter, they will not flower in the spring.
Winterizing perennials is essential for their success, but only if the plants are winterized at the right time. Allow the soil to cool down before covering the plant. This should occur after a few frosts in the late fall or early winter. Winterizing too soon will result in premature flowering.
Avoid pruning, as sensitive perennials do not need to be pruned before they are covered, and the extra material works as additional coverage.
Choose an appropriate cover for the plants, such as a cloche or mulch. A plastic or glass cloche works well to protect the plant's crown. If a cloche is not available or not desirable, cover the perennials with mulch. If using a cloche to cover the plants, cover the cloche in pine boughs to keep the plant from getting too hot on sunny days. If using mulch, apply only 6 to 8 inches, as too much can cause the plants to rot.
As temperatures begin to warm up in the spring, gradually remove the cover from the plants to allow for a smooth transition and to avoid shocking the plants.