There are several of types of hydrangeas. Some bloom on old wood, some on new and some on both. In zone 5, buds on old wood are often frozen during harsh winter weather. The result is a green plant with few or no flowers. Covering hydrangeas is a trick used to help keep the buds from freezing. There are several types of hydrangeas, each having different requirements.
Care for Macrophylla and Quercifolia
Use large flower pots or styrofoam rose covers filled with leaves to cover smaller plants, placing a brick on top or using landscape staples to secure the cover to the ground. For larger plants, try wrapping with chicken wire or a tarp and filling with leaves or wrapping with bubble type wrap and then chicken wire or a tarp to hold it in place.Care for Arborescens
Care for Arborescens
Annabelle hydrangeas bloom on new wood and in zone 5 are treated as perennials. Cut canes to about 1 ft after leaves drop.
Care for Paniculata
No cover is needed, however a top dressing of mulch is recommended.
If you are not sure what type of hydrangea you have, try looking for pictures of the various kinds on the internet.