Experts and BHG readers answer.
Overwintering Nikko Blue Hydrangea- Zone 5
This is an old problem for hydrangea lovers in colder zones, and everyone seems to have their own methods of providing winter protection for their shrubs. Basically, you need to protect the young buds on this year's branches that should bloom in 2008.
Build some sort of a structure (4 heavy stakes for example) around the plant that can support burlap or landscaping cloth filled with dry leaves or straw inside. Avoid using plantic as temperatures heat up inside during January thaws. Some people use upturned metal trash cans perforated with lots of holes in the sides, others make a cage of wire mesh. Wrap the burlap round the stakes and fill the inside with dry leaves. Oak is especially good, maple tend to mat down and get soggy. Be sure that the branches of the hydrangea are completely covered and inspect at intervals through the winter to see if the leaves have settled and exposed the ends of the stems. Add leaves as necessary. However you do it, be alert for rodents and other critters who may think they've found the ideal winter nest. It is wise to delay the ""swaddling"" until the ground freezes.