Answer: Mulch it with several inches of organic mulch over the root area in late fall after it has gone dormant for the season. If it is in a windy location, provide a windbreak for it this winter. Wait until spring to prune. Next spring remove any winter killed branches. It will regrow and bloom in late summer.
Endless Summer is special in that it is able to bloom on both old and new growth of the season. It blooms early in the season on buds formed on last year's branches and then blooms later on in the season (until fall) on the new growth of the current year. This means it is able to bloom even in very cold winter climates such as yours -- where the old branches typically are lost to winter damage every year.
If you want to try to protect the current branches to maximize bloom for next year, you will need to enclose it in a cylinder of wire mesh, wrap the outside in burlap, stuff the inside with a dry nonpacking insulating material such as straw or oak leaves, and top that off with plastic or a tarp to keep it dry inside. There should be some gaps in the sides to allow for air flow and to prevent condensation. Put this on in late fall after it is dormant, and remove it in the spring so it can wake up naturally with the season. Also be sure to cover it if a spring frost threatens after the buds have begun to swell.
Emjoy your hydrangea!
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