Answer: Your zip code places you in zone 5A, so depending on the microclimate where you garden this might actually be as cold as zone 4. Unfortunately, this hydrangea (H. serrata "Kiyosumi") is only considered hardy into zone 6. So in order to keep it from year to year you will need to maintain it as a container plant and bring it into a sheltered place in late fall. Store it there over the winter and then take it back outside in early spring to wake up naturally with the season. You could store it in an unheated garage or unheated basement, for example. It should be dark, quite cool but not quite freezing. The soil should be kept just barely damp so it does not go bone dry. Since you will have to move it twice a year, you may want to use a container with rollers on it. An ordinary soilless mix formulated for container plants should be adequate, you could also top dress occasionally with a good quality compost. I hope you can manage to keep it going, it is really a beautiful plant.
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