Answer: For the least amount of stress to your hydrangea, move it in late fall or early winter while it is dormant. Depending upon how long it has been in its present location, the root system could be extensive and you'll want to keep the rootmass intact as much as possible when you dig. I'd start digging at the dripline, going down about 12". You'll be able to tell where the major roots are once you start digging. After replanting be sure to water your hydrangea well to help settle the soil.
Best wishes with your garden!
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