Answer: Hydrangeas produce flowers on new shoots which develop on old wood. Prune in the early spring, just as the buds begin to swell, cutting the canes down to 12-18", cutting to an outside facing bud. The new shoots which develop on the old canes will produce flowers. The problem with trying to grow hydrangea's in Michigan is that they will freeze down to soil level each year which means there will be no old wood on which new flowering shoots can develop. While the roots will likely survive, without old wood, your hydrangea will never flower. I'd suggest leaving the bigleaf hydrangea in Louisiana and purchasing a new hydrangea in Michigan. Choose "Endless Summer" or one of the other newer hybrids which flower on both old and new wood. This way, if the plant freezes down to soil level each winter you will still have flowers the following summer.
Best wishes with your move and your new garden!
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