Indoor hydrangeas rarely survive winters and are considered temporary plants to enjoy while they last. Adventurous gardeners can try to keep their hydrangeas going from year to year by simulating the changes of season they would experience in nature. Here's how:
After they have finished blooming, cut each branch back several inches.
As soon as possible, move the plant outside to an area that gets lots of light, but no direct afternoon sun.
Fertilize the plant every week through late summer.
It's best to keep the plant outdoors for as long as possible, although the plant cannot tolerate heavy frost.
Protect the plant from light frost by covering or moving it inside temporarily so that it can stay out a few weeks longer.
When a hard frost is eminent, move the hydrangea indoors to a very cool location, gradually increasing the warmth over the next month or two.
With luck, the plant will have set buds during the cool weather outdoors and will proceed to bloom inside.