Answer: You might be surprised just how much neglect a hydrangea can take. Since you've already planted them, all you need to do is to water deeply once each week until fall rains take over the job for you. The roots will establish themselves as long as the soil temperatures remain above freezing. When the ground freezes the roots will suspend their growth but they'll pick right up where they left off as soon as the soil warms in the spring.
You should see some new growth in the spring. When that begins you can prune out any winter kill and your hydrangeas should be just fine. It would be worth your while to find out exactly what kinds of hydrangeas you've planted. Some bloom on new wood and some bloom on old wood. If yours is the kind that blooms on new shoots which develop on old wood, and if the plants die down to ground level each winter, you'll get lots of lush, green growth but no flowers. If yours is the kind that blooms on new wood or on both new and old wood, it won't matter if the plants die down to ground level each year - they'll still bloom for you. So it will pay to do some research and make sure that the plants you were given were performing well in your friend's yard.
Best wishes with your new plants!
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