Do not prune hydrangeas now, unless you do not want flowers on your florist or mophead type hydrangeas this year. These are the early summer bloomers that have pink or blue flowers, and variations of these colors depending on the whether the soil is acid or alkaline. The reason is that this particular hydrangea produces its flowers on old wood, unlike the Pee Gee and Hills-of-Snow types that bloom mid to late summer. So if you cut the stems back over the winter instead of just removing the old blooms (which is called deadheading) you will have few if any blooms this year. You should prune this particular hydrangea immediately after it blooms. This is why these plants are excellent to cut their blooms for arrangements, as it is essentially pruning them at the right time. Because the weather in the next few days is going to be exceptionally warm you will see new foliage appearing on these plants. So you can see that if you prune them back, you will be cutting back their new foliage too. I would not do anything with these plants until about April 15. Then I would remove any visible dead stems and fertilize them with a product like 12-6-6. I would repeat the fertilizer application again around May 15 to get maximum bloom size on the plant.