Never prune Hydrangea Macrophylla stems now but feel free to deadhead leftover brown blooms at any time that you wish. To deadhead last year's brown blooms, cut above the first pair of leaves when the stem is green or, if the stem is leafless like in the pictures, cut the string that attaches each bloom to the stem. In the picture that was posted, I saw no brown blooms that could be deadheaded.
This species of hydrangea is known as Hydrangea Macrophylla or as Big Leaf Hydrangea, French Hydrangea, Hortensia, Milflores, Christmas Roses, Mophead Hydrangea, Snowball Plant, etc.
It develops dormant flower buds inside the stem ends around late summer/fall and these buds then open in the spring if the stems remain alive. Continue protecting these flower buds at the ends of the stems until your city's average date of last frost. To protect from late frosts now, water deeply the night before; ensure the plant has 3 to 4 inches of organic mulch past the drip line; and use frost cloth or old sheets for small dips below freezing. In areas with a hard freeze, ensure live buds/stems survive by winter protecting from your city's average date of first frost through your average date of last frost. To see the various winter protection methods, do a search in Youtube for "hydrangea winter protection". Do not remove winter protection prior to your city's average date of last frost. To protect from late frosts, start to fertilize after your city's average date of last frost. To protect from early frosts in the fall/winter, your last fertilizer application should be done 3 months before your average date of first frost.
The plant should be sited where it can attain its estimated size at maturity and then its stems should rarely be pruned as the shrub spends most of the year with flower buds or blooming. Leafless stems in spring can be pruned only if they remain leafless by the end of June in PA, not earlier than that. The flower bud at the tip of live stems will bloom this year if the stem is still alive; otherwise, one of the back up flower buds lower down the stem may open instead.
The stems that have no buds on them now will not bloom in spring. They may be dead stems that should have been pruned all the way down by the end of spring 2023 if they still had no leaves. Hard to say if that is the case now. To check if they are alive now, wait until the end of June 2024. These shrubs stems will begin to break dormancy once they have sustained, overnight soil temperatures at or above 50°F. Usually, the shrub first develops new stem growth from the crown/base of stems and many weeks/month later, the live stems leaf out.
Live stems that have buds tend to color change in the fall and again as they break dormancy so it is difficult to determine if a brown flower bud is really dead. If it is alive, the stem will leaf out and you will see tiny, green broccoli heads when it opens and blooms. Broccoli heads that are partly brown may have been injured by late frosts.
