Mountain Hydrangeas bloom on old wood so, if you a pruning them in late summer or fall, you are removing the buds for the following spring’s flowers. In winter, deer will often nibble the nutritious buds off preventing spring flowering. Also, a tough winter can also damage the buds so a burlap windbreak installed in the fall may be necessary to protect the hydrangea from the drying winter winds. Here’s a link you might find helpful: http://www.mortonarb.org/trees-plants/tree-plant-descriptions/mountain-hydrangea