A:   Most or all Hydrangea Macrophylla (mop heads)should be selectively cut back after blooming around mid June.  Two species, Hydrangea Paniculata and Hydrangea Arborescens (pee gee's), both bloom on new growth.  They can be cut back in winter.  Hydrangea Quercifolia (oakleaf hydrangeas) set buds and bloom on old growth so they should should only be pruned when absolutely necessary to remove dead wood.