Photo 1 is hydrangea arborescens before pruning which has become floppy and spread beyond its intended range. Many of the outer canes are bending and need to be removed. Photo 2 shows the same hydrangea after drastic reduction in height as well as diameter of the clump and thinning for appropriate spacing of canes. Many new basal shoots will emerge this season.
Photo 3 is a closeup shot from the other side before pruning. Notice the density of growth and the bent canes. Photo 4 is the same plant after pruning. The remaining canes are strong and straight. It should be noted that hydrangea arborescens will develop flower buds on this season's growth, so they can be pruned back as low as 6" and they will flower beautifully the same season. Some people cut them all the way to the ground, but I believe it's best to keep them at least 6" high for stronger cane development. Hydrangea macrophylla, comprised mainly of lacecap and mophead forms, cannot be pruned in this manner because they develop their flower buds on last year's growth.