The common hydrangeas grown here belong to the macrophylla family. These give us most of the traditional mop-tops along with the flat heads of many lace-cap varieties. Less common hydrangea species (the ones with oak leaves, cone flower heads, evergreens and the like) often have different pruning requirements.
1) It is not essential to prune hydrangeas. They will still flower if not pruned but you will usually get many small flowers on a bush which grows ever larger. Pruning takes place to keep the bush smaller and tidier and to encourage bigger blooms.
2) Most hydrangea stems will have a series of buds in pairs visible down their length. The fat buds are flower buds. The thin, small buds are leaf buds. Ideally you want pairs of fat buds, because that will be two flower heads but sometimes you find one fat bud paired with a small leaf bud. You will only get one flower from that spike.
3) Using secateurs, prune back to the lowest pair of fat buds. If that is still much taller than you want, trim back to the lowest single fat bud.
4) After you have reduced the height of each stem, look at the clump and take out any really old, thick, woody stems and any spindly weak ones. You can also take out stems which are headed sideways and those with no flower buds if you want to keep your bush more compact.
5) Because most hydrangeas flower on last year’s growth, if you cut too low down and without taking any account of the difference between leaf buds and flower buds, you will have cut all this summer’s flowers off. You can cut off near to ground level if you want to rejuvenate an old plant and it will shoot again but you will have to wait 18 months for flowers. We have pruned for flowers on this plant.