Many gardeners make the error of pruning back hydrangeas in winter when they are dormant. This initiates healthy new spring growth, but unfortunately no flower buds are produced by the time summer flowering season arrives.
Hydrangeas must be pruned immediately after flowering is complete, which is usually around mid-January to mid-February. Firstly, remove all spent flower heads by cutting the stem back to a strong double bud (dormant growth buds on the side of the stem arranged in opposite pairs). Then remove any dead wood on the plant. Lastly, cut out any weak or straggly growth.
Remove spindly stems by cutting them down to ground level. This process will leave a network of strong, healthy stems to set flower buds for the blooming season.
After pruning, apply a dressing of general purpose garden fertiliser (chemical or organic depending on your personal preference). A thick mulch layer over the surface of the roots is highly beneficial. Use pine needles, pine park or leaf mould for this purpose. Water well until the onset of autumn when growth slows down. This should ensure a bumper amount of top quality flowers in the summer of 2019.