I have a large hydrangea hortensia, the common mophead type. It’s 5-6ft high and the flowers are too heavy for the stems so it falls over into a floppy mess when in flower. I want to get it back to about half its current height so it might maintain its shape when in flower. How/when could I cut it back without losing a year’s flowers, as I understand it flowers on the previous year’s growth? I normally prune it in late spring by cutting back two or three buds – could I cut back, say five or six buds without losing a year’s flowers? Pat Griffin, Co Dublin, Ireland
While most species of hydrangeas benefit from some kind of annual maintenance pruning to keep them floriferous and in good shape, the most suitable technique varies according to the particular species. In the case of mopheads or “hortensia” types (Hydrangea microphylla), you’re correct in worrying that incorrect pruning (too harsh, or at the wrong time of year) will impact upon your plant’s ability to produce a great display of the very large, globe-shaped flower heads for which this shrubby, deciduous species is so well-known. That’s because unlike other popular species such as Hydrangea paniculata or Hydrangea arborescent, which flower on new wood, mophead hydrangeas flower on old, ripe wood produced the previous year. If you remove too much of the latter during pruning, the plant won’t flower again until the following year.
Regenerative pruning of overly-large, established mophead hydrangeas should be carried out in March/early April before the plant comes into leaf. It’s important to use a sharp, clean secateurs or loppers to do this, cutting back up to a third of the stems to 15cm above the base of the plant to prevent growth becoming congested. Choose the oldest, thickest, woodiest stems which are typically grey in colour rather than young growth which is dark brown. The remaining stems should also be reduced in length by a maximum of 30cm to just above a strong, fat pair of buds, again prioritising the older wood. Any dead, diseased or damaged stems should also be removed at this point to encourage healthy growth.
By following the steps described above, you’ll reduce the overall size of your plant and help avoid what’s commonly referred to as “Hydrangea flop”. Other possible causes for the latter include overfeeding plants with fertiliser (this can produce a lot of soft, weak, sappy growth); and growing the plants in a very free-draining soil prone to drying out in summer (mophead hydrangeas like a rich, free-draining but moist soil that remains damp).
Another more extreme way to get overgrown or neglected mophead hydrangeas back into shape is take a sharp secateurs or loppers to the entire plant, cutting all stems right back down to 15cm above the ground. This will prevent the plant from flowering until the following year but it will have the desired effect of reducing it in size as well as producing lots of young, strong, shorter stems requiring only lightly spring pruning for several years.
Whichever pruning method you choose, finish off by generously mulching around the roots to help lock in soil moisture but note that fertiliser isn’t recommended for hydrangeas growing on heavier, richer soils.
