I get asked about pruning of hydrangeas more than any other single subject – apart from how to change their colour. In fact I decided not to post about the latter ever again after my last post of the subject which you can see here.
But pruning may merit a recap as we are coming to pruning time (March in this part of the world – basically when hard frost are probably over for the winter).
There are many species and kinds of hydrangeas so, for my purpose, I will deal only with the H. macrophylla – hortensia/mophead/lacecap (bigleaf) hydrangeas – the ones everyone knows, with colourful pink, blue or white flowers, and the tougher H. paniculata and H. arboresecens hydrangeas which, though two different species, have similar needs.
Hydrangea macrophylla types are slightly tender and new shoots are vulnerable to frost damage. Recent breeding has altered things slightly but the basic rules are that new shoots that grow from the base each spring will not have a long enough growing season to produce blooms at their tips – except in very mild or maritime areas. The most blooms are carried on sideshoots that grow from the upper parts of the shoots produced in previous years. So pruning takes place in March and we cut back the stems to the uppermoist pair of fat buds. In some cases this will be removing the old flowerheads with a small amount of stem. The old flowerheads can help protect the lower buds from frost.
After about five year, some of the old, very twiggy stems can be removed at the base to prevent congestion.
If the whole plant is cut down the result is a beautiful, refreshed, dome of growth and no flowers. This will not actually harm the plant itself and it will bloom the following year – if you leave it alone.
I have to add the caveat that some new kinds, such as ‘Endless Summer’, will flower the summer after hard pruning but just be careful.
Then there are the H. paniculata (PeeGee type) and H. arborescens (such as ‘Annabelle’). These produce their large flower clusters at the ends of the new shoots, that start to grow in spring. Their growth mode and pruning, is the same as for common buddleia. So in March, all the growth produced last year is cut back, possibly to the lowest pair of buds. You can also prune less severely. The very hardest pruning results in fewer new shoots but the biggest flower heads. This sounds wonderful but the big flowerheads are sometimes too big for the plants to support and the result can be a bit floppy. If you don’t prune at all you end up with lots of dead, twiggy wood and small flowerheads. This type of hydrangea is much hardier than the mopheads and seems to do better for me at the moment. The colour range is limited – basically all start lime or cream, mature to white and age to pink – in various degrees.