Every spring, as the first green shoots begin to appear on hydrangeas across Irish gardens, the same question comes up: should I cut it back, how far should I go, and have I already left it too late? The honest answer is that late is almost always better than not at all when it comes to hydrangeas, and the damage most people do is not in the timing but in cutting too hard. There is a simple rule that will keep you right, and once you understand the logic behind it, you will never be uncertain about your hydrangeas again.
First, a word about the dead blooms that have been sitting on the plant all winter. If you have not removed them yet, do not feel you have done something wrong — in fact you may have done your hydrangea a favour. Those dried flower heads that persist through autumn and winter are not just a cosmetic feature. They sit directly above the new growth buds that will carry this year's flowers, and through the coldest nights of winter they offer those buds a degree of protection from frost. In an Irish garden, where we regularly get cold snaps well into March, leaving the dead blooms on until the weather has properly settled is the sensible approach. Remove them too early and you expose that tender new growth to a late frost. Early spring, once you are confident the worst of the cold is behind you, is the right time to take them off.
The job that most people get wrong with hydrangeas is not leaving it too late — it is cutting back too hard and losing the flowers for the entire season as a result.
When you do prune, the seven node rule is the number to hold in your head. A node is the point on a stem where a leaf or leaf bud joins — you can see them clearly on a hydrangea stem as small swollen joints, often with the beginnings of a bud already visible in early spring. Count seven nodes up from the base of each stem. Do not cut below that point. If you cut further back than seven nodes into the plant, you will remove the growth that carries this year's flower buds, and the result will be a plant with plenty of new foliage but no flowers until the following year. The flowers on a lacecap or mophead hydrangea are produced on growth from the previous season, not on new growth from this year — which is why cutting too hard has such an immediate and visible consequence.
What you are doing when you prune correctly is removing the spent material from last year while leaving enough of the previous season's growth to carry new flowers. Cut the dead bloom stem back to just above a healthy pair of buds, keeping those seven nodes intact below your cut. If the plant has become genuinely congested over several years without pruning, you can also remove some of the oldest, woodiest stems entirely at the base — this opens up the structure and encourages stronger new growth from low down. But do this selectively, taking out no more than a third of the oldest stems in any one year, rather than cutting the whole plant back hard at once.
One question that comes up regularly is whether it matters if you prune later than ideal — if, for instance, the new growth has already come on quite strongly before you get to it. The answer is that it is still worth doing, even then. You may lose a small amount of the new growth when you remove the dead stems, but that is a minor setback compared to leaving the plant unpruned entirely. A hydrangea that is never deadheaded or lightly pruned will become increasingly congested, the flowers will get smaller and weaker over time, and the plant will eventually start to look tired. The annual tidy in early spring, following the seven node rule, keeps it performing consistently year after year.