Get the timing right and the rest is easy

Hydrangeas have a reputation for being awkward to prune, and most of that reputation is deserved — but not for the reason people think. The trouble is almost never the technique. It is the timing. Every hydrangea sets its flower buds on one of two kinds of growth: old wood, meaning stems made last season, or new wood, meaning stems made this spring. Cut away the growth that was going to carry next year's flowers and you get a handsome leafy shrub with nothing on it.

Learn which group yours belongs to and pruning becomes a fifteen-minute job you can do with confidence.

Old wood or new wood: how to tell

Watch when the plant flowers. The big mopheads and lacecaps begin opening in July and are in full colour by August. Those buds were formed the previous summer, on stems that have since overwintered, so the plant flowers on old wood. The cone-flowered and smooth-leaved kinds are later, building their heads on shoots made in the same season — new wood — and carrying on into autumn.

A quick rule of thumb: if it is in flower before high summer, treat it as an old-wood plant. If it only gets going in late July and peaks in September, it is a new-wood plant. When in doubt, leave it unpruned for a year and note exactly where the flowers appear.

Pruning hydrangeas that flower on old wood

This is the group that catches people out, because the instinct is to tidy everything in autumn or early spring. For mopheads, lacecaps and the smaller serrata types, prune in the fortnight after flowering, ideally finishing by the end of August. The plant then has six to eight weeks of warmth to push new shoots that ripen before the frosts and carry next season's buds.

• Cut each spent stem back to a strong pair of buds, just above where this year's leaves joined the stem.
• Remove one stem in four or five at ground level to keep the bush young and open in the middle.
• Take out anything thin, crossing, rubbing or damaged.
• Do not cut into the fat, pointed buds lower down the stem — those are next year's flowers.

Oak-leaved hydrangeas and the climbing kind flower on old wood too, but need only light attention: remove dead wood and shorten wayward shoots after flowering, and nothing more.

Pruning hydrangeas that flower on new wood

The cone-flowered hydrangeas — 'Limelight', 'Vanille Fraise', 'Pinky Winky' and their kin — along with the smooth-leaved 'Annabelle' types, are far more forgiving. Cut them in late February or March while they are still dormant, and cut hard.

• Cone-flowered types: cut last year's stems back to a permanent framework, leaving two to four pairs of buds on each branch. Reducing the whole plant by half to two-thirds is not too much.
• 'Annabelle' types: cut to 15–30 cm above ground, or right to the ground in a sheltered spot. Thin the new shoots to the strongest half-dozen in May so the heavy heads do not flop.
• Afterwards: feed with a balanced fertiliser and water well in dry spells — new wood means a lot of growth in a short time.

Skip this and the plants still flower, but on thin crowded stems, and the cone-shaped heads droop badly after rain. Hard pruning gives fewer, larger heads on stiff stems that hold themselves up.

Rebloomers and the in-between cases

Some newer mopheads are sold as reblooming types, flowering on old wood in early summer and again on new wood into autumn. Treat them as old-wood plants but prune lightly: in late March, once you can see which buds have come through the winter, cut back only the dead tips to the first live pair of buds. Never hard-prune these in spring.

A few late-flowering species flower on growth made in the same season and can be trimmed in spring, but they are slow to establish and are best left alone apart from the removal of dead wood.

Cold-climate timing and aftercare

In colder UK gardens the real problem is not pruning but winter kill. Frost and cold wind kill the top buds on old-wood hydrangeas, and plants pruned hard in August have less in reserve to cope with it.

There are two sensible compromises. First, leave the spent flowerheads on over winter — they shield the buds below, and they look wonderful rimed with frost. Then, in late April or early May when the buds begin to swell, trace each stem down to the first plump, green bud and cut there. You lose the top of the stem but keep the flowers.

Second, shelter the plant and feed with the season in mind. A hardy evergreen screen — yew, holly or laurel — on the windward side makes more difference than any amount of pruning. Mulch in spring with well-rotted compost, water in dry spells, and switch to a high-potash feed in late summer to ripen the wood rather than push soft growth into the frost.

Always cut with sharp secateurs to a bud, at a slight angle, and clean the blades between plants. A cut just above a bud heals quickly; a stub dies back and invites rot into the stem. Get the wood type right, prune in the right window, and next August you will have a bush loaded with flowers rather than a mound of leaves.