Nearly every pruning problem I get called to is a timing problem rather than a technique problem. The cut was fine; it happened in the wrong month. The shrub then sulks for a season, or flowers at half strength, and the cause is nine months back so it never gets blamed.
There are only a few rules worth knowing, and mostly they tell you when to leave something alone.
The flowering rule
This covers most shrubs in most gardens.
If it flowers in spring or early summer, it is flowering on wood it grew last year. Prune it straight after it finishes flowering, and it spends the rest of the season building the wood that carries next year's flowers. Cut it in winter and you are cutting off the flower buds you were waiting for. Forsythia, philadelphus, weigela, ribes and kerria all sit here.
If it flowers from midsummer onwards, it is usually flowering on wood it grew this year. Prune it in early spring, before it starts moving, and it grows fresh flowering wood in time. Buddleja davidii, hardy fuchsia, lavatera, late clematis and bush roses.
Two things to know about that second group:
Timing within it varies. Late clematis and Buddleja davidii are fine in February. Hardy fuchsia and lavatera want longer — leave the old top growth on over winter and cut back in April once new shoots show at the base. A hard frost on a fresh cut sets them back or kills them.
Not every buddleja belongs there. Buddleja davidii, the common butterfly bush, flowers on new wood. Buddleja alternifolia, the one with arching branches and flowers all along the stems in early summer, flowers on last year's wood and is pruned after flowering.
And two plants break the rule outright:
Mophead and lacecap hydrangeas flower from mid to late summer but on the previous year's growth. Leave the faded heads on over winter — they protect the buds below — and cut back to the first fat pair of buds in early spring. Prune one hard in late winter and you get leaves and no flowers.
Rambling roses mostly flower once, around June, on last year's wood. They get pruned after flowering, not in winter.
If you do not know what a shrub is, pruning it just after it flowers is the safest guess. It fails on the three cases above and on anything in the cherry and plum family, which is the next section.
Do not cut cherries or plums in winter
If you take one thing from this, take this.
Prunus should be pruned in the growing season, not in winter. That is plums, damsons, gages, sweet and sour cherries, and also apricots, peaches, almonds and the ornamental flowering cherries people have in front gardens.
The reason is silver leaf disease and bacterial canker. Both get in through fresh cuts. Silver leaf spores are about from September through to May and are released mainly in the damp autumn and winter months; bacterial canker does its damage in wet spring and early summer weather. Cutting in July or August puts the wound in the gap between them, when the tree is growing strongly and seals fast.
Timing in practice:
Established trees: mid-summer, in dry weather. July and August are the safest weeks, and stone fruit work should be finished by the end of August.
Young trees still being shaped: early spring, once the buds have opened, is the usual advice.
Cherry laurel and Portuguese laurel are Prunus too, but they are grown as evergreen hedges and are treated as hedges: trim in late spring or early summer, and do a hard renovation in mid-spring.
This is the opposite of the advice for apples and pears, which are winter-pruned, and that is exactly why people get it wrong. If you are pruning apples and pears in the dormant season, it feels natural to do the plum at the same time. Don't.
Two more to leave alone in late winter
Birch, maple, walnut, hornbeam and magnolia bleed sap heavily if you cut them as the sap starts to rise in late winter and early spring. Do birch between late summer and midwinter, and Japanese maples after leaf fall and before January.
What you can do most of the year
Dead, damaged and diseased wood comes out whenever you find it on most plants. There is no season for a snapped branch.
Two things change that:
On cherries and plums, save the routine tidying for the summer window too. Every cut on those trees is a way in, and the advice is to do all pruning on them in July or August. Wood that is actually infected or actually dangerous still comes out the day you see it — cut 10 to 15cm back past any stained wood, take the prunings away, and paint that cut.
Hard frost. Cut tissue is damaged tissue, and freezing it before it seals does no favours. Dead wood is fine to take out in frost; live wood can wait a week.
Evergreens and hedges
Evergreen shrubs generally want mid-spring, once the worst frosts have gone and before they put on their main growth.
Hedges are a separate job with a separate constraint, and it is a legal one rather than a horticultural one: it is an offence to damage an active nest, which puts a hard limit on hedge work through spring and summer regardless of what the plant would prefer.
How to make the cut
Less to say here than people expect.
Cut back to something — a bud, a side shoot, or the branch collar where a limb meets the trunk. A stub left in mid-air has nothing to grow from and dies back.
Don't cut flush to the trunk. The slightly raised collar at the base of a branch is where the tree seals the wound from. Cut just outside it.
Don't paint the wound, with one exception. Sealing compounds are not recommended on ordinary cuts; they do not help the wound close and can trap moisture against it. The exception is plums and cherries, where a wound paint on a fresh cut helps keep silver leaf spores out, and it is worth doing on any tree that has had silver leaf or canker before.
A sharp blade beats a big one. A torn cut takes longer to seal than a sliced one, and dirty blades carry disease from one plant to the next.
If something has got away from you entirely, cutting it back hard is a different job with its own rules, and it is usually better done in stages over two or three years than in one go.
I do pruning and shrub work through the winter across Ledbury, Bromyard and out towards Newent. Give me a call if you would rather it was done right the first time.