Every hedge cut between March and August gets covered up. The plant is growing, so a heavy hand or a wandering line disappears under new growth within a few weeks and nobody ever knows. The last cut of the year is different: whatever shape you leave it in is the shape it holds until about May, because there's no growth left to soften it. It's the one trim where a rushed hour is on display for seven months.
That's the real reason the timing matters, and it's a better reason than the one usually given.
There isn't one date, and that's the whole problem
People want a single cut-off — "finish hedges by the end of September" — and it doesn't exist, because the right answer depends on what the hedge has to do afterwards. Take the RHS's own timings for the hedges actually in gardens round here and the spread is obvious:
Beech — once a year, late summer.
Hornbeam — once, mid- to late summer.
Holly — once, late summer.
Privet, box, Lonicera — two or three times through the growing season.
Cherry laurel — twice during the growing season.
Leylandii and Lawson's cypress — two or three times in the growing season.
Yew — twice: summer, and again in autumn.
Hawthorn — twice: summer, and again in autumn.
Thuja — spring, and again in early autumn.
Read down that list and the pattern isn't a date, it's a question: does the plant need to grow again to cover what you just did?
Yew and hawthorn are dense and forgiving, and a tidy autumn cut on either just looks tidy. Beech and hornbeam get one cut, and it's in late summer for a specific reason (below). Conifers are the ones with a genuine deadline — the RHS is direct that trimming conifer hedges after August can encourage bare patches, and a bare patch on a conifer is not a temporary problem. That is the same brutal fact behind cutting a hedge back hard: most conifers will not reshoot from bare wood, so anything that leaves them thin leaves them thin for good.
The beech and hornbeam trick
Beech and hornbeam both do something unusual: the dead leaves don't fall. They go coppery brown in autumn and hang on the branch until the new buds push them off in spring.
Cut those hedges in late summer, once the year's growth has firmed up, and the leaves the hedge is holding when it stops growing are the ones that stay on it. You get a solid brown screen from about late October until March — half a year of privacy for the cost of one August afternoon.
Cut the same hedge hard in October and you take that screen off and bin it, and then you look through the hedge at the neighbours until spring. Nothing is damaged. It just isn't doing the job it was planted for. This is the clearest case of the general rule: on the last cut, you're not maintaining the hedge, you're choosing what it looks like all winter.
September is not automatically clear of nests
This one is worth being blunt about, because the assumption runs the other way and the law does not.
There's a widespread belief that nesting season finishes at the end of August and September is open season. The RHS puts the main UK nesting season at early March to early August "but it can go on for longer," and the legal position has no dates in it at all: under the Wildlife and Countryside Act 1981 it is an offence to damage or destroy the nest of any wild bird while it is in use or being built. Whether that's April or October is irrelevant to the offence.
And there is a specific late-season culprit. Wood pigeons breed from March right through to October, and their peak fledging months are July, August and September — they're nesting latest exactly when everyone assumes the coast is clear. Collared doves run late too. Both like a dense garden hedge, and a wood pigeon nest is a flat, flimsy platform of twigs that is genuinely easy to put a trimmer through without registering it.
So the September check is the same check as the June one, and it takes two minutes: walk the length of the hedge and look into it properly before anything is switched on. The reasoning behind that habit — and why the farmland ban people half-remember doesn't apply to gardens at all — is set out in when to cut a hedge and the law about nesting birds.
Laurel: put the trimmer down
Cherry laurel is the hedge most often ruined by the right job done with the wrong tool, and it shows worst on the last cut of the year because it doesn't get grown out.
Laurel leaves are big. A hedge trimmer slices straight through them, and every sliced leaf browns off along the cut edge and then stays there — brown-edged, ragged, and very visible — for months, because the plant isn't producing new leaves to hide them. The RHS recommendation is secateurs or loppers on laurel rather than shears or a powered trimmer, cutting whole stems back so you remove entire leaves instead of halves of them.
It takes longer. On a big laurel hedge, noticeably longer. But a September trimmer cut on laurel is a brown hedge from October to May, and there's no fixing it in between.
What I actually do, and when
Rough working order through the back end of the year:
Late July to late August — conifers get their final trim now, not later. Beech, hornbeam and holly get their one annual cut in this window.
Late August into September — privet, box and the other fast evergreens get their last tidy. Laurel by hand if it needs it.
September into October — yew and hawthorn take their autumn cut. These are the ones that still look good cut late.
After that — nothing until the winter work starts, which is a different job with different rules (when to prune what covers the shrubs, and it is mostly about leaving things alone).
Three things worth doing on the last cut specifically
Since it's the cut that stays visible, it's worth spending the extra twenty minutes on the bits that are easy to skip.
Get the top level and the sides slightly battered. Battered means tapering in a little towards the top, so the base is wider than the top. It looks better, but the real reason is that a vertical or top-heavy hedge shades out its own bottom, and the base thins over years. On a hedge that's about to sit still for seven months, this is the moment to correct the profile — run a line if the top has wandered.
Take the arisings away properly. Clippings left banked against the base sit wet all winter against the stems. Rake them out from underneath, not just off the top.
Clean and oil the blades before they go away. Sap dries hard over winter and the first cut of next spring is miserable on a blunt, gummed-up trimmer.
And if it's already too late
If it's late October and the hedge is shaggy, the honest answer is usually to leave it. A hedge carrying a season's extra growth through winter looks untidy and is otherwise fine — and the growth takes a bit of the worst weather instead of the plant. Cutting a conifer at that point to make it look neater for Christmas is the one version of this that does lasting damage.
Wait for spring, cut it then, and it grows away and covers the wait in a fortnight. That's the whole advantage of the growing season, and it's exactly what the last cut of the year doesn't have.