This is the one where getting it wrong is permanent, so it's worth two minutes before the trimmer comes out.
A hedge that's got too wide or too tall has two possible futures. Either you cut it hard and it reshoots from the old wood, or you cut it hard and you're left looking at a brown wall for good. Which one you get is decided almost entirely by what the hedge is.
The dividing line
Most broadleaved hedges reshoot from old wood. Hawthorn, beech, hornbeam, holly, privet and laurel will all break out again from bare stems. It looks alarming for a season and then it fills in.
Yew is the conifer that behaves like a broadleaf. It breaks from old wood and takes renovation as well as anything. An inherited overgrown yew hedge is recoverable.
Leylandii and Lawson cypress won't. Cut past the green on those and the brown stays brown, permanently. That's the expensive mistake, and it's why so many leylandii hedges have one bald flat side.
Thuja sits in between and I'd not bet a customer's hedge on it. It takes a hard reduction far better than leylandii, which is why it gets sold as the trimmable alternative — but taking a hard reduction isn't the same as breaking from bare brown stems, and I've not seen enough of them come back to promise it. Treat it as cut-within-the-green until you've watched one recover.
Box is on the list but wants looking at twice. The species reshoots fine. The problem is that blight and box tree caterpillar have got into most established box, and a hard cut opens a lot of wounds on a plant that's already struggling. If it's thin, bronzing or patchy already, renovation isn't the answer and replanting probably is.
How much you can take in one go
More than people think, on the right species. Beech, box, hawthorn, holly, hornbeam and yew will stand losing about half their height and width in a single cut and come back from it.
It's only when you want more off than that — back to the main stems — that the job wants breaking up. Then it's three years, not one weekend:
Year one: cut one side hard, to about 15cm inside the width you actually want. Trim the other side as normal. Leave the height alone.
Year two: the other side, the same way, once the first has broken out and greened up.
Year three: take the height down.
The hedge keeps enough leaf to feed itself the whole way through, which is what it needs while it's rebuilding. An old hedge often hasn't got the reserves to lose everything at once.
Afterwards is where these are won and lost
Mulch it, every stage, with organic matter over damp soil either side.
Water it through any dry spell in that first growing season. This is the one that decides whether it comes back, and it's the one people skip. A renovated hedge that dries out in its first summer is a dead hedge.
Don't feed it unless the regrowth is actually poor. Pushing fertiliser into something that's just lost half its leaf gives you soft growth that the frost takes.
Timing, and the legal bit people get backwards
Deciduous hedges — winter, dormant, when you can see the framework.
Evergreens, yew included — mid-spring, once the hard frosts have gone, so it has the whole season to respond.
Now the part worth being straight about. The mid-spring cut sits inside the nesting season, not outside it — April and May are the peak of it. Avoiding summer does not get you off the hook, because the garden rule follows the nest rather than the calendar: it's an offence to damage a nest while it's in use or being built, any bird, any hedge, any month. The fixed March-to-August ban people half-remember is the farmland rule and doesn't apply to a garden hedge.
So the winter cut on a deciduous hedge is clear. The spring cut on an evergreen is check-first, every time, and the check decides whether the job happens that day.
Conifers you can't renovate — what you can still do
If it's leylandii or Lawson cypress, the sides are fixed: only ever cut within the green. There's no way back from brown on the sides.
The top is different. You can take real height off an overgrown conifer hedge — up to about a third, in April — and it will cope. Past a third and you're into wood that stays bare. So an oversized leylandii isn't necessarily a write-off; it's a hedge whose height you can fix and whose width you can't.
Worth knowing there's a legal reason to deal with one rather than let it run. Under the high hedges rules, a line of two or more evergreens over two metres that a neighbour says is spoiling their light can end up with the council involved, and they can order it reduced. Cheaper to keep on top of it.
Get the shape right while you've got the chance
On anything above about a metre, give it a batter — a taper, so the base is wider than the top.
The commonest fixable reason a hedge goes bare at the bottom is that the top overhangs the base and the bottom never sees light. A tapered hedge keeps light on its lower branches and stays furnished to the ground, and it sheds snow instead of being splayed open by it. Below about a metre — box edging, a low privet run — keep the sides vertical; there isn't enough height for shading to be the problem.
Rule the other causes out before you blame the shape, mind. Brown or thinning conifer is more often cypress aphid than shading, and that wants treating rather than cutting.
A renovation is the one moment you can reset the shape, so do it then rather than promising yourself you'll fix it later.
Ordinary trimming, for the rest of the time
None of the above applies to a hedge that's already the right size. How often depends on what it is:
Privet and box — the frequent end, two or three cuts through the growing season.
Yew and hawthorn — twice.
Beech, hornbeam and holly — once, in August. The browned leaves that beech and hornbeam hold through winter sit on that season's shoots, so a later tidy takes the shoots and the leaves with them.
Informal and flowering hedges — once a year, but when depends on what you're keeping. Grown for flower, cut straight after flowering, on the same logic as any other shrub that flowers on last year's wood. Grown for berries — cotoneaster, pyracantha — leave it until the birds have had them.
Bottom line
Work out what it is first. Leylandii and Lawson cypress never get cut past the green on the sides, though you can still take a third off the top. Everything else will forgive a hard cut — up to about half in one go, staged over three years if you want more than that. Then mulch it, water it through its first summer, and put a taper on it while it's open.
If you've got a hedge that's beyond a trim, that's the same conversation as a garden that's got away entirely — worth planning rather than attacking.
Hedge work is a good part of what I do around Ledbury, Bromyard and out towards Ross-on-Wye. Give me a call and I'll tell you honestly whether yours will come back.