In short
Brown patches on a conifer hedge are usually cypress aphid, a trim made too late in the year, drought or waterlogging, or a cut that went back into old wood. The RHS says conifers other than yew have little or no ability to regrow from old wood, so brown areas caused by hard cutting will not green up again.
This is a job for my Hedge cutting service.
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What causes the brown patches
The RHS investigated brown patches on conifer hedges and found cypress aphid responsible for around half the cases. The aphids feed on the sap through spring and summer and the damage shows as yellowing, then browning, often low down and on one side first. Scale insects and fungal diseases such as Pestalotiopsis and Coryneum canker cause the rest, usually on hedges already weakened by something else.
The something else is nearly always growing conditions or how the hedge was cut. Drought, frost and waterlogging all brown a conifer, and on the heavy, wet ground we get in parts of the Fens around Upwell, Outwell and Emneth, waterlogged roots are a common one after a wet winter.
Why it matters when you cut
The RHS is clear that trimming conifer hedges after August encourages bare patches. Cuts made in September or October do not have time to be covered by new growth before winter, and the exposed inner branches brown off. I trim leylandii and other conifers between late spring and August for exactly that reason, and I will not do a late-autumn conifer trim for a customer even if asked, because I know what it will look like in March.
Will it grow back?
This is the question everyone asks and the honest answer is: only if there is still green growth on the browned branches. The RHS states that, with the exception of yew, conifers have little or no capacity to regrow from old wood. So:
- Brown tips on green branches (aphid, drought, frost): usually recover over a season or two once the cause stops.
- Brown, bare, woody inner hedge after a hard cut: will not regrow. The best you can do is let the neighbouring green growth spread across it.
- A whole plant brown from top to bottom: dead. It needs taking out and the gap filling.
What I do on hedges like this
- Look for the aphids and the honeydew and sooty mould they leave. If it is aphid, the hedge needs feeding and watering to grow through it rather than spraying.
- Check the last cut. If someone has taken the hedge back into brown wood, I say so, because no amount of feeding will fix that.
- Trim only the green outer growth, keep the sides tapered so light reaches the bottom, and keep to the spring-to-August window.
- For an overgrown conifer that has to come down in size, the RHS advice is to reduce the height by up to a third and thin selectively rather than try to renovate it. Anything more and you are better replacing the hedge.
I look after a lot of leylandii boundaries around Wisbech and Downham Market, and most of the brown ones I see were cut too late or too hard. If yours is browning, get it looked at before the next trim rather than after.