There's not much else that would cause that kind of damage to both the grass and the hedge. I would start from the assumption that it was council weed spraying, and send them the pictures asking what type of weedkiller they used. If it's one that instantly kills the green top growth but isn't systemic, you might get some regrowth on the hedge.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
An update. I did get in touch with the council and they said that they didn't spray any weedkiller. And I think I tend to believe them. The dying off process seemed to stop during the winter. Indeed only the west side of the hedge (next to the road) seemed to be dead. The top and east side appeared healthy. But now we are in spring time again the problem has taken a vengeance. Previously it was just one side and about a 10 feet long. Now the whole hedge is dying (30 feet) on both side and the top. The grass is dead or dying up to about a foot and a half away from the hedge on both sides. Could it be some sort of larvae. A few years ago part of the hedge died off and when I dug it up to replace there were stag beatles larvae there but in that case the grass hadn't died. Please help.
I think it will be impossible to ascertain what's wrong only from photos we can only make suggestions based on what knowledge we have.
It's been very dry so lack of water deep down. Grass shows up lack of water quicker especially if there isn't much soil depth. Griselinia isn't totally hardy so maybe frost a contribution. Don't know how long lived they are.
Could it be Honey Fungus ?? Seeing the remnants of Bracken may indicate former old heathland . This may have contained trees that were cleared prior to building .
A bit of a long shot here , but maybe Honey Fungus was present then , the 'bootlaces' or rhizomorphs have survived to find other hosts ; unfortunately they may be your Griselinias.
I think the dead grass is just from dryness and proximity to the hedge .
Griselinia is on the RHS list of hedging plants resistant to honey fungus, so I'd be surprised if that was the cause of the problem... if it's not weedkiller, I'd suspect severe drought, I think. It could take a long time to kill an established hedge.
Stag beetle larvae only eat dead wood, I believe.
You haven't got any vindictive neighbours who don't like griselinia, I suppose??
Since 2019 I've lived in east Clare, in the west of Ireland.
Is there anyone that I can pay to analyse the soil? It would be pointless going to all the trouble time and cost to plant new if the same is going to happen to the next hedge
I agree with everyone else it's difficult to pinpoint the cause. If you draw a blank with the RHS maybe CABI could help? I once attended a lecture and a quick look at their website suggests you might be able to send in samples. Here's a link https://www.cabi.org/products-and-services/bioscience-services/
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It's been very dry so lack of water deep down.
Grass shows up lack of water quicker especially if there isn't much soil depth.
Griselinia isn't totally hardy so maybe frost a contribution.
Don't know how long lived they are.
Can't suggest anything else but maybe others can
Stag beetle larvae only eat dead wood, I believe.
You haven't got any vindictive neighbours who don't like griselinia, I suppose??