Some odd search engine terms bring people to this blog – this question is one of today’s.
I’m not a lawyer, so take that as a legal disclaimer for the following – but my answer would be that the boundary of a property is not materially affected by hedges – a hedge need not mark an exact boundary in any case (this is part of the judge’s definition of a hedge in the landmark case Stanton vs Jones). Boundaries are marked fairly clearly on the deeds to a house.
There are situations in which the shifting of a boundary marker such as a fence or wall might in the long run affect the exact location of a boundary if a judge concluded that long usage had established a new boundary but the law is very much geared to respecting the original boundary, to the point that it is possible to get a judgment that incorrectly positioned walls and fences must be demolished. But the growth of a hedge is different to moving a wall or fence because it is a natural process and doesn’t actually shift the location of the hedge to any specific degree (assuming it is growing out in both directions). Also, unlike a fence or wall, you have an immediate remedy open to you if your neighbour’s fence is expanding – you can trim overhanging growth on your side of the hedge (though not to cut the hedge down from the top) – you have the right to trim it back to your property line if you can do
it without killing the plant.
Note that there is a minor legal quirk here – technically the hedge trimmings belong to the neighbour and should be offered back to them before being disposed of. It’s highly unlikely they would want them, but it never does any harm to talk about these things before taking action to avoid any conflict.