Posted: 23/10/2014 at 07:28
I would consider a shrub rather than a tree. In my experience Hornbeam hedges need to be wide and tall to look good....
I'm a little puzzled that you should say that AWB - hornbeam is well known for working well as a narrow hedge. See my earlier post about the way MD uses it at Longmeadow and also an extract from this article http://www.telegraph.co.uk/gardening/3344586/Adventures-in-hedges.html
" ,,,Hornbeam (Carpinus betulus) is tolerant of a wide range of conditions, and its dense twiggy growth is ideal for cutting into detailed shapes, making it possible to create "windows", "doors" and interesting finishing touches to tops and edges. It is also possible to shape it into a very narrow hedge that produces a delicate, curtain-like effect...."
It's this aspect of its growth that makes it so suitable to create many well-known hedge mazes. It can also be successfully kept as a short hedge - see the restored hornbeam maze at Woburn Abbey http://www.woburnabbey.co.uk/gardens/explore/hornbeam-maze/