Yew hedge on top of retaining wall
Hi everyone, new-ish gardener here - I've been interested in gardening for a long time, but until now I've always lived in flats, so haven't had much opportunity for gardening beyond pots on the windowsill. However, in a few weeks my partner and I will be moving into our first 'proper' house with a garden, and so I'm starting to think about plans for my new plot.
We have a brand new, new-build garden about 10 metres long by 6 metres wide. I'd love to plant a hedge along the far back end of the garden, to screen the bottom fence and provide cover for wildlife - ideally something evergreen, but I'm not too bothered about it being quick growing, so I've been thinking that a Yew hedge might fit the bill.
However - the back fence that I want to screen sits on top of a 5 ft retaining wall that drops away into a neighbour's garden below us. It's fairly solidly built with breeze blocks and red brick facing, having been put in as part of the housing development, but I want to take extra care not to plant anything that could cause damage to it with its roots in years to come. I was wondering if anyone can tell me how far away from the retaining wall I ought to plant the Yews for my hedge, so as to reduce the likelihood of root damage. I'm planning on keeping the Yews clipped to no more than 2m in height once they've matured, and I already assume I will need to plant at least 3 feet away from the fence/wall to allow space for clipping the back of them - will this be a safe distance from the wall, or could the roots reach further underground than this, and damage the blockwork from behind?
Also, can anyone tell me what sort of minimum width/depth (ie from one side to the other) a Yew hedge can be clipped to, while still being evenly and densely green?