Hello , They are pleached Photinia, I admit not my favourite evergreen pleached yet more cost effective than trees such as Holm Oak. There are also quick growing in the scheme of things. I generally will decide depending upon the space, symmetry of a space and exactly how they are being used ~ purpose & long term vision. It also depends upon location, site. One can `play` with the spacings to fit into a space as in here we had 3 zones on boundaries where we were planting them, All were slightly different in length. In certain gardens one may plant at approx. 1800mm. Although they can be spaced a little more apart as we did here. They were actually 2000-2100. Many would think this is too far apart yet as we had so many I did not want to plant closer than 1800 to fit spaces and have `too` many verticals as I feel in some spaces it becomes unbalanced. My vision was that once this wider spacings were mature with substantial stems they would add structure without being too many and prevent being an over fussing space.

So I would always evaluate the Big picture.

I do one off consultancy if you need to evaluate spacings and what works best in your space. This can be via the telephone and sharing details, images etc of your project.

Thank you for your response. I am in the UK. Although I have consulted out of the UK also. I work with a large percentage of clients & home owners who are renovating, restoring, rebuilding, extending etc. Glad that this info above may be useful.