pruning/trimming pleached hornbeam
 

A friend of mine has a screen of about 5 or 6 of these providing some
privacy in her otherwise overlooked garden. About 3 years old, I think
they're pretty well established now but they're tending to grow out
like small trees rather than being constrained the way I've seen them
on garden centre ads, shaped like a playing card on a stick.


Does anyone have any advice on optimum time for pruning/trimming?
They're now out fully in leaf and I'd like to run the hedge trimmers
over them to flatten them off but wonder if this would be untimely or
too aggressive.

Thanks for any tips.