Question
We have a hedge row of fully grown barberry.
They are about 6 feet tall and would like to cut  them to the ground so that thet may re new themselves. If this is ok what height should we cut at and when?  

Answer
Hi Jack
This shrub does not regrow very fast. Most overgrown ones are just replaced insted of cut back. once the shrub reaches the desired size, then all that needs to be done is seasonal trimming where just the new growth is removed.
it is possible to cut them down and regrow them but recovery will take many seasons to restore them.  Good luck
Larry  :-)