How do I prune pompas grass and when do I prune it?

How do I prune pompas grass and when do I prune it?

 

Some would say two inches below soil level. Less murderous folks prune about six inches above the soil.
If you are on the edge of it's hardiness you may want to wait for the first signs of growth in Spring to avoid Winter-kill. Wear safety glasses and gloves as well as a long sleeved shirt to avoid cuts from the leaves. http://en.wikipedia.org/wiki/Cortaderia_selloana http://en.wikipedia.org/wiki/Pampas