I have this ,and its best to leave it till spring,when you see new shoots coming from the bottom..The foliage will turn to a beige/creamy colour in the winter,which is very attractive,and will protect next years new growth..It is called,Miscanthus Zebrinus..but most of us know it as Zebra grass..

I also leave mine till the early spring, any later and it will blow around and make a mess.

Thanks guys, I've been doing it wrong for the last 4 years. (cutting it down to ground level about now !)