When can I prune/divide ?

LilyandPoppy.

I bought this ornamental grass as a tuft, but over the last few weeks it's gone a bit mad and straggly.  I've googled it but can't seem to find anything about giving it a haircut or even divide it. When is the best time to do either ?
It is Corynephorus canescens "Spiky Blue"

K67

Probably needs more watering.
Most grasses you don't divide until autumn.