If it's really Pampas Grass (Cortaderia), the best time to divide it is in late winter/early spring. Give it a haircut, and dig up and divide the rootball. Pay close attention to it from now until then, and watch for any weak sections of the clump (like the center), and cut them out when you divide.

thank you donn for ur help, may i please show my
ignorance, when i bought my first 5 from my local
nursery, i was told that it was pampas grass. so
thats what i automatically believed. it is now a
very large clump of grass approx 7 foot tall. with
blonde plums, when you deal with the blades, if u
go with the grain its smooth but go against, it will
cut you. so is this not pampas grass, it really
doesn't matter, whatever it is I LOVE IT!
thank you again for ur delp.
gwen

The nursery should know, Gwen. Your description sounds right, especially the sharp blades. I mentioned it because the term 'Pampas Grass' is frequently used (like Xerox) as a catch-all name for any tall ornamental grass.

okey dokey, just wanted to make sure, i wasn't doing that.
(that xerox thing) :-)
thank you again for your help.
gwen