Divide Pennisetum the same way you do other warm-season grasses, and at the same time. How often is up to the size and condition of the plant, and your wishes. If the plant as any problems, like center die-out, you'll want to divide it, eliminating the problem section. If you simply want more plants, divide a large crown into several pieces. They will become round mature clumps in their time.

I chop sections out of my P. orientale to give to people and you can't tell when they leaf out. You can see the chunks I've chopped out during the winter though.

I haven't had any center bare spots like I do get with my Miscanthus 'Flame'.

My other fountain grasses are too young to be divided yet. I don't do them every year. I don't divide any of my grasses every year although I might if I needed the divisions for another spot. I have mostly smaller well behaved grasses so I can cram in more on my small lot.

Oops, discovered I had name wrong yesterday.

Miscanthus sinensis var. purpurascens not Miscanthus 'Flame'