I tend to do this in the spring when they are dormant. but i dont have many grasses so perhaps someone else who grows more will have better advice.

It is possible to split many grasses now, especially if they are established clumps, and by zebra grass I assume you are referring to one of the banded Miscanthus cultivars. However as Seaburngirl says spring is really the best time, March-April

Definitely Spring when the grass is producing new roots. The roots on them now are beginning to go dormant and the plants often do not survive an Autumn split.

Just to clear up any confusion here ...warm season grasses, i.e. Miscanthus, Panicum, Molinia, and Pennisetum, should be divided in spring to early summer. Cool season grasses, i.e. Carex, Deschampsia, and Stipa, can be divided in spring or autumn.....although there are always exceptions to the rule.

Thanks very much for your inputs - I will move and split in the Spring!