The best time to divide your ornamental grasses is in the spring around Mar. or Apr. when you cut back the prior years growth. I found the best way is to dig the entire plant up and then divide it into how many sections you want depending on the size of the original plant. Give them some root stimulator, no fertilizer. You may not get a lot of fall show the first year after dividing but they will be their showy selves the 2nd year on.