I haven't tried cutting any Fountain Grass back in summer, but I think what you'd get would be unattractive. Chances are, the cut-off blades would continue to grow, and have squared-off and brown ends. I don't know if it would still bloom or not. I traded for some divisions of 'Karley' early this summer, and they bloomed, in pots.

Maybe you should consider dividing it next year. Also be sure to plant it in lean soil. Frequently too-rich soil will make a grass flop.