Cutting most types of ornamental grasses during ... fountaingrass (Pennisetum spp.), and muhly grass (Muhlenbergia spp.) do most of their growth in summer when temperatures range between 80 ...

You may even resort to a chain saw to cut very large clumps (clean the saw carefully after use). To make clean-up easier, bind each clump of grass tightly with twine, a bungee cord, or painter's tape.