when cut back fountain grass
The best time when to trim fountain grass back is in the late winter or early spring. The exact timing is not as important as just making sure that you prune fountain grass back before it starts actively growing. You want to avoid doing fountain grass pruning in the fall, as the plant has not yet died back all the way. If you try to cut back fountain grass in the fall, you may cause it to go into a growth spurt, which will make it more vulnerable to the coming cold weather and will reduce its chances of surviving the winter.