It depends on the Hardiness zones.
For zones 1 through 8, where the temperature can be below 20 degrees Fahrenheit in winter, Purple fountain grass cannot survive outdoors. So, growing them in containers indoor for during the winter is a good option.
In the spring, when the temperatures start rising, they can be brought back to the outdoors.
For zones 9 and 10, where the winter temperatures do not reach lower than 20 degrees Fahrenheit, cut the Purple fountain grass back to the ground in early spring. The new growth will initiate as the temperatures start rising.
Plumes make a great dried flower arrangement in the house. So, at the end of the season, cut them off and make them into a bunch and use them as a nice decoration in the Vase.