Purple fountain grass (Pennisetum setaceum "Rubrum") enhances the garden with purple clumps in spring and summer, followed by burgundy plumes in autumn. The plant is perennial in U.S. Department of Agriculture plant hardiness zones 9 through 11. With ample protection, purple fountain grass grown in a container may survive winters in climates as low as USDA zone 7, as container growing drops hardiness by about two zones. Otherwise, grow the plant as an annual or bring it indoors for the winter.
Water purple fountain grass normally throughout autumn if moisture isn't provided by rainfall, as healthy, well-hydrated plants are more likely to survive winter cold. Withhold fertilizer after midsummer, as new growth is tender and easily damaged by freezing temperatures. Winterize the plant after the first light frost.
Bury the Pot
In chilly climates, burying the pot is often the best way to protect ornamental grass and other perennial plants during the winter. Dig a hole deep enough to bury the pot up to the rim, and then place the pot in the hole. Fill in around the pot with soil and cover the pot with a thick layer of mulch such as dry, chopped leaves, shredded bark or straw. Check the pot throughout the winter and replenish the mulch if it blows away.
If your climate experiences frost but no hard freezes, purple fountain grass may survive the winter in a protected location. Move the pot to a sheltered area next to a building, preferably a location on the building's east or north side. If you have other containerized plants, group them closely together, and then cover the entire group with a thick layer of mulch.
In cold climates, growing purple fountain grass indoors during the winter is the most dependable way to ensure the plant survives the winter. Check the plant carefully for signs of pests and disease first, as those problems will continue indoors and may spread to other indoor plants. Place the pot in your brightest window and water as needed to keep the potting soil moist but never soggy. Move the plant outdoors when you're sure all threat of frost has passed.