Purple fountain grass (Pennisetum setaceum “Rubrum” and "Purpureum"), which is also called red fountain grass, is something of a multiple personality in the garden. Not only can it be grown as a perennial or an annual, but the attractive, droopy grass with its feathery purple plumes is considered an invasive pest in some areas while welcome in others. Purple fountain grass has a clumping habit and grows up to 3 feet tall. The plumes appear in summer and fall. The ornamental grass is rarely bothered by pests or disease.
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Perennial or Annual?
Purple fountain grass grows as a perennial in U.S. Department of Agriculture plant hardiness zones 8 to 11, but the plant dies back in winter in colder climates. Because of this, most gardeners outside the hardiness zones grow purple fountain grass as an annual. Dwarf or standard varieties in containers are perennial in colder climates only if the containers spend the winter indoors.
Divide and Conquer
As a perennial, purple fountain grass continues to expand year after year, forming large, dense clumps. Sections of these clumps can become choked and wilt or fail, making the grass planting appear sickly and unkempt. Dividing the clumps in spring alleviates the crowding while doubling the number of grass plants decorating the yard. Simply dig it up, being careful not to damage the roots, and carefully separate the root ball into two or more parts, depending on how large it is. Replant one in the original spot, and plant the others about 3 feet apart in holes as deep as the roots originally were and twice as wide. Backfill the holes and water thoroughly to keep the soil moist but not wet until the roots re-establish themselves.
Take a Bit Off the Top
While it's perfectly fine to leave fountain grass alone, trimming old or dead growth keeps the plant looking neater and healthier. Because it is a warm-season grass, the best time to cut back fountain grass is when it is dormant, in late fall or winter. Fountain grass may be pruned down to the crown, if desired. Pruning keeps the old, faded foliage from marring the beauty of fresh new growth. Sterilize pruning shears in a solution of 1 part bleach and 9 parts water for five minutes, allowing them to air-dry, before and after pruning.
Light, Water, Food
Purple fountain grasses thrive in full sun and are drought-tolerant. Virtually care-free, the plants normally need no fertilizer and do well on less than 20 inches of water annually. During extreme heat or extended dry periods, provide enough water to keep the soil moist but not wet. A 4-inch layer of organic mulch surrounding but not touching the base of the plant helps keep the roots cool and conserves moisture. Organic mulch also adds nutrients to the soil as it decomposes, further reducing the need for added fertilizer. If the plant needs more nutrition, evenly spread 1/4 to 1/2 cup of complete, all-purpose granular fertilizer around the root zone and water in well. Fertilize in spring, when new growth appears.
Cautions and Warnings
Purple fountain grass cultivars rarely set the seeds that allow the plant to spread willy-nilly all over the landscape. Planting in containers or in sites that have natural barriers such as curbs also keeps fountain grass under control. In winter, fountain grass can become dry as tinder and burns easily. Other types of ornamental plants such as flax, lavender or clumping bamboo may be better choices in areas prone to wildfire. Ornamental grasses can have sharp edges, especially when the leaves are dry and stiff. Wear work gloves or heavy gardening gloves when handling fountain grass.