The term "ornamental grass" covers a wide variety of perennial and annual plants with narrow foliage. Perennial types, such as "Gracillimus" Eulalia grass (Miscanthus sinensis "Gracillimus"), which is hardy in U.S. Department of Agriculture plant hardiness zones 6 through 9, can be split and transplanted in either spring or fall to reduce congestion or to create new plants. The timing of grass splitting or division depends on whether the grass is a cool-season variety, which begins new growth in fall, winter or spring, or a warm-season variety, which begins new growth in spring.
Cool-Season Grass Division
Cool-season ornamental grass should be divided in early fall, after the hot summer is gone. Varieties include "Karl Foerster" feather reed grass (Calamagrostis x acutiflora "Karl Foerster"), hardy in USDA zones 5 through 9 and featuring pink-purple flower stalks and an arching habit. Another cool-season ornamental grass is low-growing fescue, such as the "Elijah Blue" cultivar of blue fescue (Festuca glauca "Elijah Blue"). Fescues generally top out at about 12 inches tall. "Elijah Blue" is hardy in USDA zones 4 through 8 and features narrow, blue-green leaves.
Warm-Season Grass Division
Very early spring, when the ground becomes workable but before or just as the grass' new growth starts, is the time to divide a warm-season ornamental grass. Varieties include big blue stem (Andropogon gerardii), which is hardy in USDA zones 4 through 9, tops out at 6 feet tall and changes color as the growing season progresses. Another common warm-season variety is switch grass (Panicum virgatum), a moisture-lover with a columnar form. Switch grass is hardy in USDA zones 5 through 9.
Division and Replant Techniques
Whether division takes place in early spring or early fall, watering the ornamental grass' soil thoroughly about one hour before dividing the plant softens the ground. Cut the plant's foliage to about 6 inches tall. Using a sharp spade, dig all the way around the grass clump, and lift it from the ground. Divide the clump into two or more pieces by using the spade, a sharp garden knife or a small axe to separate roots. One of the pieces can be replanted in the original hole, and other pieces can be planted in other locations. The plants should be at the same soil depth at which they grew previously. Fill the remainder of each planting hole with soil, and firm the soil around the plants.
Many ornamental grasses, especially large types, should be divided every three or four years. If a clump has not been divided in many years, then its middle portion may be dead. If that is the case, dig up the entire clump, but take divisions from the healthy root areas, discarding the dead middle section. Extremely hot weather makes division hard on a plant and the task of dividing the plant hard on the gardener. If the weather is very hot, wait until temperatures cool before dividing grass.