Ornamental grasses provide low-maintenance accents to mixed borders, meadows and other areas of the garden landscape. The foliage color and texture as well as the seed plumes are the primary ornamental features, and well-formed fibrous root systems contribute to stabilizing soil. Annual grasses die after one growing season, but perennial grasses continue to get larger over the years, eventually needing rejuvenation to maintain vigor and overall beauty.
According to the University of Rhode Island, ornamental grasses generally fall into two types: clumping and rhizomatous. Clumping grasses grow into a compact, rounded clump of stems and leaves and get larger in clump diameter and circumference each year. Rhizomatous grasses grow with spreading underground rhizomes or surface-running stolons. They spread laterally in the landscape; some species of ornamental grasses can be invasive and weedy.
All ornamental grasses benefit from an annual pruning or rejuvenation at the beginning of their growing season. Removing the dried, dead biomass allows new leaves and stems to emerge from the root mass without being blocked or hidden. Cut back the plant to a height of 6 inches, or higher if the density or toughness of the stems makes cutting difficult. This annual practice makes the grass grow freely and more attractively in the growing season. For cool-season grasses, that means cutting back all leaves and stems in late summer; for warm-season grasses, prune back the grass in late winter. Annual grasses are not maintained after they die after the growing season.
Depending on ornamental grass species, perennial plants tend to lose their vigor or overall beauty after three to five years. Digging the plant up and dividing the root ball mass and replanting the sections rejuvenates them. Clumping grasses are easier to dig and divide since the roots are concentrated and finite. Rhizomatous grasses can lose vigor at the center of the thicket, with lush plants and roots at the edges. Divisions of these grasses can be made at the periphery and transplanted into the center of the thicket to rejuvenate.
Digging and dividing grasses can be done any time of year, theoretically, as long as the soil is not frozen. The least amount of stress from water loss or heat occurs when the ornamental grass is dormant. In cold winter regions, digging grasses for rejuvenation is best done in early spring before the new growth has barely started. In milder regions where the fall and winter months are moist and cool, fall digging and planting can be undertaken after the first fall frost kills the grass foliage. According to Plant Talk Colorado, cool-season grasses are best divided and replanted in late summer just before the cooler, moist growing season begins.
Since the root systems of the divided ornamental grasses was disrupted during the rejuvenation task, it is important to water the newly planted grasses. Keep the soil moist but refrain from over-watering to create a soggy soil which can lead to fungal rot. Conversely, do not allow the root ball of these transplants to get too dry as the root ball health and growth rate will diminish as it tries to survive. Stressed, unhealthy grass roots will not create strong and abundant new leaves and flower/seed plumes.