Muhly grass is the common name given to any of the ornamental plants in the genus Muhlenbergia. Perhaps the most showy species is pink muhly or Gulf muhly (Muhlenbergia capillaris), which displays rosy pink flower spikes and seed plumes in late summer or early fall. Other muhly grasses grown in the United States include pine muhly (M. dubia), Texas muhly (M. lindheimeri), bush muhly (M. porteri) and bamboo muhly (M. dumosa). Trim back these perennial grasses in late winter to early spring. In regions with winter temperatures that drop below minus 10 degrees Fahrenheit, muhly grasses die by spring and don't need further maintenance.
Muhly grasses can be trimmed back to the ground, or stumps of stems about 2 to 6 inches tall anytime after fall or winter frosts and freezes turn foliage tan and dead. Gardeners allow cold-killed muhly grass seed plumes and foliage to remain through the winter, adding a wispy texture to the barren winter landscape, and interest when covered with frost or light snow. Trim back dead foliage on muhly grasses before any new leaf blades start sprouting from the roots by spring. You can be tardy in the trimming, but the already-emerging leaves will display blunt-cut wounds if trimmed too late in spring.
Muhly grasses remain evergreen in regions with frost-free winters, such as Southern California or peninsular Florida. Some leaf blades may remain green across the winter. Wait until mid to late February and then trim back the muhly grasses within 6 inches of the ground each year. This removes any old, tired-looking tissues and results in rich green plants in March and April with the flush of new leaves.
Avoid trimming back muhly grasses when plants are actively growing during the summer. Cutting back the leaves will not kill muhly grass, but the plant will look coarse until the newest leaves grow above the level of the trimming cut. Trimming back muhly grasses too late in summer can prevent the production of the wispy white or pink flower plumes, which are the most ornamental parts of the grass.
Rather than relying on trimming clumps of muhly grass to keep plants smaller and in scale with your garden bed, dig up and divide the root clump every three to six years. In late winter, dig up the large muhly grass clump and slice the root ball into halves or thirds. Replant the healthiest-looking root ball segment in the original location and allow it to grow normally the rest of spring, summer and fall. The remaining sections of root balls can be composted or thrown away, given to a gardening friend or planted in a new area of your landscape.