An expansive carpet of mondo grass (Ophiopogon sp.) makes an attractive, fine-textured ground cover in a garden setting. When soils are fertile and overly moist, conditions favor the establishment of the grass-like weed known as nutsedge (Carex sp.). Once established, these weeds are difficult to eradicate since they grow from tiny bulb-like tubers deep in the soil. Pulling is partially successful, as is using typical garden herbicides that will kill the mondo grass along with the nutsedge. Using a nutsedge-specific herbicide kills the weed with minimal or no detriment to the many tufts of mondo grass.
Purchase a herbicide specifically formulated to kill sedges. These herbicides often contain the active chemical ingredient halosulfuron. One product available to home gardeners on the market in the United States is called "Sedgehammer."
Read the selective herbicide's product label fully to determine best application. Take note of any cautions such as the time of day or temperatures in which to apply the herbicide as well as dosage formulas.
Mix a small batch of the herbicide per label directions and apply it to a small area, about 1 square foot, in the mondo grass patch.
Monitor this "test area" over two to seven days primarily for any adverse effects on the mondo grass. If you find that the mondo grass yellows and looks sickly, discontinue use of the sedge-selective herbicide altogether. Conversely, if the mondo grass remains green and healthy-looking, plan on applying the chemical to the entire planting area in an attempt to rid it as much of the nutsedge as possible.