Lush green, prolific and looking like turf grass, nutgrass or nutsedge (Cyperus spp.) is a pesky weed that proliferates in fertile, moist soils. Not a grass but a sedge, it gets its name from the underground tubers that were once commonly referred to as "nutlets." Proper garden watering is the best way to control nutgrass, as over-watering encourages the spread of tubers across the landscape. Digging up tubers is a labor-intensive and seemingly unfruitful option, but persistent and ethical use of herbicides is your best bet.
Pre-emergent herbicides are spread as dry granules or in a water-soluble barrier on the soil surface to kill germinating nutgrass seeds. Post-emergent herbicides are used when foliage is present. Two types of post-emergents are referred to as "selective" or "non-selective." Selective herbicides are those that effect certain types of plants, such as only broad-leaf weeds in a lawn and not harming the lawn grass at all. Non-selective herbicides will act upon all green plant tissues, damaging or killing whatever the chemicals touch regardless of species.
Application of post-emergent herbicides attempts to get chemicals absorbed into the nutgrass foliage and then transported via the vascular tissue to the underground tubers. Once the tubers are killed by the herbicide, nutgrass plants neither rejuvenate nor multiply further. Pre-emergent herbicides act to prevent the germination of any kind of seed in the soil. The University of Hawaii website mentions that although nutgrasses do flower and produce seeds, these pesky weeds reproduce most regularly from underground with branching stems from tubers, making pre-emergents marginally important in overall control.
Post-emergent herbicides are most often employed to combat nutgrass infestations. Glyphosate is the only non-selective herbicide useful in fighting this weed; it is the active chemical ingredient in products like Round-Up. According to the University of California, Davis, many more effective selective herbicides are better: Halusulfuron is sold as Sedgehammer/Manage, penoxsulam as Green Light Wipe Out, sulfosulfuron as Pennant, trifloxysulfuron-sodium as Monument, imazaquin as Image and MSMA (monosodium methyl arsenate) under several brands.
Always follow product label directions for personal safety, dosages and application rates and frequency. Unfortunately, all post-emergent herbicides have limited effect on nutgrass. Repeated treatment slowly diminishes the dominance of nutgrass in your landscape. The University of California, Davis comments that the best result occurs when chemicals are applied to nutgrass plants before they reach the "five-leaf stage." The University of Hawaii, Manoa, adds that a repeat application of herbicide on nutgrass needs to occur within a three-month time frame to destroy underground tubers.
Read labels of the herbicides carefully to learn if the chemicals have harmful effects on other garden plants in your landscape, including turf grass. For example, MSMA shouldn't be applied to a lawn comprised of tall fescue, St. Augustine or centipede grasses, and improperly applied amounts can still harm Bermuda or zoysia grasses. Where shrubs or flowering annuals/perennials grow, application of some herbicides may prove disastrous; spot digging of tubers works best.