Purple Nut-sedge
Nutgrass
Cyperus rotundus
Photograph is hosted at Wikimedia where there are more photos of sedge species.
Flowering stem cross-section.
This plant has been labeled as one of the world's worst weeds. In the Sonoran Desert it is an exotic, introduced weed found primarily on irrigated land because it requires plenty of moisture. It is frequent in urban lawns of Phoenix and Tucson - mostly those that are over watered. They survive mowing and can grow close to the soil - rarely blooming and almost never going to seed and indeed look rather like grass. Reproduction is almost exclusively vegetative by means of nut-like tubers on the underground stolons.
In cross section the stems are clearly triangular, as shown in the image at lower left, unlike true grasses.
Close-up of an inflorescence. The filaments emerging from the spikes are the female stigmas - their feathery structure is designed to capture airborne pollen released by other nut-sedge plants. The reddish-purple hue gives the common name. Flowering in the Sonoran Desert comes late in the season Sept. to Nov. Seed rarely developes however.