Native Perennial Grasses in Highway Medians: Pre- and Postplant Techniques for Establishment in a Mediterranean Climate

Within highway rights-of-way, native perennial grasses provide desirable services to support natural and human constructed ecosystems. However, native perennial grass establishment in annual grass dominated roadsides of semiarid and Mediterranean climates of the western United States requires specific cultural and chemical management treatments to control weeds. In 2004, field studies were conducted in Sacramento Valley, California to determine the effect of herbicide, disc cultivation, and species selection on native perennial grass establishment and annual weed persistence. Perennial grass species mixes common to drier and wetter upland areas in northern California were drill seeded at two sites (I-5 North and I-5 South) that had been burned in 2003 and received weed control (i.e., herbicide, cultivation, mowing) in spring 2004. Herbicides were the most important treatments for native perennial grass establishment and weed reduction. Native perennial grass species persistence was largely unaffected by cultivation or native plant accessions at these sites. Native perennial grass density increased at I-5 North in the second year of growth (2006) resulting in a plant density totaled across all herbicide regimes of 3.9 plants m 21 compared to 2.5 plants m 21 at I-5 South. Vigorous native perennial grass growth in the more fertile and less droughty soils of I-5 North helped to limit annual weeds through competition, which is anticipated to reduce the need for chemical and mechanical control in years following early establishment. Weed management is required in the first few years after the establishment of native perennial grasses along roadsides in the arid (250 to 500 mm [9.8 to 19.3 in] precipitation per year) and Mediterranean climates (hot, dry summers and cool, wet winters) typical of the western United States (Wrysinski 1999). Roadside revegetation projects that receive little management for controlling weeds and promoting native perennial grasses have greater potential for reverting to stands of the preexisting weeds (see Rentch et al. 2005). Because of this common trend, successful native perennial grass establishment requires active weed control (Anderson and Long 1999). Roadsides can be difficult locations for establishing native perennial grasses. Site conditions that can limit the success of any revegetation effort include topography impacts (traffic, maintenance activities). Brooks (1995) found revegetation treatments for visual impact mitigation were unsatisfactory on 72% of the cut slopes evaluated in the Tonto National Forest in central Arizona. In the Sacramento Valley of northern California, Bugg et al. (1997) report resident weeds influenced native perennial grass establishment, but …