Determining the suitability of native grasses for highway revegetation sod

Abstract

In past years, the California Department of Transportation (Caltrans) has used hydroseeding, imprinting and drill seeding methods to revegetate highway construction sites with varying degrees of success. Ecological concerns have led researchers to consider using native species for revegetation as they are better suited to local environments, require less maintenance and do not pose a threat to adjacent ecosystems. In addition, the urgency for stabilizing areas of high erosion potential and that of establishing native plant cover quickly in order to prevent non-native plant and weed establishment, have led researchers to consider using native grass sod for highway revegetation. Twenty-one species of native grasses were selected in order to determine their suitability for sod production. Grasses were grown in six growth chambers; each mimicking one of the climates of the six selected California ecoregions. Mixtures of varying species included either one rhizomatous species with three bunch grasses, one rhizomatous species and five bunch grasses, two rhizomatous species with three bunch grasses, or two rhizomatous species with five bunch grasses.

The mixtures were grown and tested for yield, species composition and percent cover over time. At the end of the seven-month production cycle, a final harvest evaluated root architecture as well as sod strength. Results varied between mixtures and from ecoregion to ecoregion. However, 18 of the 20 native species included in the sod mixtures seemed to be tolerant of sod production methods and became established. With few exceptions, total ground cover was similar between mixtures for each ecoregion. Root architectures for individual species varied, but they balanced each other such that there were few significant differences in total root mass between mixtures. Sod strength readings varied from ecoregion to ecoregion depending on sod composition. Mixtures with more species tended to have lower sod strength. Sod strength tended to increase as the percent composition of rhizomatous species increased. Mixtures with the rhizomatous species Festuca rubra tended to have greater sod strength than mixtures with other rhizomatous species.