A Comparison of Inter- and Intraspecific Interference on Broom Snakeweed (Gutierrezia sarothrae) Seedling Growth

Broom snakeweed (snakeweed) is a native range shrub found throughout semiarid rangelands of the western United States, which increases and dominates plant communities after disturbances such as overgrazing, drought, or wildfire. The objective of this study was to compare the ability of selected grass species and prostrate kochia to restrict establishment and growth of snakeweed seedlings in potted-plant and replicated field studies within two sagebrush ecological sites. In the potted-plant studies, single snakeweed seedlings were grown with seedlings (seedling neighbor study) and established plants (established neighbor study) of three cool-season grasses (crested, pubescent, and bluebunch wheatgrass), prostrate kochia, and snakeweed at increasing densities (1, 3, 5 plants/pot). Interference from crested wheatgrass in the seedling neighbor study, and both crested and bluebunch wheatgrass in the established neighbor study, induced the greatest mortality of snakeweed seedlings, and snakeweed growth was suppressed more by interspecific (grass) than intraspecific (snakeweed) neighbors in both potted-plant studies. Snakeweed establishment was also evaluated at two field sites: Howell and Nephi, UT. Snakeweed and downy brome were controlled by picloram (0.25 kg ae/ha) and glyphosate (1.5 kg ae/ha), then three native and three introduced grasses were drill-seeded, and prostrate kochia was dribble-seeded in replicated plots (3 m by 15 m) at both sites in October 2003. Snakeweed seedlings were transplanted into seeded plots and a bare ground control plot in autumn 2004. Snakeweed mortality was greatest (73%) in crested wheatgrass plots at Howell, but there were few differences among species treatments at Nephi. Of the snakeweed seedlings that survived, there was relatively little growth in any of the seeded plots compared to those in the bare ground control plots. These results indicate that seeded cool-season grasses interfered with and reduced establishment of snakeweed seedlings. Nomenclature: Glyphosate; picloram; bluebunch wheatgrass, Pseudoroegneria spicata Pursh 'Goldar'; broom snakeweed, Guterrezia sarothrae (Pursh) Britt. & Rusby GUESA; crested wheatgrass, Agropyron cristatum (L.) Gaertner X A. desertorum (Fisch. Ex Link) Schultes 'Hycrest'; downy brome, Bromus tectorum L; prostrate kochia, Kochia prostrata (L.) Schrader 'Immigrant'; pubescent wheatgrass, Elytrigia intermedia ssp. trichophorum (Host) Beauv. 'Luna'.