The objective of the thesis was to gain an understanding of how soil amendments with biosolids controlled Scotch Broom. Scotch Broom is an invasive woody species in northern California and the Pacific Northwest typical of well-drained, nutrient-poor mineral soils. Management of Scotch Broom or other invasive species, is a growing design and maintenance challenge for landscape architects. Current methods of control are based on one or more of the following methods: physical, chemical, biological, burning. These methods are often ineffective, labor intensive, and cost prohibitive, or have adverse side effects to ecosystem resources and processes.

Cases in the Pacific Northwest have shown success in controlling Scotch Broom or undesirable tree species by using soil amendments with biosolids. Biosolids contain high amounts of nitrogen and, unlike commercial fertilizers, organic matter. Application rates of biosolids are based on theories which consider the concentrations of and relationships between carbon (C) and nitrogen (N). Research showed the importance of nitrogen availability in creating self-sustaining herbaceous plant communities which were competitive with woody species.

In the absence of any research regarding the use of soil amendments, with or without biosolids, to control Scotch Broom, or any other invasive species, a field study was designed as part of this thesis to test the relationships between C, N, the C:N ratio, herbaceous cover, and Scotch Broom occurrence. The results of the field study suggested that as C, N, and herbaceous cover increased, and the C:N ratio decreased, Scotch Broom occurrence decreased. These results combined with observations from two case studies suggest that both high amounts of C, N, and organic matter, at low C:N ratios will control Scotch Broom either directly from the biosolids application, or indirectly from competition resulting from the biosolids application. Separate studies are necessary to determine the role of competition. In either case, these findings provide another method to control Scotch Broom that is valuable for landscape architects when designing or managing rehabilitation projects, parks, and open space.