Scotch Broom isn't a native Pacific Northwest species; it's originally from Western Europe. Scotch Broom was first planted as an ornamental shrub on the east end of Canada's Vancouver Island in 1850, by Captain Walter Grant who planted broom at his farm as an ornamental shrub. From there, the plant quickly spread to the rest of Vancouver Island. From there, it spread to the mainland in British Columbia, and Western Washington state, especially in the Puget Sound area. The shrub's tolerance for sun, deep roots, and ability to thrive attracted the attention of state and province highway departments, and they planted Scotch Broom along roads as a soil retention measure.
Unfortunately, Scotch Broom has no natural enemies in its North American habitat, and it has not only thrived, it is pushing out the native plants. It's a nitrogen fixer, like most legumes, which means it can thrive in very poor soil, and it changes the chemical composition of the soil, making it difficult for many native species to thrive. It spreads rapidly because a single shrub produces thousands of fertile seeds, which remain viable for decades. The seeds form in pods and ripen in late summer, when they split their pods and are ejected as much as twenty feet away. Scotch Broom rapidly began to take over where it was planted, and essentially choked out native vegetation. Efforts to control the shrub by chopping it down prove ineffective because the deep roots allow the plant to resprout. The very rapid growth means that the shrub easily overtakes native plants that feed wildlife who do not eat the seeds or stems of Scotch Broom.
In Washington state and in several other states and Canadian provinces, Scotch Broom is officially recognized as a Class B noxious weed. It's every bit as pernicious as Gorse, another invasive species from Europe that is sometimes taken for Scotch Broom. It is not legal to sell Scotch Broom in Washington, and the Washington State Department of Agriculture urges homeowners not to plat it even as an ornamental shrub. There's a downloadable .pdf file on "Best Management Practices for Scotch Broom" that describes methods of curtailing the spread of Scotch Broom.