Thank you for your question.
Scotch Broom is a perennial problem when it seeds itself beyond its place. I have a couple of links for you to look over and perhaps save as PDFs for future refernece, as Scotch Broom is often a recurring problem. For information on the life cycle and conditions that abet Scotch Broom, I suggest Scotch Broom: Biology and Management in the Pacific Northwest, catalog Number PNW 103. It can be downloaded free of charge here: https://catalog.extension.oregonstate.edu/sites/catalog.extension.oregonstate.edu/files/project/pdf/...
In addition, for more information on strategies to eradicate or manage
the infestation that you have, there is an online article called Scotch Broom Smackdown! It can be seen online here: http://extension.oregonstate.edu/question-of-the-week/scotch-broom-smackdown
In addition, we have information on other Broom varieties in Invasive Species in Forest Land: Brooms Catalog number EC 1598E. It can be downloaded free of charge here: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/19804/ec1598-e.pdf
That's a lot of information, but i think that you will find it helpful in weighing your options when dealing with the tough growing Brooms.
Thank you again for your question and for using Ask an Expert..