How can I get rid of invasive Acanthus mollis? It was present in my yard when I bought the house.
Multnomah County Oregon invasive species
Thanks for your question. I had not known that the Acanthus mollis could be invasive. A quick look on the internet showed you are not alone in your assessment. While the plant is not on the Oregon noxious weed list or the US list of invasive species, it is on the Global Invasive Species Database. You can see the description of Acanthus mollis on the database at: http://www.iucngisd.org/gisd/species. It is considered an invasive species in Australia.
Discussion of the plant's invasive nature in the Pacific Northwest is on the University of Washington's Elizabeth C. Miller Library web site that you can view here: http://depts.washington.edu/hortlib/resources/resource_search.php?userTerm=acanthus+mollis&submi...
An interesting discussion of the invasive nature of Acanthus mollis is debated on Dave's Garden website. You can view it at http://davesgarden.com/guides/pf/go/177/#b. Contributors to the site share both praise and disgust for the plant and, pertinent to your query, ways to get rid of it.
What I learned from these various web sites and in response to your question: to eliminate Acanthus mollis, you will need to dig up the plants and then studiously remove any root fragments from the soil. You may have to do that repeatedly for several years to achieve success. One gardener recommends totaling removing the soil.
Check out what these sources reveal and pick a method with which you are comfortable .