It's tomentosa, not "tomentosum." In this case, the Catalogue of Life shows it as a provisionally accepted name, a synonym of A. leptophylla, and a synonym of A. millefolium. It's clearly unresolved here, but the ITIS search page says it's not accepted and is a synonym of A. millefolium.
http://www.catalogueoflife.org...
http://www.itis.gov/servlet/Si...
