Thank you to DarinAz@Flickr for sharing one of his excellent photographs (contributed via the UBC BG Botany Photo of the Day Flickr Pool | original). Appreciated once again! If you love plants, I do recommend visiting Darin’s photograph sets on Flickr — there’s much to enjoy.
Silver senna or feathery senna is endemic to much of mainland arid Australia, with the exception of the state of Victoria. It seems to have naturalized in both Arizona and California. Previously thought to be a member of the genus Cassia, its former scientific name and related common names (e.g., silver cassia or feathery cassia) still persist in many online references.
The Germplasm Resources Information Network (GRIN) recognizes seven subspecies and three nothosubspecies, or naturally-occurring hybrid subspecies. I’m somewhat confused as the Wikipedia page for Senna artemisioides mentions both a subspecies artemisioides and a hybrid subspecies artemisioides, whereas GRIN only recognizes the latter. What I’m not confused by, though, is the fact that there seems to be much taxonomic work to be done with Senna artemisioides and all of its subtaxa. It seems to me that this is one of those instances whereby the process of presently-occurring speciation is denying taxonomists the ability to sort everything into little boxes.
If you are interested in growing this plant, the Master Gardeners
of the University of Arizona Pima County Cooperative Extension provide this growing guide. FloraBase, the Flora of Western Australia, provides a few more photographs of Senna artemisioides.