Charles Day is Wave Hill’s Ruth Rea Howell Horticultural Interpreter.
This plant has several common names, including “dirty thoroughwort,” “blue mist flower” and “purple torch of the clouds.” It has more than one scientific name, too: it is now known as Bartlettina sordida but previously was classified as a eupatorium, hence the inclusion of Eupatorium sordidum as the synonym above.
A change of name is not unusual in the world of botany. It can result from new research (often via genetic testing), which might show that a certain species has more in common with a different genus, or even different plant family, than the one it to which it was originally assigned. Hence it has to be reclassified and given a new name.
Another common reason is that more than one name may have been given to the same plant species. Different botanists might have discovered it on different occasions with each discoverer ascribing it a different name. Confusion can reign until it is established (by clearly defined scientific protocols) which name is officially recognized.
Nomenclature aside, this handsome plant is great for a cool greenhouse—such as our Palm House, the central section of the Marco Polo Stufano Conservatory—where its large, slightly raggedy, purple flowers make a wonderful contrast against the tidier blooms of the orange clivias nearby.
It is native to the cloud forests of Mexico and it looks like a larger and wilder cousin—which indeed it is—of the more familiar ageratum (Ageratum houstonianum), a common summer bedding plant used in gardens and containers.