Silphium, known as an ancient method of birth control, was such an integral part of the Cyrenian economy that it was inscribed on their coins. It was likely farmed to extinction, and reportedly was a bit finicky and cultivation attempts failed.

It was featured on a several Greek coins over the years, but whether it was an advertisement or a nod to its importance is not entirely clear. At any rate, that's the first example of a plant having such importance that it became a national symbol that came to mind for me.

Dalby, Andrew (2002). Dangerous Tastes: The Story of Spices