Strange question, @Avalon: I wouldn’t call anybody “caléndula”, whether male or female. But, to humor you, I would say “el caléndula”, using the masculine form of the article (el) even if the noun caléndula is feminine, to mark that I am referring to a man. I would not put a masculine ending to the noun, turning it into the non-existing “caléndulo” unless I wanted to mock the man it refers to. Do I make sense?