The answer would be: When the people you're talking to don't know anything about trees or  botanical nomenclature.

A Sierra Club forestry expert in Michigan wanted to honor his state tree, so he did what anybody with a passion would do: He ordered vanity plates for his car. But, because Marvin Roberson requested the tags read "PINUS" (the latin name for the white pine, pinus strobus), and not, say, "WHITEPINE," the Michigan Department of State rejected his application on the grounds the word could be mistaken for the name for a certain male body part, according to an report this week.

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This got me thinking about obscene plant names -- and there are many. For instance, the plant at the top of this page is Trillium erectum, aka "Stinking Willie." I kid you not.

Carl Linnaeus, the father of botanical nomenclature, was considered a pretty rude and offensive guy in his day. His classification of plants largely focused on their similarities to human genitalia and sexuality. Back in the 1700s, he referred to stamens as husbands, and pistils as wives, shocking the community at large. Pretty much, he was regarded as a pervert.

But who among us can say orchids don't give them pause -- a sort of speechless-what-am-I-looking-at pause? Linneaus just called them as he saw them.