Thanks a lot for taking the time to delve into this question. You are right: a common plant must have a name. But names do change, even with common plants. So it could well be possible that the Romans (or rather some pre-Roman Italic tribe?) had a different name for the plant which was later replaced by the Greek expression - along with its mythological and practical implications. This is bare theory, I must admit, and I would have to find other examples and tangiable evidence.
Unfortunately I am packing up my whole house at the moment (including a family of six) to set off for the States for one year two weeks from now. This will also keep me from taking a look at the books you mention (thanks for that!) - unless I get to visit a good library. But maybe I'll take a book or two on Indo-European languages with me.