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"Fern" is a very broad category, but for the species name (the 'imagined' part) I would go with either fictus or conceptus. The fern that you think of when you think "fern" is probably of the Athyrium genus. So I would say Athyrium fictum or Athyrium conceptum (you have to change the ending of the adjective to match the (probably) neuter gender of athyrium). One of the Latin words for fern is filix, which is feminine, so yo could also do Filix ficta or Filix concepta.
EDIT: The genus you'd want is actually Polypodium, so you want Polypodium fictum or Polypodium conceptum. (source: my Latin teacher, who's also into botany).
Source: William Whitaker's Words and two years of Latin.
Thanks! I think I will go with "Fernus imaginatus", because I wanted just something fern-like but without allusions to real plants - a name for a spherical fern in vacuum, if you wish.