This is a very nice question^^
As Mrs. Elizabeth said, for plants you should use "aru".
However, for ghosts, of course they are not alive, "iru" is used.
and fairies, spirits (including spirits of trees) and vampires are iru. (...inai^^?
anyway, what i really wanna insist is that most of the definitions in the dictionary is not always right and there are some exclusions.