Try to make your search as simple as possible:
Rehmannia will find all items including that sequence of letters.
Search results for "Rehmannia":
"Rehmannia angulata" - Rehmannia piasezkii
"Rehmannia angulata Rufus" - Rehmannia piasezki Rufus
"Rehmannia chinensis" - Rehmannia glutinosa
"Rhus rehmanniana" - Searsia rehmanniana