Of course, "palm" is neither type nor genus. But whether it might be a freshly invented leave_type instead, has no consensus. Moreover, undiscussed mass edits are no-go.  https://community.openstreetmap.org/t/leaf-type-palm-is-anything-wrong-with-it/109646

Particularly in this botanical garden with name tags on a quarter of the exhibited plants, osm tagging should conform with botanical classification by genus and species (or at least by taxon). A remote mapper may be unable to determine ground truth. Ok, then, please, leave detailed tagging to local mappers. 
By the way, why not travel to Cologne and visit our wonderful Flora? It's worth it.