Try to make your search as simple as possible:
Diplarrhena will find all items including that sequence of letters.
Search results for "Diplarrhena":
"Diplarrhena latifolia" - Diplarrena latifolia
"Diplarrhena moraea" - Diplarrena moraea
"Diplarrhena moroea prov.Tas" - Diplarrena moraea prov. Tasmania