Try to make your search as simple as possible:
Thalictrum will find all items including that sequence of letters.
Search results for "Thalictrum":
"Thalictrum dipterocarpum (hort" - Thalictrum delavayi
"Thalictrum dipterocarpum Franc" - Thalictrum delavayi
"Thalictrum fendleri var. polyc" - Thalictrum polycarpum
"Thalictrum minus ssp minus" - Thalictrum minus
"Thalictrum sylvaticum" - Thalictrum minus