Try to make your search as simple as possible:
Dipsacus will find all items including that sequence of letters.
Search results for "Dipsacus":
"Dipsacus fullonum v sativus" - Dipsacus sativus (hooked napping teasel)
"Dipsacus sylvestris" - Dipsacus fullonum (not hooked fullers teasel)