George,
The Search bar in the plant database has worked fairly well for me, once I get past the idea that I can't specify genus or species in their own fields.
I just type whatever names I know, whether Latin binomial names, or cultivar name, or comon names and then sort through the list that comes up.
Often it helps to go to just one particular genus or specialized database, like "Lettuce" or "Adeniums". It is a general feature in this site that a "Search" is location-sensitive. If you Search from the Plant database, only the plant database is searched. If you search from "Forums", only forums are searched. If you Search from the "Lettuce" database, only lettuces are searched.
I'm not sure, but it is possible that if you search from inside one forum, maybe only that forum is searched.
Some admins have suggested that if I have trouble finding a specific cultivar name, it may be best to go to that genus or database, then "Browse all Cultivars". Then you get an alphabetical list where, if the first few letters are known, you can scroll past them all looking for alternate spellings.
For sure, in cultivar names, try combining or splitting words. For example, "Oak Leaf" and "oakleaf".
I hope any of that is helpful. Sometimes i wish for the kind of search where you can enter
Genus: _______
Species: _______
Cultivar: _______
However, if someone only has one word and doesn't know yet whether that is a genus or a species, this site's search style of "type it all in and Dave will find all matches" may be easier to use.