This one took a while to get added and to those who were waiting for it, I'm sorry for the delay. But it's here now, better late than never!
The Also Sold As (ASA) field appears when you edit the name of the plant, and it acts just like the common names or cultivars. You click the green plus sign to add a new field and then type away.
It is a free form field and you can type whatever you want into that field.
The ASA field is indexed by the plants search engine, so you can do searches and find plants by their ASA names.
