Well, cultivar names are cultivars - they are usually (always?) registered.
Trade names are copyrighted and trademarked names that are used in place of the cultivar name. As far as I know, you can't have a trade name without first having a cultivar name, since you can't trademark a non-cultivared variety (that I know of).
Evan, I do believe that common name is the right choice when you have a name that is not a cultivar and not a trade name. Those common name fields show up right at the top and if a particular name has more thumbs than any other, then it becomes the default name that is shown when you're looking at the plant lists.
