It's an automated thing, based on the scores of the plants in each plant specific database. Each plant has a score based on how many different people have posted stuff to that plant. Taken all together, then, daylilies has the highest score of them all since that is the database that has the most different people posting. Then comes irises, followed by roses, etc.
The top 10 scoring databases are shown first in order of their score, then the rest are alphabetical.
And actually I haven't updated that list in a while, and I just now updated it and the salvias (as well as dahlias) actually dropped off and were replaced by daffodils and tomatoes.
