The Irises Database, like most custom databases on ATP, has it's own unique architecture to accommodate the particular features of the genus. The vast majority of information about irises, including hardiness, has been recorded in the customized portion of the database. If you want to include hardiness in your search, you'll need to check one of the boxes in the Hardiness field (6th one down, after Classification).
http://garden.org/plants/group...
Please note that if you are searching for tall bearded iris, for example, checking "Zone 5: -20 to -10" won't work because they all have the "Tall Bearded Zone 3-9" box checked, not the individual zone boxes.
For what it is worth, checking the "Fragrant" box in the Flowers field only works because, as I recall, Evan asked Dave to program the Iris Database so that whenever any of the boxes in the Fragrance field (except "Absent") the "Fragrant" box in the Flowers field would automatically be checked as well. However, there are just a handful of fields in General Plant Information part of the Iris Database that get used.
