Considering that the iris roots hanging down from the rhizome never seem to be longer than about 3" long when I receive them from growers, or dig them up myself, I think that the AIS recommendation to cultivating to 10" is probably needless overkill.
Also, any experienced farmer will say that you don't till when the soil is wet or soaked, because that creates a lot of hard clumps, particularly if you are trying to dig up hard-pan clay. I think it's best just to till down to the hard-pan, and add amendments to what you have managed to till. Our topsoil in this area of Maryland is only about 4" deep, if that, before we hit the impossibly hard hardpan clay. When making my gardens for the first time, I ran a Rototiller over the area 3 times and could not penetrate the hardpan. My garden has done fine, nevertheless. If I need a deeper hole than 3", a combination of shovel, or even a 6' iron prybar can be brought to the task!
