here's a link that may help with your "culture" question:
http://irises.org/About_Irises...
as to the "rhizomes going deeper".....how 'deep" is deep ? I've found that , in my VERY loose, heavily amended soil, there appears to be several factors involved: As the rhizome grows in length, and pushes itself along the ground, the base of the leaves (fans) actually act much like a bulldozer blade, pushing the loose soil into a berm (ridge) around the base of the plant. If I don't either dig, divide, and reset the plant, every 2-3 years, OR remove the "berm" every year.....mine can get "too deep". Another factor, with 'looser soils" seems to be "splashing". In even moderate rains , the loose soil is "slashed" onto the leaves, washes DOWN the leaf, and , of course, piles up there.
But, if "deep" is only an inch, or less.......I don't consider that TOO deep. They will tend to "seek their own level", and, in sandy soils, with good drainage, a one inch covering of soil doesn't seem to bother them.
Another factor , at least in my beds that have been heavily amended with organic soils, is that during the freeze-thaw cycles (and we can have many of those , during "open winters")the soil surrounding the plant, "fluffs up", or "heaves", while the soil IN the clump is actually "held down" by the roots and rhizomes....in effect, the base of the plant is growing in a shallow "bowl"....then, rains wash some of the surrounding "fluffy soil" onto the base of the plant.
Bottom line, if you aren't experiencing any "woes" from the "deep" ones......rot issues or extremely "goose-necked" rhizomes......it's probably a "non-issue".If you ARE having rot issues.....reset the plants a bit more often, and plant on a slight "mound "of soil....it only needs to be a couple inches higher than the surrounding area. That *should*keep the rhizomes from becoming "too deep", at least long enough to reach your next "dig & divide" time.
