I have no exact way of proving it, but the deep long roots, may be a way, for the Iris, to survive some prolonged periods of drought.
While shallow roots, may be another way, of adapting to the environment, by collecting precipitation, that is reduced , or overall, in lesser total annual amounts. Including from dew at night.
As long as the rhizome itself, does not become dessicated while nearly dormant in summer, it can grow new roots, but then I wouldn't expect it to increase as well either , when it does so.
Or , as it otherwise might, to recover from drought, in poor soils & low moisture conditions, unless it does produce roots located near the surface to adapt to the situation. I've seen the Iris rhizomes before, where they were actually nearly growing completely, at the very surface of the soil & still bloom away, during spring.
