Maybe (probably) either different soil conditions or iris varieties, but our iris here take over any patch of soil if they're left undisturbed for several years. I've learned that evey other year I need to dig them up, replant half (wait, half? crap, how many ARE there? fine, one-third, make it one-third) and find new places for the remaining plants. They only thing that slows mine down are deep shade. DEEP shade.