It's a variety thing. Some have bigger blooms, some have
sturdier stems. It's pretty much your choice - stake 'em,
let 'em lie, plant a sturdier variety...

In article ,
says...
Is it normal for iris to fall over? I mean the flowers, of course. They
were fine until the flowers came out and then boom, down they went.
Should I tie them up somehow? The flowers are HUGE.