Oh -- I hope not, Julia. Especially since I have my Daylilies mixed in with my Irises in several locations.
Polly -- I do the same thing -- plant my Iris rhizomes, then spread Preen, then water. I forgot to do that on a bed I planted in August -- I now have a bed full of annual weeds that will have to be hand-weeded. Can't take a chance on hitting the Iris leaves with Round-Up spray..............