Should iris rhizomes be mulched?
Transplanting Iris Rhizomes
The point of winter mulching is to prevent freeze and thaw cycles which can heave the rhizomes. So if you live in a climate of on and off freezing temperatures, wait until the ground freezes (Dec/Jan depending on location) and then cover with a few inches of mulch. You are trying to prevent a warm day or sunlight from thawing the soil and rhizome. In spring, remove the mulch.