Hi, Heather.
to ATP.
Is it a TB iris? Is the plant bare-root now? If so, I'd think it wouldn't matter either way. I'd be very cautious about mulching it, however; if the mulch blows over the crown and settles in it may invite rot. Do watch it for excessive frost heaving, whichever planting method you choose. If it heaves, press it gently back into the soil until it makes firm contact, but don't let the crown sink below the soil level.
