In your zone after the ground has frozen, I would lay down mulch over the iris area. Iris are notorious for frost heaving because they are so shallowly planted. Frost heaving happens when there are freeze/thaw cycles over the course of the winter season. Even in my zone 6a the Iris will heave out of the soil.
If you lay your mulch down AFTER the ground has frozen it will help keep the ground frozen until you remove it. Just don't lay it down over the iris before the ground has frozen to avoid rot problems (that goes for any plants that are susceptible to crown rot from excessive and prolonged moisture. You will also want to remove that mulch/rake it aside, ect. as soon as possible in spring for that very reason so moisture does not get trapped and the ground can warm up faster. BTW I use wheat straw often with a mix of fall leaves for winter mulch.
variety of colors depending on type: white, blue, purple (some with yellow markings), canary yellow (may have green or orange-brown markings).
green; leaves tend to be short at flowering and elongate after bloom fades to a length of about 12 inches.
beds, borders, rock gardens, around trees and shrubs, containers and indoor forcing
Plant Dwarf Iris in the spring or summer from potted plants or bare-root divisions. Bulbs should be spaced 3 inches deep and 6 inches apart in a sunny location with well-drained soil. Water regularly and do not cut back the foliage until it dies back naturally. Give them time to store enough nutrients for next year's season. Divide plants in the spring every 3 to 4 years to keep them looking their best.
Two of the most common species of the Dwarf Iris are Iris reticulata and Iris dandordiae. The first originates from the Caucasus Mountains in the Middle East and is the variety most commonly sold commercially. It comes in beautiful shades of blue and purple. Iris dandordiae exhibits all of the same characteristics as Iris reticulata, but is a bright canary-yellow color.