Susan, I pot up all my plants I get over the summer because it's just too hot with our intense sunlight. With irises I just use cheap potting soil with NO fertilizer in it that I get at Lowe's, and I put the 1 gal. pots on a makeshift plywood table in the shade (pergola plus large umbrellas) and I wait to plant them in the ground until Sept-Oct. when they've rooted and the sun is further south in the sky and I can just plunk the rooted contents in the ground. But if you can't do that you should probably store the boxes the way they came in the shade to wait for some cooler weather for planting. I don't know about keeping them in the boxes for a long time, though -- I'm thinking maybe 10-15 days tops.
Thomas Johnson included a note with the Mid-America order for the first time that they understand that much of the country has been experiencing a heat wave, whereas in Salem it has been cool, maybe 60s-70s, so their plants are full of moisture and if people plant them right away in their drought conditions the irises might go into shock, so he recommended doing pretty much what I said above, with temporarily potting them up so they can slowly adapt being the best option. We've always had those hot, dry drought conditions here in the summer, so I learned the hard way years ago to pot mine up to transition them into my climate.
