I got some rhizomes but was only able to plant a few. I left them outside and now the roots are dry. Can I soak the roots and go ahead and plant them.
This seems to be one of those questions which has a 'What's the alternative?' answer. You can do as you suggest and they may well grow. But if you don't do it they certainly won't. It will depend on how dry they are, how cold it gets and how soon. So I'd just do it and see what happens.
Bearded iris (if that's what you have) is absolutely the toughest plant I have ever seen. I've dug them up in the spring and completely forgot I had done so, realized that they were sitting in a plastic bucket in the shade, planted them in fall, and they grew and returned to full size the next year.
I've also dug them up and set them aside on top of mulch and the irises on the the bottom of the stack rooted and were growing up through the others.
It's gotten to the point where I've gotten lazy and don't even "plant" bearded iris anymore. When I want to plant some I scrape off the mulch, rough up the soil a little bit, set them on top of the soil and then re-cover them in much. As soon as it rains they root and grow.
Not suggesting you do this in this particular situation where they're already weakened, but if they are not completely dry through-and-through or rotted, they will live. In severe stress the older part of the rhizomes will die first and it will progressively advance to the newest portions, so there could be dead portions of rhizome but the plants might still survive.
I don't think you need to soak them. They are either alive or dead and soaking doesn't change it. It's already cool-weather season so if you water them in when you plant them, the water will last in the soil so they can contact it and it will trigger root growth.
The old-fashioned or smaller varieties of bearded iris are much tougher in this way than the big showy kinds are. But either way I say try it out, it can't hurt.
If you have another kind of iris, you can pretty much disregard all this.
If they are tall bearded Iris! Plant them now and they will be just fine.