The nice thing I have learned about my chickens is that if it is not good for them they leave it alone. For instance we have some barley mix that a brewing friend gave us (he had extra he wasn't going to use). It is a mix of roasted and dried. They eat all the white ones and leave every single black one. Won't even eat them out of my hand. Funny.
About plants. I have had horrible luck with planting and having my chickens kill in two scratches all of my work. This last summer I started putting chicken wire on the ground so the plants could grow up and out but chickens couldn't scratch very well. It has been working with my clamatis vine! I will try some sweet peas this year, sounds pretty.