I think the previous posters were right on, you need to be worried more about keeping them cool. I'd especially second Elmo's suggestion of putting the coop in the shadiest part of your yard. I didn't have enough shade in my yard and built the run around our grapevines, which provides great cover and forage for the chickens. There is also a toxic plant list here on byc, but I can't remember where I saw it. Here is a list of plants that have been successfully grown in the runs of BYC members: https://www.backyardchickens.com/web/viewblog.php?id=22588
