Hi.... is Cosmos safe to plant around the outside of a chook run?
Cosmos…Is It Safe for Chickens
Certified GKH Gardening Expert
As a former chicken owner and gardener, I will say that chickens will eat nearly anything they are presented. Food is food and they don't care if it's your pretty flowers.
As for the safety of Cosmos for the hens, I would suggest contacting a vet or someone more specialized on chicken care.
This link you may find interesting.
I would suggest keeping hens confined to the coop until plants are a bit taller and bigger in spring. Hens can be less interested in bigger plants then tender seedlings.