hehehe...Saw the title of your post and I thought, oh, lord, that's just the kind of thing Huey would do. He wants to put every little thing in his mouth. Leaves, pieces of wood, I've never found him with a rock in his mouth, but I'm sure that's just a matter of time...he wants to nose in the muck bucket in case there might be a few pieces of hay in there that haven't been through anyone's digestive tract.
I'm hoping by now you've passed an uneventful night and that your boy is showing no ill effects from the flower. Probably find it in his manure today, looking just like it did yesterday, only a bit worse the wear for the trip down his system. I'm sure that some day you will look back on this and laugh, but in the meantime, know that you have plenty of company with the rest of us who lose sleep at night worrying about our horses. :)