When people say that Poppy straw is legal, they are wrong
All parts of Papever Somniferum, excluding the seeds, are considered Schedule II controlled substances, and therefore cannot be legally possessed without prescription
People just are not generally prosecuted for possession of the Poppy plant, except for in cases of large scale growing operations
I would think that you would be much more likely to get in trouble at an airport though, since they search people specifically for things they are not allowed to have on their person. But that's just my thinking
So whatever you decide, know that it's still illegal