Subject: best method to hang a sign from a tree

We made tree label signs identifying tree species. How do we hang them without 
harming the tree? So far we have eye hooks screwed into each side of the sign. 
Then we added a spring on one eye hook to provide expansion as the tree grows. 
Now we want to use fishing line. Is that OK? if not how about loose chain? 
Please advise. Most ppl say to just nail it on the tree. Activists will not 
appreciate that. 

Re: best method to hang a sign from a tree