Metal  label tags for mushroom logs (nails included) 
These aluminium plant labels are the best thing we've found for permanently keeping track of what species of mushroom you've got inoculated in your logs.
Simply write on the label with an old ball point pen (doesn't have to be working) to create a permanent indentation on the tag that will never wear off no matter how much weather  it gets exposed to. Then attach to the end of the log with two tacking nails (included).