It's a smartweed (Polygonum or Persicaria, depending on who you believe) of some kind. I would have guessed Jumpseed (Polygonum virginianum), but it's not supposed to occur anywhere near Idaho. I wouldn't be surprised if there was a similar species growing in your part of the world, but I don't know which one it might be.
Or, maybe, it was tagging along in the pot with the hosta?
