Ferns in a town.
Ferns displaying the "unharvestable" message.
Ferns are a plant within the game that can be destroyed with your fist, and doing so will yield medical plants, leaves and fibre. They are usually found in towns. If you try to destroy them with any other tool, E.G: Axe or Fireclub, it will come up with a message saying, "A different tool is needed for the task..."
TRIVIA:
-Before the most recent update, these plants couldn't be destroyed with ANY item, including your fists.