Thoroughly wash and rinse a clay pot. Air-dry the clay pot for a minimum of 72 hours to make certain it is completely dry.
Gather moss from the shady side of a tree, the woods or purchase from your local gardening store. Rub the bottom of the moss with a clean, dry toothbrush to remove dirt, debris and sticks.
Break the moss into very small pieces and place them in a small glass bowl. You cannot make the pieces too small for growing on a clay pot. Break up enough moss pieces to cover the different areas of the clay pot that you want to cover with moss.
Pour enough buttermilk in a glass bowl to get the moss soggy. Stir with a wooden spoon.
Dip the sponge brush into the moss mixture. Brush the mixture on the outside rim of the clay pot. Repeat dipping the sponge in the mixture and applying to the entire surface of the clay pot.
Set the clay pot inside a plastic grocery bag. If the grocery bag is too small, use a small plastic trash bag. Loosely tie the bag shut.
Place the clay pot in a dark area for 3 days. Remove the bag. You will see moss growing on the pot and it's ready to use for plants or to display.
Making Stenciled Designs
Follow the steps in the first section to make the moss and buttermilk mixture.
Tape your stencil to the clay pot with painter's tape.
Dip the sponge brush into the moss mixture from Step 4. Brush the mixture into the open area of the stencil. Repeat Steps 5 and 6 to all areas of the clay pot to make designs.
Set the pot in a plastic bag to age as described in the previous section.