Cut into some moss growing underneath trees or another shady area, and use the tip of the trowel to lift it from the ground. Pick up enough to fill the blender about halfway.
Put the moss into the blender with the yogurt or buttermilk. Blend until you have a thick, soupy mixture, adding more liquid if necessary.
Smear the mixture onto the outside of your concrete planter, using a wide paintbrush or a thick piece of old cloth. Brush it onto all the areas where you want moss to grow; make a mottled pattern, covering about a fourth of the total surface area. If you cover the whole planter, it will look moldy instead of charming.
Set the planter in a cool, shady area. Check on it every two days to make sure your moss is growing. Squeeze a wet cloth over the painted areas if the moss seems to be growing too slowly. Moisture and shade are the keys to growing a good layer of moss on the planter.