Depends on the type of pot. Not likely to have much effect except on unglazed pottery (terra-cotta). A 50% mix of beer & yogurt or buttermilk is an excellent way to get moss growing on outside of the container, especially if the container is shaded by the plant material, or is, in fact, in the shade.

Al

And do think about how often you want to move the pot. That algal bloom is awful to clutch and hold - and makes a nasty stain if brushed against.

For unglazed terra cotta - quite often the salts from the fertiliser leaching through the pores in the pot will be enough to create a favourable environment for some mosses and algae. ;-(

Good points made, but I understand the fastest effect is gotten when you mix bits of moss into the yoghurt [or buttermilk] solution just before painting it onto the pot. The pots are generally kept out of direct sun because they need to stay slightly damp (another reason why unglazed is used). Once the moss is established, it may be somewhat sun-tolerant provided it doesn't dry out.

Beer? I dunno - I've had success while being non-alcoholic - both me and the pot, lol.

Catherine, how long will depend on your humidity/weather. A general 'recipe' is

Clump of moss (remove dirt) Add moss to a blender. One can of beer + 1 tsp - 2 tsp sugar, 1 cup buttermilk, 1 cup yogurt

Blend thoroughly to pulverize the moss
Spread the soupy mixture onto your pot with a brush wherever you want the moss to grow

After you have applied your mixture, place pot in a cool, shaded area, and mist it often enough to keep it slightly damp - if it dries out the moss dies. Just think about how moss grows in the wild - found in damp, shaded spots.

You can create the effect of the same patina using three or four colors of latex paint and a sponge (shades of green from pale blue-green through black green). Or you could just send your pots here and I'll leave them on my deck - and I'll mark them as yours so I don't scrub off the algae and moss as I do my own in Spring so it doesn't make green circles on my deck :)