Yogurt works but you should protect your piece from rain and sprinklers. They wash off the little mosses before they can get a firm grip. So keep it wet but out of the rain and moss should take root.

One of the best moss & algae formulas involves making a "moss-smoothie". Take a cup or so of live culture yogurt and put it in ablender along with either some sphagnum moss or a sample of live moss from your yard (which is considered better because it is obviously adapted to your environment) and blend the mix together. You can also add a little buttermilk and/or flat beer, both of which contain live cultures. Once blended paint it on with a brush and keep it moist and out of direct sunlight. NOTE: Moisten it carefully with a spray bottle to avoid washing away the mix. You should see growth within as little as a couple of weeks. At least that's the story from my professional landscaper friends, and they are sticking to it.

Thanks for that Tango! I am going to give the "moss smoothie" a try!

btw... is it true that the lime in crete/tufa, discourages the growth of moss? Should a piece be treated with vinegar before applying the moss smoothie?

Thanks

To be honest I don't know how the alkalinity affects moss & algae. I do know that many of the sealers on the market today contain a small amount of copper which will absolutely prevent algae growth. Any of you algaeafficianados or moss mavens out there have the answer to this one? Are moss & algae alkaline sensitive to the degree that fresh cement paste inhibits growth?

I think damp is the key here.
Damp concrete is going to allow the lime to leech away rather quickly.
So damp concrete conducive to growing moss will also have quickly declining levels of lime.
If you can keep the concrete damp and relatively cool it should grow moss.