"You can cultivate moss indefinitely, by planting it directly on a sticky soil-mix we call 'muck' - which can be made with 1 part potter's clay (or natural clay) and 1 part humus, such as: milled sphagnum moss/leaf mold etc... mixed thoroughly to a consistency of soft putty. It will adhere to rock or concrete and hypertufa

First, moisten the surface to be planted and coat it with the muck in the desired pattern; then press the collected and cleaned moss into the muck and keep the area misted a few times a day.... voila! - the moss will continue to grow and multiply. Once consolidated, the moss will tolerate periods of drying out before re-watering.. "

I followed the instructions and until late in September, I wondered if it was going to work on this particular rock because it's in sun for about half the day. But then the weather cooled off & we've been having the seasonal grey skies and quite a lot of prolonged drizzle. And Jack's formula is obviously working. Here's what it looked like today -