Answer: In many cases moss will appear on its own if the conditions are right for it to grow. To introduce moss, the old rule of thumb is to find some moss growing in a similar situation (on a similar type of surface in similar lighting), mix it with either buttermilk or yoghurt (the kind made with live cultures in it) in a blender, then paint that onto the surface. The area should be damp and humid for best results. Most mosses grow well in the cooler moister months and by taking it from a nearby source you will know it is suitable to your area.
Since your fence is wooden you may not want to keep it wet as this may cause it to rot and deteriorate. It may also be difficult to get moss to grow on it if it is made of treated wood. Instead, you might consider using a soft sage colored or mossy green colored stain or applying several colors in a mottled effect. This, combined with a climbing plant or two, should give a similar visual "feel" and be safer for the fence. It is also faster and more predictable.
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