air roots
The best way to create air roots, according to Jerry Meislik in his book Ficus, the exotic bonsai. is to create a microclimate on the tree. First it is best to nick the bark of the trunk where you want the roots to develop to the phloem level therefor disrupting the flow of sap and then wrap the trunk with a plastic back and water as usual, the bag creates the desired microclimate and the roots begin to develop.