That Raintree is awesome! The following as to how it is made is a guess, but it would work. First, I gather that water drips through it so this would mean that the concrete trunk base would be hollow enough for a tube to pass through it and the fountain base broad enough to house a pump in it. The trunk would be formed, flared on top a bit and holes pierced around the top, outside edge. Using these holes a wire frame would be closely woven in the desired shape. Think chicken wire type closeness. The form would be stuffed tightly with unmilled spaghnum moss with drip tube in center. You will note that the planting extends over the edge of the form a bit hiding wires. Then moss, baby tears or whatever is attached to the form and held in place with floral pins. Hairpins work also. Here you are using normal topiary practices. I suspect sculpture wire was used as it shapes easily. Available at art stores.