Not all Ficus will grow from root cuttings but one fig that does particularly well is Ficus tremula.

Left root cutting, middle same cutting after 6 months, right trimmed back to start the bonsai training
Not all Ficus will grow from root cuttings but one fig that does particularly well is Ficus tremula.
Left root cutting, middle same cutting after 6 months, right trimmed back to start the bonsai training
In this case a root cutting of Ficus ‘Mystery’ was used to create an interesting bonsai.
On the left a root cutting of Ficus ‘Mystery’, center is the cutting wired for shape, right is the bonsai design beginning to take shape
On removing some of the larger and more shapely roots the root is potted up with the root hairs in the soil and the cut end sticking out of the soil about 1-2 inches. After 2-6 months the end of the root will sprout new leaves and stems. Over time the new growth can be shaped to be an interesting bonsai. In addition most of the root can be lifted out of the pot to make a longer trunk.
Not all fig species will sprout from roots but natalensis is one that sprouts very easily.
Ficus natalensis, root cutting that has sprouted new stems and leaves
The same root cutting after a year of growth and wiring for shape
To develop a new apex and create a better transition to the new apex, two of the sprouts were pulled together with electrical ties. A month or two passed and the ties were removed. The lower part of the fusion appears to be nicely grown together but the upper portion is not fused. So several new electrical ties were placed in areas adjacent to the old ties.
Root cutting of Ficus natalensis
Side view shows three sprouts have grown from the root cutting
To develop a thicker transition to the trunk two of the sprouts were tied together with electrical ties
After a month or two the ties appear to have worked
The ties were removed and the upper part of the fusion was not together. Some mild scars from the ties will be present for about 6 months. With growth they will disappear.
Two new ties were placed adjacent to the old ones and growth will be allowed to speed the fusion more completely
Some basic wiring was done to give the young bonsai a bit of shape
The images show one of mine that is only 2 years from a root cutting.
Root cutting in 2013
Same cutting in 2015
A root cutting is shown from a Willow Leaf fig. The bonsai is now about 10 years old.