One of the more unusual bonsai styles is neagari. A Japanese word for exposed root, sometimes called octopus style bonsai.
This style is very different from most traditional bonsai shapes when it comes to the roots. On most bonsai, nebari is important (the spread of the roots at the base of the tree.)
As shown above, this unusual bonsai can be combined with other styles. In this case - cascade.
Since bonsai imitates nature in many ways, you may ask, how would this happen in the landscape? Floods, mud slides and avalanches often wash away the soil from the base of trees. Eventually roots are exposed. In tropical climates “aerial roots” can sometimes create "legs" to support trees such as Ficus.
The following graphic is borrowed from the Chile Bonsai Society.
This is the traditional bonsai way to obtain the long roots. It is still a perfect way to create a neagari. After planting and allowed to grow, the top boards are removed and the new exposed roots begin to harden. Then continue to remove boards as time goes on.
Shown here is an imported Chinese elm.
When it first arrived from China, it had many more exposed roots. Over time, they were gradually removed.
Less traditional bonsai techniques are also used.
John Callaway uses clear plastic bottles (soda, water etc.) and cuts off top and bottom. He places this “tube” partially in a container of soil, then fills the tube with bonsai soil. John plants cuttings and/or starter plants in the top of the “tube” and lets them grow.
No matter the method you use, it will most likely take years to have a good example of the neagari style bonsai. Be patient and create other styles while you're waiting.
Each of these trees will have a very different look. It will be
important for the roots to be sturdy enough to hold the tree up. The
roots should also be substantial enough to look like multiple trunks. With
neagari, roots are supposed to extend from one half up to two thirds of
the height of the tree. In the final design the trunks should appear
in odd numbers.
It's best to wait until the tree has somewhat matured before removing those roots unnecessary for styling.
In addition to the above 'in the box' technique some neagari are found growing naturally with exposed roots.
An experienced bonsai artist can often make it look very natural, so it's often hard to tell which is which.
Occasionally, you will find a plant that the roots have a natural tendency to form this way. Severinia buxifolia is one of those species. Related to Citrus, its common name is Box Orange or Boxthorn, and yes, it has thorns.