As a prequel to this post, you should probably read my June 2014 post, "Exposed Root Pine Bonsai #1". It contains some more background information about this 2006 batch of trees. This tree is possibly my favorite…
Making exposed-root pines is a relatively simple process; it can take only a couple years to create the longer roots. After that, depending on how you treat the tree the roots may fatten or even fuse together as they did in the tree in the post I mentioned above. One thing to consider is that a few properly trained exposed roots may fuse to look exactly like a wider base. Additionally, rather than discarding the part of the tree that is growing under the soil you are utilizing the entire growth of a young tree, which effectively makes a larger tree more quickly than is possible with a normal informal upright tree.
In March, 2008 I repotted this tree from a 4″ container into this much larger container. The cylinder of plastic is a repurposed akadama bag which is filled with pumice; I used a thin layer of regular soil and sphagnum on the top to keep the roots from drying out. Under the cylinder is a 10″ square pond basket with regular bonsai soil. Taking the tree out of the 4″ container, I cut the roots short and arranged them to come out from the trunk in a harmonious way. At the same time I wired the trunk to add some movement – this is a step that’s very important; no movement in the trunk following a lot of nice movement in the roots means you’ll have a composition that doesn’t harmonize with itself.
This is a different tree that I did recently, but it illustrates the technique that can be used to create a similar tree. The two year old tree is taken out of a small container, roots pruned and arranged and then the tree is arranged into a large tube filled with coarse pumice. The tube in this case is made up of two one-gallon containers with bottoms removed and stuck together. The containers can be cut away in stages to expose roots.
In an exposed-root composition the roots effectively become the trunk of the bonsai; they are the visually interesting part of the tree. The substrate that you use in the section where you want the roots to travel to make your “trunk” should be coarse and stay relatively dry and not retain nutrients. You want the roots to go through it, not stay there and divide and multiply. If you can’t find pumice, try large lava, or stones. The advantage of pumice is that you can easily cut through the pieces to remove them from between the roots when needed. Lava and stones will create movement in the roots in the same way, but they will be heavier and more difficult to remove later.
The movement that you get from the large particles in the tube is just small wiggles in the roots normally. If you use a finer substrate you may end up with roots that are completely straight. Once the pumice is removed you should immediately evaluate the roots that you have and then bend them in some way to add some movement. The roots will take a few years to fatten up and get more stiff, so you can re-evaluate later. The tree may need to be supported until the roots become woody enough to hold it up.
I moved this tree into a 14″ colander after three growing seasons in the smaller one to try to accelerate the growth. I let the “top” run, which was interesting on this style because it was actually growing sideways mostly. Each spring it would try to grow upright, but then as the weight increased the branch would sag more.
Summer 2014. The tree has been cut back a little and soil scraped from the surface to expose more roots.
Finally, after seven years of growing in large containers, with some pruning in places and wiring in others I decided that it was time to repot it into a bonsai container. The pond basket was falling apart after years in the sun and the roots around the section where the original smaller basket was were overly dense; with a lot of old dead feeder roots.