bonsaiTALK Community - View Single Post - Repotting an Established Bonsai (5 parts)

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Other root work should also be done at this time. This tree has a good deal of dead wood on it, even at the roots. Some of that is reduced with a drawknife (Figure 19) to give the nebari a better, more tapered line, and to get rid of some of the soft, rotted wood. This is an older fukien tea with large, established areas of dead wood. The wood of tropical trees tends to be softer than that of conifers or even deciduous trees, so extra care must be taken to preserve it both for aesthetic reasons and to maintain the structural integrity of the tree. This tree will eventually need wood hardener or a marine epoxy resin to achieve this.

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Following this, it is time for a little more delicate work underneath the root ball with my tweezers, looking for pockets of bad soil, etc (Figure 20). You should be able to see clearly the bottom of the trunk at this point. The base of the tree should rest solidly on new bonsai soil. Leaving any remnants of bad soil in a root ball is a common mistake of beginners ad experienced enthusiasts alike. It will eventually cause your tree to fail, so do not miss any!

Set the Tree in Its New Pot

The next obvious step in the process is to secure the tree in its new home. The next few steps are crucial to making the tree stable and secure, and to ensure that it will thrive.

Drainage Layer/Soil Mound

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(Figure 21) Traditional Japanese bonsai teaching indicates the need for a drainage layer at the bottom of your pot. The deeper the pot, the thicker should be the drainage layer. Try to use a slightly larger particle for this than your regular soil, and in a shallower pot, it may only be one or two particles deep. This will help prevent soggy soil and root problems including fungus and rot.

“Perched” Water Tables

There has been some controversy as to the efficacy of drainage layers. A great deal has been made of the notion of “perched” water tables. A perched water table is an aquifer that occurs above the main water table. This occurs when there is an impermeable layer of rock (aquiclude) above the main aquifer but below the surface. Water percolating down to the main aquifer gets trapped above this second impermeable rock layer.

It must be noted here that perched water tables require a constant supply of ground water and an impermeable layer of rock or clay, neither of which are possible in a bonsai pot. An engineered sort of perched water table has been employed by Turf Diagnostics and Design. This seems to be the closest thing to what is claimed for bonsai pots:

Perched Water Table

The USGA system provides maximum removal of water during heavy precipitation events, and it stores water above the gravel during periods when the ground is not saturated. How does it achieve this effect? Because the USGA system is based upon a concept known as the perched water table, which is also known as an inverted filter design. It is called an inverted filter because of the presence of the fine sand particles over the more coarse gravel. This design allows water to be held (or perched) in the root zone layer. This perched effect occurs because the primary driving force for water movement during periods of unsaturation is the capillary effect of the particle voids.

The large voids of the gravel result in a reduction in the capillary effect. At the root zone/gravel interface, these larger voids effectively create a barrier to further downward water movement during unsaturated times. As saturation is approached, additional pressure is applied allowing water to move into the larger voids of the gravel layer, and further down through the sub-surface drainage system.[ii]

There are a couple of interesting thing about the USGA system. One is that it is engineered to ensure that water [i]does perch, as this is beneficial to the plants (sod) above. Another interesting note is that the sod is planted in fine sand, and the gravel below is quite large. And while it is not stated in the above reference, my guess is that some sort of filter fabric is used to keep the two layers distinct.

In the bonsai pot, none of these elements pertain. The two layers of soil are in contact, and the difference in size must not be so extreme. Because there seems to be no proof for a deleterious effect, I choose to follow the Japanese tradition for its self-evident history of success.

Mound the Soil

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The movement of this tree is to the left, so it must be planted toward the right in its pot. A largish mound of soil is placed directly under where the trunk base will sit (Figure 22). The tree will be placed here and wriggled firmly into place, removing air pockets or voids underneath it. This mound should not be so large that the tree will sit too high in the pot, nor so small that it will allow the tree to sit too low.

Position the Tree

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Correct placement of the tree in its pot is a crucial stage of the repotting process (Figure 23). Unless we are restyling an established tree, it should be replaced with the same orientation it had previously. With a solid nebari and established position such as this tree, it is a fairly simple matter. Position yourself at eye level with the rim of the pot to check the rise of the nebari from the pot. Keep an eye on previously styled branches to be sure they are in their proper place. Settle the tree and prepare to tie it in.

Bunjin or other trees with unusual planting positions can pose a challenge when trying to repot in an identical orientation. How does one make certain a return to the original position? A small carpenter’s plumb bob is indispensable. Before removing the tree from its pot, tie it to a prominent branch, allowing the tip to almost touch the nebari. Place a single dot from a Sharpie at that point will help line up the tree correctly in its new pot. Simply be sure that the plumb bob touches the dot in just the same way when repotting the tree.

Wire the Tree into the Pot

Many trees, especially prebonsai, have prominent roots, roots that are too high, or a developing nebari that makes it difficult to get a grip with the wire without damage to the bark. Protect your tree in this case with a portion of old garden hose cut for the purpose, or some other firm but flexible product to avoid wire scars. Place it carefully between the wire and the bark. Be sure it is not going to let the wire slip off when you tighten the tie wires. One excellent product for this is the used serpentine belt from a late-model car. These have deep grooves running lengthwise which allow the wire to settle in, avoiding mishaps.

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Plan your work here so that your final tie, the one which you will tighten in the final step, bears on the portion that would tend to tip up if there were no wires present. This will give you your best holding power. Begin with the wire just to the right or left of that spot (Figure 24), depending on which direction you will be proceeding, and bend the wire across the nebari, far enough out that it doesn’t rub the bark or show above the soil. If necessary, a stub of chopstick can be driven into a firm root ball farther away from the trunk to keep the wire to the outside.

[i] Perched Water Table. Wikipedia, the free encyclopedia. Available at http://en.wikipedia.org/wiki/Perched_water_table. Accessed 7/23/06.
[ii] Perched Water Table. http://www.turfdiag.com/perched_water_table.htm. Accessed 7/23/06.