A New Technique for Bending Trunks and Branches

Dr. Rick Wagner, President
Dai Ichi Bonsai Kai
Photographs by Becky Wagner
Copyright 2002 by Dai Ichi Bonsai Kai, all rights reserved.

Nomenclature: I use a number of engineering terms here in what is essentially an engineering application to bonsai. Some bonsaists may not be familiar with these technical terms so I define them here first:

Note, then, that by these definitions, that wire on a branch generally will have both an axial and tangential component to its direction.

One technique for bending curves in trunks (or heavy branches) is to wrap the trunk with wet raffia first. Binding the trunk with raffia keeps it from breaking so more severe bends can be formed. However, the raffia technique is difficult and time consuming. First an axial layer of wet raffia is applied. Then the trunk is wound with a tangential layer. Soaking, wrapping, and tying the raffia is troublesome, but works well when done properly. A tree bent with the raffia technique is shown below in Figure 1:

Figure 1: A small oak tree with the trunk bent using the
traditional raffia technique.

Raffia is a natural fiber used for making baskets and hats. Noted for its wet strength, raffia is obtained from the stalks and leaves of a Madagasdar palm. Raffia works well for strengthening trunks and branches for bending for bonsai, but is difficult to apply properly. It occurred to me that fiberglass strapping tape might afford the strength of wet raffia and the stickiness of the tape might make it easy to apply. I started my experiment with a rather straight and uninteresting valley oak I had grown from an acorn (Figure 2).

Figure 2: A five year old valley oak tree.
Note the rather uninteresting trunk.

First I ran lengths of the inch-wide strapping tape up the trunk covering all sides (Figure 3). The axial layer adds longitudinal strength. This is what keeps the bark from rupturing during bending.

Figure 3: First I ran axial plies up the length
of the trunk. No helper is required because the
sticky tape holds itself in place.

Next I wound a tangential ply layer around the trunk, winding it around and around. No need to stop and tie in more raffia! The tape roll holds many feet and the wrapping process goes quickly. I held the tape fairly tight as I wrapped (Figures 4-6).

Figure 4: Winding the tangential layer. Starting with
the base of the trunk, I overlapped the layer as I wound
up the trunk.

Figure 5: Winding the tape up the trunk continues.

Figure 6: The trunk is now completely wrapped in
strapping tape.

Next I wound the trunk with a thin aluminum wire. I am going to counterwind a slightly heavier wire tightly over that, and the first wire ensures that I will be easily able to pass a wire under the top wire for later tying.

Figure 7: I wind the trunk with a thin aluminum wire.

Then I counterwind a slightly heavier wire over the first wire. This deliberately violates the "no crossed wires" rule, but these wires are not holding wires, but provide tie points for tensile members (wire stays) to later hold the bent shape.

Figure 8: I counterwind the trunk with a thicker
aluminum wire.

Finally, I start bending the trunk. Starting with the thickest part (base of the tree), I start making an S curve. This is the part where I needed a helper (not shown) who tied the wire stays while I held the bends.

Figure 9: Bending the base of the trunk.

Figure 10: The bend is constrained with a wire stay,
like a bowstring to hold the curve.

I formed the second bend so the branch was at the outside of the curve. The first bend stay had relaxed somewhat and would later be tightened to hold a more severe curve.

Figure 11: Making the second bend. The bends are planned
so that branches are at the outside of the curve.

I continued the process with the third bend also having a branch on the outside (Figure 12).

Figure 12: The bending process continues.

I repeated the S curve at the top of the tree where it was considerably easier to bend.

Figure 13: The finished tree. The tape and wire will
stay on for about a year.

John Naka said in his Bonsai Techniques that a tree bent with a repetitious S curve like this one could be considered monotonous and therefore also uninteresting. I did the bending in this manner to illustrate a new techniqe. It is quite likely that the future tree will be shortened considerably, so that repetition might not be present in its future form.

I was quite pleased with the way the bending with strapping tape and wire stays came out. Raffia will disintegrate on its own over time, but the fibergalss will not. It has been about three months now (as of this writing) since the bending was done. The tree has new growth and shows no ill effects from the tape or the bending. Next spring I will remove the wire and tape and I hope to have photographs with which to update this article.

Here is the tree in 2010, ready for a bonsai pot.

Email Richard dot J dot Wagner at gmail dot com

This page created August 16, 2002, by Rick Wagner. Last updated June 10, 2013 by Rick Wagner.