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A multi-trunk or clump-form bonsai has three or more odd-numbered trunks growing from the same root base. This article describes one method of developing a multi-trunk bonsai; it also illustrates a technique for developing the nebari (surface roots) that can be used for trees with single trunks in development for bonsai.
This bonsai started in early 2003 as three Field Maple (Acer campestre) bare-root saplings. All three trees had roots unsuitable for bonsai as is typical with bare-root stock.
As these trees were to be planted into the ground for a number of years to thicken and develop the trunks, I took the opportunity to develop a suitable nebari (surface roots) at the same time.
The following technique uses the same principle as using a tourniquet to ground layer a new rootsystem and combines it with planting the tree on top of a tile to ensure a flat root system.(See related article: Creating new Nebari for Bonsai by Layering)
January 2003: Three field maples of different sizes have been selected for this project and each is threaded through holes drilled into a piece of wood. The holes were made large enough for each tree (and its branches) to pass through the wood with the minimum of room to spare.
I have used a wooden board for the trees used in this article; though the board has produced satisfactory results, it has begun to rot and break up after 4 years in the ground. A better alternative is to use an old ceramic tile though these can be more difficult to make holes through.
Choose the positions of the holes carefully as they will dictate the final positions of the trunks themselves. Remember that the trunks will swell over time and the trunks should initially be placed further apart than you eventually want them to be.
A useful alternative for single-trunk trees is to use an old CD; saplings of less than 1.5cm diameter can be threaded through the hole in the middle of the CD prior to planting.
The trees are then potted up into a suitable container or into the ground where they must be encouraged to grow as vigorously as possible. Great care must be taken that no air pockets remain underneath the tile/piece of wood and that a layer of at least 3" of soil covers the surface of the wood/tile.
The idea of this technique is that as the trees grow and their trunks thicken and expand against the sides of the holes, each tree begins to layer itself; in the same way that a ground-layered trunk will begin to produce new roots above the wire tourniquet.
As the new roots emerge above the wood, they grow laterally across the surface of the wood (or tile), unable to travel downwards until they reach the edge.
February 2004: The trees are lifted from their pot and bare rooted. The layering process has already started and the new roots have grown across the surface of the wood. Having arranged the new surface roots and removed any inward growing or crossing roots, the trees are replanted (this time into the ground but a pot would have been adequate). Again, the surface roots are covered by a around 3" of soil.
The depth of the soil above the tile must be deep enough to stay moist during the hottest days of the year otherwise the new surface roots are liable to dieback if allowed to dry out.
I prefer to lift trunks being developed this way every year so that the new surface roots can be arranged to ensure they are lying flat on the board and growing outwards in a radial fashion; this is not absolutely necessary as any ugly roots can be pruned away every 2 to 3 years if preferred.
November 2004: All three trees have grown well through the year and they are briefly lifted so I can see how well the surface roots are developing. Development looks fine though the smallest tree has failed to root as well as the two larger trees. After arranging and photographing the roots, the tree is replanted again.
February 2007: Just over two years later the trees are lifted again; trunk and root development has been excellent though there is disappointment that the bare portion around the smallest trunk still exists.
The speed and rate at which new roots appear above the board depends entirely on the rate of top growth (new shoots and leaves) of each tree. The more top growth there is each year, the faster the trunk swells and the more root growth that occurs.
In order that these three trunks are of differing sizes, the smallest trunk has been repeatedly pruned whereas the largest trunk has been allowed free and vigorous growth. The result of this pruning regime can be seen above; the largest trunk has produced the most new roots, the thinnest trunk that has grown comparatively little has produced much fewer new roots.