Hollow Trunk

by Rudi Adam

Hollow trunk (Uro) is a 'feature style' meaning that it can occur with any of the five major styles. The feature or focal point of the tree becomes the old looking hollow or rotten trunk so often observed in old Oak and Willow trees as well as many other species. Although it is more commonly found in broad-leafed species and soft-wood evergreens, in bonsai it may be used in any type of material (deciduous, broadleaved evergreens, evergreens) provided the material has a sufficient trunk diameter. Thin trunks are not enhanced by the edition of a hollow. These can bear Saba-Miki or Split trunk.

hollow-trunk-bonsaiUro have their origin normally due to trunk or branch damage. Trunk damage - naturally caused by a loose boulder careering down the mountainside striking a glancing blow to a large tree and causing an open jagged wound. Its torn and jagged wood fiber now no longer protected by cambium and bark is exposed to fungal and insect infestation; this in time will rot into a large hollow. A less romantic cause is when a car careers off the road into a tree. The hollow may also be caused by animals, deer eating the bark for food in winter; the might elephant ripping the giant Baobab in order to get to its soft juicy wood; the tiny termite finding a juicy morsel, beginning only with a small dead wood area, they damage more and more fiber close to the living area and this will die in turn, just to be cleaned out again by the termites.

Branch Damage

This can be caused by natural die-back due to lack of sunlight, to storms ripping off large branches or even secondary or major trunks; lightning strike scarring the trunk right down to the soil.

Whatever the reason, if the scar is large enough it forms a feature and therefore must be prominently displayed except if the Uro is of an uninteresting nature or the fluid tapering of the tree is jeopardized. In this case the Uro would be placed to one side or partially hidden by or with a live branch or Jin.

The creation of Uro in Bonsai

In collected as well as nursery grown· trees it may often be necessary to remove extremely large branches, and although given enough time any wound may heal over, in our art form we may wish to depict the extreme - in this case the hollow trunk.

Remove large branch or upper section of the tree to be shortened - do not seal - and grow the tree for a period of one or two years until the upper section of the tree has enlarged. Carve out the stump avoiding the natural new cambium and only carving in this area to create a more elongated and natural looking scar.

uro-bonsai-treeAvoid extension of the Uro into the soil region as rot may set in in an uncontrolled fashion; chisel out all soft dry wood until live wood is reached. If the cavity is large enough stop there and then and wait another three to four months before treating the wood. A natural barrier is usually formed by the living tissue of the tree. Thereafter treat with a solution of lime sulphur. If hollow is not yet large enough fill the cavity with fresh compost, keep it moist and let nature take its course for another period of time (natural decay) then continue to enlarge by artificial means - carving, dremmel-drill. Thereafter proceed as before.

Allowing excessive growth above and beside the scar (without jeopardizing the shape) will promote the natural rounded healed-over edge, suggesting that the scar has been there for a long time. Remove excessive growth as soon as the desired effect is achieved. Face the hollow towards the sun in order to promote bark formation on the new cambium.

In instances where the carving is done all in one operation the carved area as well as the cambium must be protected with tree-seal to avoid any further decay. This may be removed one year later when the wood can be exposed for one or two months and then treated with Lime Sulphur.

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Rudi should have mentioned that before you start working on your Hollow Trunk, make sure it is not 'PRE-USED'. You never know who got there before you.