Trees affected by development could face three fates: preservation in situ, transplanting to another location, and felling. Especially for large trees in robust health, the option of felling is usually not recommended. It is desirable to keep them in their original location. Due to site constraints and other reasons, sometimes it may not be possible to preserve trees, in which case the moving option has to be considered. Transplanting should only be adopted as the last resort, because it invariably entails massive loss of roots and would often gravely weaken and deform the tree.

Transplanting a tree means moving both the stem (the above-ground parts, including the trunk, branches, branchlets, twigs, leaves, buds, and any flowers and fruits that may be present), as well as the root system. The difficult part is root retention which must be moved together with the enclosing soil without disturbance in a rootball. Tree roots normally spread beyond the drip line (the outer edge of the tree crown). Ideally, all roots should be kept; in reality, it is impossible. Usually a compromise is made between engineering and site feasibility, and tree requirements for long-term survival.

In practice, a rootball containing the soil and the embedded roots is prepared. If the rootball is too small, too many roots are lost and the tree may not survive the transplant. If the rootball is too large, which will contain more roots, it may be too big and too heavy to be moved. The drip line usually indicates the suitable diameter of the rootball. As most tree roots do not grow below about 2 m depth, even for large trees a 3 m rootball depth should be adequate. The rootball must be properly wrapped and secured within a temporary container that will hold the soil during the course of moving. Small rootballs are usually wrapped with hessian and ropes, or enclosed in a timber box. Large rootballs have to be secured in a steel box reinforced with steel beams.

Transplanting must be prepared meticulously and conducted at least one year in advance of the actual move. A hasty job may irreversibly damage tree vigour and ruin the tree form, and the affected tree may not recover from the transplant shock and associated injuries. Small trees are easier to transplant, because less roots will be lost and they have the vitality to recover from the harms. Large trees are more difficult to move. The heavy cost of transplanting could be gauged against the long-term benefits.

Professor C.Y. Jim
March 2004