Karen Russell, East Malling Research & David Thompson, The Irish Forestry Board
Clonal plant propagation is a very useful way for creating exact copies of important trees for conservation and future research. The type of method used to propagate trees clonally depends on the number of plants required, and the age and size of the ‘mother’ tree (from which the propagating material is collected), time of year and the ease of propagation of the species.
Seedlings and young trees are relatively easy to propagate from by a variety of methods.
However, for many species, as trees mature, it often becomes increasingly difficult to propagate them from cuttings and so grafting is often the preferred technique. Grafting also has the advantage of maintaining maturity so the grafted plants can often produce seed straight away. Shoots (scions) from selected trees are grafted on to rootstocks (plants used to provide the roots and stem to support the growth of the scion) of the same species. It is important that both rootstocks and scions are at the same physiological stage, ideally both dormant, to maximize success.
When the mature “plus” trees (outstanding timber trees) are selected in the wild, scion wood (ideally vigorous, one-year-old shoots) is collected in the winter when the tree is dormant. This is done by climbing or by shooting shoots down with a shotgun (see photo). The scions and rootstocks are specially cut so that their cambium layers are in direct contact with each other. In sycamore, the grafting technique used is known as a ‘whip and tongue’ graft which allows the cambiums contact in two places and also provides better support at the graft union for tying the scion to the rootstock. The top of grafted plant is then dipped in hot wax (see photo) to seal the union and prevent water loss.
After grafting, the plants are placed for about two weeks in a ‘hot pipe’ which provides 20°C heat to the graft union, encouraging rapid callousing (healing) to join the shoot to the rootstock, whilst keeping the roots and shoot cold. Grafting success varies depending on the quality of the plant material, facilities, timing and the skill of the propagator – it can be as high as 85%.
The grafted trees can be used to establish clonal seed orchards, trials and create clonal genebanks to provide good quality seedlings for future woodland plantings.
Fig 1 The collected Scions are quickly sent to the Irish laboratory for propagation.
Fig 2 Lined up cambia
Fig 3 Young grafted tree
Fig 4 Dipping shoot tip in hot wax to prevent water loss.