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This article describes the technique of approach grafting new roots in an otherwise blank area of the nebari or trunkbase.
It is strongly advised that the article ‘Approach Grafting’ is first read and understood before applying any techniques described in this article. A second useful and related article is ‘Threadgrafting New Roots’; both threadgrafting and approach grafting new roots are equally viable techniques and should be seen as techniques that can be used individually or in conjunction with each other.
Example Approach Grafting of an Acer palmatum Sapling
This is the trunkbase of an Acer palmatum/Mountain Maple in development. Having been grown in the ground for several years to thicken the trunk to its present 3"-4" diameter, the tree is now being refined in a large nursery pot. Part of the process of refinement is the continued development of the nebari.
At the front of the trunk is a large gap in the nebari or rootspread. Roots could be prompted to grow from this point using a number of hit-and-miss techniques, however, these new roots would be very thin and would take many years to thicken to the same girth as the surrounding surface roots.
The fastest way to introduce a suitable root in this position is to either threadgraft or approach graft a young tree into the gap in the trunkbase. Once grafted, the trunk of the young tree becomes a root of the main tree itself.
Due to the thickest of this trunk and in particular its trunkflare, approach grafting is by far the easier of the two techniques to apply in this situation.
Not long after midsummer (see timing notes below), a channel that will receive the sapling (scion) is cut into the front of the trunkbase . Sufficient soil is removed from the area where the scion will be planted. The roots of the main tree are disturbed as little as possible.
A small amount of disturbance, to a tiny fraction of the overall root-mass of a healthy tree during the growing season doesn't cause any problems.
If one is fearful of disturbing the roots of the main tree during the growing season, one alternative is to place an object (such as a stone or small plastic pot) into the soil area during repotting in Spring. The object can then be removed during the Summer to leave a planting pocket for the scion.
A pencil-thick Acer palmatum scion (in this case taken as a small airlayer during the previous year) is then inserted into the channel and securely fixed into position using a brass screw and an aluminium wire staple.
As can be seen in the image above, the scion still has the green bark seen on Mountain Maples for the first decade. In this case, the scion will be covered with soil to speed up the maturing of the bark so that it quickly forms the same colour as the surrounding roots and trunk.
As with newly approach-grafted branches, having been sealed the graft with wound sealant, the scion will now be allowed to grow freely and vigorously until such time that the cambium layers of the scion and main trunkbase graft together.With this tree this will take between one and two years. The top of the scion will then be removed leaving the base of the scion as a new root.
UPDATE July 2008
Early July 2008 and the graft has healed well. Both of sides of the cambium layers have grown tightly together
An additional 4 approach grafts have now been made to further enhance the nebari of the tree
Preparing a Scion for Approach-Grafting New Roots
The scion or seedling/sapling tree to be grafted needs to be prepared and shaped in the months prior to grafting. This is necessary so that it will fit into position at the base of the main trunk. Suitable scions can be obtained from seed, cuttings or airlayers.
For the majority of approach grafts, the scion should be:
(a) bent so that there is an 'elbow' in the scion that can be inserted into the channel in the main trunk
(b) the part below the elbow grows into the soil surface at the same angle as the surrounding roots of the main tree.
(c) the part above the elbow has room to grow freely (allow it to grow out, away from the trunk and branches of the main tree)
(d) though not essential, if you have the opportunity, train the roots of the scion as shown in the diagram above
(e) prune the scion roots in the Spring before grafting so that they are reasonably compact and can be replanted into the main tree's soil later on in the year. I would not recommend any rootpruning of the scion at midsummer itself.