The other day, I was unwrapping some bark grafts when I noticed big differences in graft union appearance. The photo above shows two Kanza grafts made the same day on similar-sized seedling rootstocks. I used the exact same bark grafting technique on both trees. The graft on the left seems to be growing from just the outside of the stock tree while the graft on the right has grown callus over the top of the stock tree's stump. The tops of both grafts are growing vigorously and have made good diameter growth. Why such a noticeable difference in graft union appearance?
It all comes down to cambial contact. As you might remember, the cambium is the layer of cells found between the bark and the wood. During the spring flush of new growth, cambium cells grow and divide to create a new layer of wood and a new layer of bark. This annual flush of cell division is how trees grow in diameter. When grafting trees, cambial tissues of the scion and the stock are placed so close together that the cells grow together and form callus tissue. Once callus is formed the trees starts to lay down co-joined layers of new wood and bark to form the graft union.
The tree on the left made good cambial contact under the bark of the stock tree but callus did not form along the upper edge of the stock tree. This usually happens when an arched scion pulls away from the upper edge of the stock during the grafting process. When the scion lays tightly against the stock all the way to the top of the stock, callus forms along the upper edge of the stock and the graft union heals over rapidly. Just look at the tree on the right. The cut surface of the stock is almost completely covered in just one growing season.
Although I would prefer every tree to look like the graft pictured on the right, I can live with a less that perfect graft like the tree on the left. As long as I'm getting good top growth, a poorly formed graft union can be managed by pruning dead tissue off the stock and the tying the scion to a strong stake.