A plant graft is the union of parts from two or more plants into a single plant. The technique of grafting requires a knowledge of which plants can be grafted, which ones benefit from grafting, and how to do the numerous types of grafts. Knowledge of how to graft plants has existed for centuries. Knowledge of why only certain plants can be grafted and

why they grow as they do is more recent and is still incomplete. Much of the research into grafting has been accomplished with citrus and other fruit trees because of the commercial importance of those grafted plants.

Ornamental propagators graft either when the resulting plant is superior to those provided through other methods of propagation or when no other propagative method will work. Other ornamental horti­culturists such as groundskeepers turn to grafting to repair, brace, or otherwise save damaged plants. Occasionally, grafting is used to create special and unusual plant forms, such as tree-form shrubs or weeping- form trees. While grafting can be done on both woody and herbaceous plants, it is most common to woody plants.

A typical plant graft involves the following parts (Figure 14-5):

• Stock: the portion of the graft that will develop into the root system of the new plant

• Scion: the portion of the graft that will develop into the branches and foliage of the new plant. It may constitute some or all of the canopy depending on how much of the top growth of the stock was removed. The scion may be a small branch or a bud. It should not be a sucker or water sprout.

• Graft union: the point where stock and scion join. It usually remains visible throughout the plant’s life.

• Interstock: a piece of plant stem placed between the stock and scion, resulting in two graft unions. It is not a part of every grafted plant.

It is used to join two species that will each unite with the interstock species but not with each other. It may also contribute additional characteristics to the new plant.

figure 14-5. Typical plant grafts with and without an interstock (Delmar/Cengage Learning)

How the Graft Forms

While methods are varied, the goal of the propagator is always to place the separate cambiums of the stock and scion into close contact in order that the two may fuse. The joining of the separate parts is made possible by the production of callus, or wound tissue, from the paren­chyma cells of the cambiums of both stock and scion. The cells of the callus tissue intermingle until a binding results. Cell differentiation follows and parenchyma cells become cambium cells. The new cam­bium, a product of the two previously separated plant parts, produces new vascular tissue. The formation of a vascular connection between the stock and scion is essential for the success of the graft. When the graft union is successful and the two different plants become one, it is termed a compatible graft. If the two plants do not unite, even though there was no error in the technique used, it is termed an incompatible graft. In some instances, the graft may appear successful until months or years later when the plant suddenly dies or snaps off unexpectedly at the graft union. Delayed incompatibility is the term used to describe such a situation.