Tree grafting is a horticultural practice that involves joining parts of two different plants so they grow together. This technique allows a grower to combine the desirable characteristics of a shoot from one plant (such as fruit quality or flower color) with the beneficial root system of another. The rootstock may offer disease resistance or control the overall size of the tree.
The Essential Components and Biological Fusion
The two components joined in the process are the scion and the rootstock. The scion is the short piece of stem or bud material selected for its desirable aerial characteristics, representing the future canopy of the tree. The rootstock is the lower part of the graft, providing the root system and sometimes the lower trunk, chosen for its vigor, soil tolerance, or ability to dwarf the resulting tree.
Successful fusion relies on the precise alignment of the cambium layer, a thin, actively dividing band of cells located just beneath the bark. When the scion and rootstock are cut and pressed together, the cambium layers must be in direct contact. This contact triggers a healing response where specialized cells near the wound begin to divide rapidly, forming an undifferentiated mass of tissue called callus.
This callus tissue emerges from both components and acts as a biological glue, filling the space between the scion and rootstock. Over a period of several weeks, the callus cells differentiate, forming new vascular tissues that bridge the gap. The inward-differentiating cells become the water-conducting xylem, while the outward-differentiating cells form the nutrient-transporting phloem. This regeneration of a continuous vascular system allows water and nutrients to flow seamlessly between the rootstock and the scion.
Common Grafting Techniques
Whip-and-Tongue Graft
The Whip-and-Tongue graft is frequently used when the rootstock and scion are of similar, small diameters, often about the thickness of a pencil. This method involves making identical, long, sloping cuts on both parts, followed by a small, downward-pointing “tongue” slice in each. The resulting interlocking cuts provide mechanical stability and maximize the surface area of cambial contact, promoting rapid healing.
Cleft Graft
The Cleft graft is preferred when the scion is much narrower than the rootstock, often used to change the variety of an established branch or tree. The technique requires splitting the rootstock down the center and inserting one or two wedge-shaped scions into the resulting opening, ensuring the cambium layers align along the edges. The pressure exerted by the rootstock’s split holds the scions securely in place as they heal.
Bud Grafting
An alternative method, Bud Grafting, only uses a single, detached bud from the desired variety instead of an entire stem section. This small bud, often accompanied by a sliver of bark, is inserted into a matching T-shaped or shield-shaped cut made in the rootstock’s bark during the growing season. Because only one bud is transferred, this method is highly economical with scion material and is frequently employed for fruit trees like peaches and citrus.
Factors Determining Grafting Success
Timing and Dormancy
The timing of the grafting operation significantly influences the outcome. Many techniques are performed in late winter or early spring while the scion is dormant but the rootstock is beginning to activate. Grafting during this period minimizes water loss from the scion before the vascular connection forms. Bud grafting, however, is typically done in the summer when the bark is “slipping,” meaning the cambium is actively dividing and the bark separates easily.
Biological Compatibility
Biological compatibility is a requirement for a permanent union. Successful grafts generally occur only between plants within the same genus or, ideally, the same species. Different plant families often have physiological differences that prevent the formation of a functional vascular bridge. Incompatibility can result in a weak, brittle union or delayed failure that may take years to manifest.
Aftercare and Environment
Environmental conditions and aftercare are important for successful fusion. The graft site must be sealed immediately after the cuts are made using grafting wax, tape, or specialized wraps to prevent desiccation and pathogen entry. Maintaining moderate temperatures, often around 60 to 70 degrees Fahrenheit, encourages rapid callus formation without causing the scion’s buds to break prematurely. Sharp, clean tools are necessary to create smooth cuts that maximize cambial contact and minimize the risk of infection.