A grafted tree is a product of horticulture, combining the tissues of two separate plants so they heal and grow together as a single organism. This practice, known as grafting, is an ancient technique used for centuries to propagate and improve fruit and ornamental trees. It merges the desirable traits of one plant with the robust foundational qualities of another. Through this process, growers reliably reproduce specific varieties that would otherwise be difficult or impossible to clone from seed.
The Essential Components of a Grafted Tree
Every grafted tree is composed of two distinct botanical parts, each contributing unique characteristics. The upper portion, which forms the canopy, is called the scion, and is selected for desirable traits, such as a specific type of apple or rose color. The scion determines the genetics of the fruit, flower, and above-ground growth, maintaining the exact genetic makeup of the parent plant.
The lower portion of the union is the rootstock, providing the root system and the base of the trunk. Rootstocks are chosen not for their fruit, but for their foundational strength and environmental adaptability. They anchor the structure and are primarily responsible for traits like disease resistance, tolerance to poor soil conditions, and the ultimate size of the mature tree. A successful graft requires the scion to provide the desired yield while the rootstock provides the necessary hardiness and support.
The Biological Mechanics of Grafting
Graft success relies on the precise alignment and fusion of a specific tissue layer within both plants. This thin, actively dividing layer is the vascular cambium, located just beneath the bark and outside the woody xylem tissue. When the scion and rootstock are joined, their cambium layers must be placed in direct contact to initiate healing.
If the cambial tissues are properly aligned, they produce undifferentiated cells that form a protective layer called a callus. These callus cells intermingle and bridge the gap between the two pieces of wood. Over time, the callus differentiates, forming new vascular tissue: the water-conducting xylem and the nutrient-transporting phloem. This re-establishment of vascular continuity allows the scion and rootstock to function as one, permitting the efficient flow of water and sugars.
Primary Goals of Tree Grafting
Grafting is primarily employed to maintain the genetic consistency of commercially important fruit varieties, a process known as cloning. Since most fruit trees grown from seed produce highly variable and often undesirable fruit, a scion taken from a ‘Honeycrisp’ apple tree ensures the new tree will produce genetically identical ‘Honeycrisp’ apples. This asexual propagation guarantees the continued availability of specific, high-quality cultivars.
Another application is modifying the tree’s overall size through specific rootstock selection. Certain rootstocks possess natural dwarfing characteristics that restrict the scion’s growth, allowing for high-density planting and easier harvesting in commercial orchards. This size control allows growers to manage the tree in a smaller space than its natural genetic potential would allow.
Rootstock selection also boosts the tree’s resilience against local environmental challenges and pathogens. Growers can choose rootstocks naturally resistant to soil-borne diseases, such as Phytophthora root rot, or those adapted to adverse soil types like heavy clay or saline conditions. A robust root system allows the tree to thrive in areas where the scion’s own roots would quickly fail.
Common Grafting Techniques
The physical method used to join the two plant parts is determined by the wood’s diameter and maturity. The Whip-and-Tongue graft is favored for joining small-diameter rootstock and scion wood (typically less than one inch thick) because the interlocking cuts provide high stability and cambial contact. This technique involves a long, slanting cut on both pieces, followed by a small, downward-facing cut—the tongue—that locks the two parts together before wrapping.
For thicker rootstock, often when top-working an established tree, the Cleft Graft is utilized. This method involves splitting the large diameter rootstock and inserting two wedge-shaped scions into the opening. Alignment ensures the scion cambium meets the rootstock cambium along the edges of the split. Budding, such as T-budding, uses only a single bud taken from the scion. This bud is inserted beneath a T-shaped cut in the rootstock bark, which is highly efficient for propagating large numbers of plants when the bark is slipping easily.