Grafting is a centuries-old horticultural technique that joins two separate plant parts so they can heal and grow together as a single organism. This method is a form of asexual propagation, meaning the resulting tree is a clone of one of the parent plants. Many fruit trees, shade trees, and ornamental shrubs found in gardens and commercial orchards are produced this way. Grafting allows growers to combine the desirable traits of two different varieties into one robust, functional tree.
The Anatomy of a Grafted Tree
A grafted tree is a composite plant made of two distinct vegetative components that share a single vascular system. The upper part is called the scion, which is the stem or bud cutting chosen for its desirable top growth, such as specific fruit quality or flower color. The scion determines the variety of the tree, producing all the leaves, branches, flowers, and fruit.
The lower portion of the tree, including the root system, is known as the rootstock. The rootstock provides the anchor and handles water and nutrient uptake for the entire tree. It is selected for traits like soil tolerance, environmental hardiness, and its ability to influence the overall size of the mature tree.
The success of the graft depends on the fusion of the cambium layers of both the scion and the rootstock. The cambium is a thin, actively dividing layer of cells located just beneath the bark, responsible for secondary growth. When the two components are precisely aligned and held together, these cambium layers produce specialized wound-healing cells called callus tissue.
Callus tissue bridges the gap between the scion and rootstock, forming a continuous vascular connection. Once established, water and nutrients flow from the rootstock to the scion, and sugars produced by the scion’s leaves travel down to the roots. The point where the two parts fuse is known as the graft union, representing the anatomical seam of the composite tree.
Horticultural Reasons for Grafting
Grafting is primarily used because many popular fruit and ornamental varieties do not grow “true to seed.” For example, a seed from a McIntosh apple will not reliably produce a McIntosh tree. The technique ensures the genetic identity of the desired variety by using a vegetative cutting as the scion, making every Granny Smith apple tree a clone of the original parent.
The rootstock offers a powerful tool for controlling the mature size of the tree, a practice called dwarfing. Specific rootstocks, often designated with codes like “M.26” for apples, restrict the scion’s growth, creating semi-dwarf or true dwarf trees. This size control is useful for high-density commercial planting and small home gardens, as it makes trees easier to manage and harvest.
A major benefit is the ability to select rootstocks that provide resistance to specific soil-borne diseases or pests. Grafting a susceptible scion onto a robust, resistant rootstock provides a defensive advantage against local environmental challenges. Rootstock can also be chosen for increased winter hardiness, allowing a less cold-tolerant scion variety to thrive in a colder climate.
How to Identify the Graft Union
The graft union is the physical marker indicating where the scion and rootstock were joined, usually found near the base of the tree. Visually, the union often appears as a distinct bulge, a slight crook, or a visible line encircling the trunk. Sometimes, there is a noticeable difference in the bark texture or color above and below this line.
For most commercially grafted trees, the union is positioned a few inches above the soil line, typically 2 to 6 inches. Locating this spot is important because the tree components have different genetic makeup and growth habits. If the union is buried, the scion may develop its own roots, which negates the dwarfing or disease-resistance qualities provided by the rootstock.
Pruning and Maintaining Grafted Trees
Maintenance for grafted trees centers on preventing the rootstock from overtaking the scion variety. The most common issue is the growth of suckers, which are vigorous shoots that emerge from the rootstock below the graft union or directly from the roots. These suckers carry the genetics of the rootstock, which is usually an inferior variety that does not produce desirable fruit or flowers.
Suckers must be removed promptly because their rapid growth diverts water and nutrients away from the scion, potentially stunting its growth or causing the graft to fail. When removing them, cut the suckers cleanly and flush with the rootstock tissue from which they originate, ensuring no stub is left that could resprout. If suckers are allowed to grow, they can quickly dominate the canopy and eventually replace the grafted variety.
When planting a grafted tree, ensure the graft union remains exposed and sits well above the final soil level. Burying the union can cause the scion to root, which eliminates the rootstockâs influence on tree size and hardiness. Proper planting depth and consistent removal of suckers ensure the long-term success of a grafted tree.