Grafting is a horticultural technique that joins two different plants so they grow and function as a single organism. This process involves fusing a desirable fruiting branch, called the scion, onto the root system of another plant, known as the rootstock. Growers use grafting for several reasons, including ensuring the new tree produces fruit identical to the parent tree and accelerating the time until the first harvest. Grafting also allows for the creation of multi-variety “cocktail” trees, controlling the mature size of the tree, or improving its resistance to soil-borne diseases. Understanding the rules of compatibility is the first step in determining which fruit trees can be successfully joined.
The Taxonomic Rules Governing Graft Compatibility
The success of any fruit tree graft is fundamentally determined by the genetic relationship between the scion and the rootstock. Compatibility is highest when the two components are closely related, following the biological classification hierarchy of family, genus, and species. When a scion is grafted onto a rootstock of the same species (intraspecific pairing), such as one apple variety onto another, the success rate is almost universally high. This is the most reliable combination for a durable graft union.
Moving to the next level, interspecific grafts—between different species within the same genus—are frequently successful, though with more variation. An example is grafting a European plum onto a peach rootstock, both of which belong to the Prunus genus. Compatibility decreases significantly when attempting an intergeneric graft, which involves two different genera within the same plant family. While some intergeneric combinations work, like pear on quince, these successes are rare exceptions. Grafting between plants of different families is generally considered impossible for woody fruit trees.
Successful Grafts Within Pome and Stone Fruit Families
The Rosaceae family contains two of the most popular groups for grafting: pome fruits and stone fruits. Pome fruits, including apples (Malus) and pears (Pyrus), offer reliable compatibility within their respective species. Apples readily graft onto virtually any other apple rootstock or crabapple, as they are all part of the Malus genus. Pears can be grafted onto other pears, but also onto quince rootstock (Cydonia oblonga), which is a different genus but still within the Rosaceae family.
The pear-on-quince combination is often used to dwarf the pear tree. However, certain pear varieties are not directly compatible and require an intermediary piece of wood to bridge the gap. Stone fruits belong to the Prunus genus, which is famous for its high level of inter-species compatibility, making it ideal for multi-variety trees. Gardeners can successfully graft plums, peaches, nectarines, apricots, cherries, and almonds onto a single Prunus rootstock. For instance, an apricot scion can be grafted onto a plum rootstock, enabling one tree to produce multiple types of stone fruit.
Compatibility for Citrus and Unique Fruit Types
Citrus, which belongs to the Rutaceae family, exhibits compatibility rules that are highly permissive across species. Virtually all common citrus varieties, including oranges, lemons, limes, grapefruits, and mandarins, can be grafted onto one another. A lemon scion can be placed successfully onto an orange rootstock because they all fall within the closely related Citrus genus. This high intrageneric compatibility allows for the easy creation of multi-graft citrus trees.
Other unique fruit types follow their own specific compatibility rules, often limiting successful grafts to within their own species. Figs (Ficus carica) are typically grafted onto other fig rootstocks. Grapes (Vitis) are usually grafted only onto other Vitis species, with specific rootstocks selected for their resistance to pests like phylloxera. The primary rule holds: the closer the genetic relationship, the greater the likelihood of a strong, permanent union.
Recognizing and Managing Graft Incompatibility
Graft incompatibility occurs when the vascular connection between the scion and rootstock fails to form properly or breaks down over time. This failure results from a lack of complete integration between the cambial and vascular tissues. Incompatibility can manifest as immediate failure, where the scion dies shortly after grafting, or delayed failure, which can occur years after planting, leading to a sudden decline or breakage of the tree.
Visual signs of incompatibility often include a noticeable swelling or overgrowth at the graft union, which indicates an imbalance in growth rates or a physical barrier to nutrient flow. Other symptoms are a weak union that breaks easily in the wind, premature leaf drop, or discoloration of the foliage. In cases of known incompatibility, growers can use an interstock, a small section of a third, known-compatible cultivar, inserted between the scion and the rootstock to bridge the gap and ensure a stable connection.