One may not graft two species of trees onto one another, as the verse states: “You shall keep My statutes; you shall not breed your animal with a different species; you shall not sow your field with kilayim” (Vayikra 19:19). Just as the first clause prohibits breeding two animal species with one another, so too “you shall not sow your field with kilayim” prohibits grafting two plant species onto one another. This includes grafting different species of trees or vegetables, as well as grafting a tree onto a vegetable or a vegetable onto a tree. All non-fruit-bearing trees are considered one species. Therefore, one may not graft a fruit tree onto a non-fruit-bearing tree. One may, however, graft one non-fruit-bearing tree onto another, since for this law they are all considered one species. Their defining feature is that they do not produce fruit, and that feature classifies them together as a single species (SA YD 295:1–3; Rema YD 295:6).
This prohibition applies in the Diaspora as well. Although at first glance grafting appears to be dependent on the Land, the prohibited act itself is performed above the ground. It therefore resembles the prohibition of animal kilayim, which is mentioned next to it in the verse (Kiddushin 39a). The two laws are similar in another respect as well: although one may not breed animals or graft trees, the resulting offspring or produce may be eaten or used even from the outset.
Grafting is used to combine the desirable traits of two species or varieties of trees. Trees that bear abundant, flavorful fruit often have weaker root systems and are more susceptible to disease and death. Hardy, resilient trees, by contrast, generally bear little fruit, and their fruit is less flavorful. To address this, growers graft one onto the other. They plant a cutting from a hardy tree in the ground; this is called the rootstock. The rootstock is cut in a particular way, and a branch from a tree that bears high-quality fruit, called the scion, is attached to it. The two parts are bound together until they fuse and then continue growing as a single tree. The roots and lower trunk belong to the hardy species, while the upper trunk, branches, and fruit belong to the species that bears abundant, flavorful fruit. Thus, even though the two trees have been grafted and joined, each part continues to function according to its own nature: the rootstock draws nourishment from the ground through its roots, while the scion produces fruit according to its own nature. As a result, there is almost no difference between fruit that grows on a scion grafted onto another species and fruit that grows on a scion rooted naturally in the ground.
If the rootstock and scion are from two different species, grafting them is a Torah prohibition for which one is liable to lashes. If they are different varieties of the same species, grafting them onto one another is permitted. Therefore, one may graft a large, red, sweet apple variety onto a small, green, sour apple variety, since both are apple varieties. However, one may not graft a peach tree onto an almond tree, because they are two different species. For the laws of kilayim, species are defined broadly. Classification depends on the tree’s full range of visible characteristics: the fruit’s form and taste, the branches and leaves, and the growth pattern. Therefore, trees classified by common usage or by scientific taxonomy as two species may nevertheless be considered one species according to halakha. When the Jewish people lived in the Land, and the Great Sanhedrin resolved every question and uncertainty, the traditional classification of species for the laws of kilayim was known and clear. Over time, however, the many exiles eroded that knowledge, and doubts arose about the status of various trees that are now difficult to resolve.[4]
Aḥaronim offered several approaches to species classification, each identifying a different feature as decisive. From the overall teaching of the Sages, however, it seems that the definition of species for kilayim does not depend on a name or on one particular trait, but on the totality of its characteristics. Rav Kook wrote similarly (Mishpat Kohen 25). Over the generations, doubts arose regarding various trees. The Sages’ formulations indicate that species categories are defined broadly. Therefore, two kinds that appear to be one species are treated as one species (Mishna, Kilayim 1:1; MT, Laws of Kilayim 3:5). Even so, some were inclined to be stringent in every case of doubt (Rema YD 295:6; Sha’arei Tzedek), while others maintained that the halakhic authorities of each generation have the authority to decide such matters (Ḥazon Ish, Kilayim 3:6).
Citrus species became a major area of uncertainty. Some maintain that grapefruit and orange are one species (Mishpetei Uziel 1, YD 24), whereas Rav Kook was stringent (Mishpat Kohen 25). Many authorities were left in doubt and therefore tended to forbid a Jew from grafting them, while permitting a gentile to perform the graft.
In practice, it appears that when uncertainty remains even after examining the full range of characteristics, the doubt may be resolved by looking at natural fertilization. If pollen from one species can, under natural conditions, fertilize the ovules of the other species, they are one species, as I wrote above regarding animal kilayim (section 2). R. Elḥanan Ben-Nun writes similarly (Teḥumin 1). The Sages gave various identifying signs because fertilization was difficult to test. In addition, the basic principle in distinguishing species is that we follow how people understand and perceive the species before them. Since today our understanding of species classification is closely tied to reproductive compatibility, doubtful cases for Jews should be resolved on that basis. This does not, however, necessarily settle the question for Bnei Noaḥ. If doubt remains because of some other characteristic, they may be lenient, as explained below in section 6, note 5.
