03 – Animal and Tree Kilayim

1. The Kilayim Prohibitions: General Principles and Rationale

God created distinct species in the world, and in several contexts commanded us not to mix them with one another. The Torah states: “You shall keep My statutes: You shall not breed your animal with a different species; you shall not sow your field with kilayim; and a garment of kilayim, sha’atnez, shall not come upon you” (Vayikra 19:19). It also says: “You shall not sow your vineyard with kilayim, lest the entire yield become forbidden – the seed you sow and the produce of the vineyard. You shall not plow with an ox and a donkey together. You shall not wear sha’atnez, wool and linen together” (Devarim 22:9–11). This category of prohibitions is generally called kilayim, a term meaning “mixture.” In other words, even when each species is permitted on its own, their combination is forbidden. Therefore, the prohibition against cooking meat with milk is also included among the prohibitions of kilayim.

There are seven prohibitions of kilayim: (a) animal kilayim – breeding two different animal species; (b) working two different animal species together; (c) tree kilayim – grafting two different plant species; (d) seed kilayim – sowing two types of seeds together; (e) vineyard kilayim – sowing grain or vegetables near grapevines; (f) garment kilayim – wearing clothing that contains wool and linen together; and (g) meat and milk.

The prohibition of kilayim applies to both plants and animals. In some forms of kilayim, only the act of mixing is prohibited. After the fact, however, one may eat or derive benefit from the result of the mixture. This is true of seed kilayim, tree kilayim, and animal kilayim, whether through breeding or labor. By contrast, in other forms of kilayim, the act of mixing is prohibited, and the resulting product is forbidden for consumption or benefit. This is true of vineyard kilayim and cooking meat with milk. In the case of garment kilayim (sha’atnez), the prohibition is only to wear such a mixture; there is no prohibition against making it for a gentile or for a purpose other than wearing.

Mitzvot dependent on the Land, such as seed kilayim and vineyard kilayim, apply by Torah law only in Eretz Yisrael, within the territory settled by those who came up from Egypt (below, chapter 12, note 2). By contrast, mitzvot that are not dependent on the Land, such as garment kilayim, meat and milk, and animal kilayim, apply in the Diaspora as well. The prohibition of tree kilayim – grafting one species onto another – also applies in the Diaspora. Although this prohibition appears to be dependent on the Land, the prohibited act takes place above ground, not in the ground itself (below, section 5). Since the law of vineyard kilayim is especially stringent, as the mixture is forbidden both for consumption and benefit, the Sages decreed that it is prohibited in the Diaspora as well (below, chapter 4, section 4).

Although we cannot grasp the full depth of the mitzvot, which are divine in origin and rooted far beyond human understanding, some mitzvot have reasons that are more readily understood. These are called mishpatim, or ordinances. Others have reasons that are less understandable to us; these are called ḥukim, or statutes. The mitzva of kilayim belongs to the category of ḥukim, as the verse says: “You shall keep My statutes” (Vayikra 19:19). Rashi likewise explains that these are decrees of the King whose reasons are not revealed. We may suggest that these ḥukim reflect the unique qualities that God implanted within all created beings, qualities that point to the distinct identity and purpose of each creation. The divine statute, whose secret we cannot fully fathom, concerns the meaning and unique purpose of every species God created. Through the prohibition of kilayim, we are commanded to preserve the special and dignified place of every species, not to blur its uniqueness by mixing it with another species (Ramban and Maharal to Vayikra 19:19; Sefer ha-Ḥinukh 244). Similarly, the Sages said: “‘You shall keep My statutes’ – the statutes that I already engraved for you in the nature of creation, you must now observe” (Kiddushin 39a).

The word kilayim is related to kele, which means prison. The mixture imprisons the distinctive qualities of the species, preventing them from being expressed properly once they have been mixed with another species (Zohar III 86a; Rabbeinu Baḥya). With seed kilayim and vineyard kilayim, the prohibition applies even when the species merely grow near one another, even though the species themselves do not actually mix or merge. These prohibitions apply specifically in Eretz Yisrael because in the Land, the sanctity of creation is more fully revealed. As a result, the uniqueness of each species is more apparent there, and its special and dignified place must be preserved.^[1]

2. The Prohibition Against Animal Kilayim

One may not breed a male of one species with a female of another, as the verse states: “You shall keep My statutes; you shall not breed your animal with a different species” (Vayikra 19:19). Although the verse says “your animal,” it refers to all living creatures, including birds and sea creatures. The Torah mentions animals by way of example because domesticated animals are commonly kept by people (Bava Kama 54b). By Torah law, one may not perform the mating directly, meaning actual insertion of the male’s organ into the female. The Sages further prohibited bringing about mating indirectly, such as by positioning the male atop the female in a way that leads them to mate. However, one may place different species together in the same enclosure. If they mate on their own, one need not separate them, since placing them in the same enclosure is not considered an act that caused the mating (SA YD 297:2–3).

One may not breed two species of domesticated or wild animals. If one transgressed and did so, however, one may derive benefit from the offspring. The best-known hybrid is the mule, born of a male donkey and a mare. Mules have long been prized for carrying loads and plowing. Since the prohibition is only to bring mules into being, one may purchase and use them.

Breeding two different species is prohibited, but breeding two breeds of the same species is permitted. Therefore, one may breed bulls and cows of different breeds with one another. Similarly, one may breed different breeds of sheep, cats, or dogs. However, one may not breed two different species, even if they are relatively similar to one another. The definition of a species is determined by the overall traits and form of each species, as explained below, section 4. As a general rule, the inability to produce fertile offspring is a sign that two animals are different species. Usually, breeding two different species produces no offspring at all. In the rare cases where offspring are born, as with a donkey and a mare, the offspring is sterile. If two types can produce fertile offspring, it appears that they are not two separate species, but two varieties within a single species. In that case, there is no prohibition against breeding them together.^[2]

3. Working Two Animal Species Together

In addition to the prohibition against breeding two species with one another, the Torah forbids working with two different species together. For example, one may not plow with an ox and a donkey together or harness a horse and a donkey to the same wagon. The verse states: “You shall not plow with an ox and a donkey together” (Devarim 22:10). The Torah uses the example of an ox and a donkey because these are domesticated species commonly put to work together. However, the prohibition applies to any combination of two animal species used together for labor. The prohibition is not limited to the driver who harnesses the horse and donkey to the wagon and directs them; anyone who assists in the work violates it as well. Even one who only calls out to drive them forward violates a Torah prohibition, since he causes them to continue pulling the wagon or plow (SA YD 297:11, 13–14).^[3]

Jews alone are bound by this prohibition. If a Jew hires a gentile to transport merchandise to another city, and the gentile chooses to use a wagon harnessed to a horse and a donkey, the Jew violates no prohibition, since he has no stake in that arrangement (Arukh ha-Shulḥan YD 297:21). However, a Jew may not sit in a wagon that a gentile drives with a horse and donkey, because the passenger becomes a partner in the labor. If he were not sitting in the wagon, the driver would not command the animals to pull it (SA YD 297:12).

The prohibition of kilayim applies only when the two species are tied together. Therefore, one riding a horse may hold a rope attached to a dog. However, he may not tie the dog’s leash to the horse’s saddle, since doing so connects the two animals. At times, the dog may move ahead of the horse and pull it forward, causing the rider to violate the prohibition of animal kilayim (Arukh ha-Shulḥan YD 297:19).

Although we cannot fully grasp the depth of the mitzvot, especially the ḥukim, the Sages nevertheless sought to understand their reasons. Rambam explains that this prohibition is meant to keep different species apart, so that people will not be led to breed one species with another (Moreh Nevukhim 3:49). Sefer ha-Ḥinukh explains that the prohibition prevents animal suffering. Each species naturally prefers the company of its own kind and suffers when paired with another species. This is especially true during labor, because animals of different species do not have equal strength; when they do not pull the wagon or plow evenly, they suffer (Sefer ha-Ḥinukh 550).

4. The Prohibition Against Grafting Trees

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]

5. Grafting and Hybridization

Prohibited tree grafting consists of joining two tree species to one another in such a way that each part retains its own qualities, with no genetic merging between the two parts. By contrast, cross-pollinating two species is permitted. Every fruit-bearing plant produces flowers, and in the flower are male stamens and a female ovary. Through insects, butterflies, and wind, pollen from the male stamens reaches the ovary and the female ovules within it. Through this process, fruit is formed, containing the seed from which another tree can grow. If pollen from the male stamens is taken from a tree of one species and brought together with the female ovary of another species, a hybrid of two different species is created, and the fruit that grows will have genetic traits shared by its two “parents.” Yet each hybridization will yield a somewhat different type of fruit, because each time different components of each species’ genetic sequence are expressed, creating a new blend.

Usually, cross-pollinating two species does not succeed: pollen from the male stamens does not fertilize the ovules in the ovary, and even when fertilization occurs, the fruit produced is usually poor. Sometimes, however, the union succeeds and produces fruit with a distinctive taste, or a tree with special qualities, such as the ability to grow in a climate in which it previously could not survive. To this end, many scientists work to create numerous hybrids. Some rabbis maintain that just as one may not graft two tree species onto one another, one may not cross-pollinate them with one another (Shevet HaLevi 9:224). The primary halakhic position, however, is that the Torah forbade only the act of grafting two different trees onto one another, where they live together contrary to their natural order; it did not forbid cross-pollination, which occurs before the tree itself comes into being. Furthermore, a stamen on its own is not a species, since it cannot grow by its own power; likewise, the ovule in the ovary is not a species. Their hybridization is therefore not prohibited. All the more so, there is no prohibition of grafting in genetic engineering, which joins genetic material from one species with that of another (see also R. Yisraeli, Ha-Torah ve-ha-Aretz 3; Kerem Shlomo 2:1; R. Ariel, Teḥumin 33).

6. Bnei Noaḥ and the Laws of Kilayim

According to most Rishonim and Aḥaronim, the prohibitions against grafting trees and breeding animals of different species apply to Bnei Noaḥ as well, although they are not among the Seven Laws of Bnei Noaḥ. The verse states: “You shall keep My statutes” (Vayikra 19:19), and the Sages derive from this: “The statutes that I already engraved for you” – even before the giving of the Torah – “you must now keep.” Similarly, the Torah states: “The earth brought forth vegetation: seed-bearing plants of every kind, and trees of every kind bearing fruit with the seed in it. And God saw that it was good” (Bereishit 1:12). From here we learn that these statutes are binding on Bnei Noaḥ as well (Sanhedrin 60a; She’iltot; Behag; Rambam; Riaz; Vilna Gaon; Arukh ha-Shulḥan; Rav Kook).

Others disagree and maintain that Bnei Noaḥ are not prohibited from grafting trees or breeding different animal species (Ritva, Rosh, and Radbaz). Even according to this view, however, a Jew may not ask a gentile to graft his tree or breed his animal. Just as a Jew may not ask a gentile to violate Shabbat on his behalf, he likewise may not ask him to perform other Torah prohibitions involving his property.

Even according to the stringent view, the prohibition may apply to Bnei Noaḥ only when the grafting is certainly prohibited. However, when there is uncertainty about the graft’s halakhic status – for example, when the rootstock and scion may be two varieties of the same species – Bnei Noaḥ are not prohibited from performing the graft (see Mishpat Kohen 13).^[5]

7. May One Plant or Maintain a Grafted Tree?

As noted, although one may not graft two tree species together, the fruit produced by such grafting is permitted from the outset. The practical question is what may be done after a prohibited act of grafting has already been performed. May one care for the grafted sapling and water it so that it continues to grow? This question arises frequently. Most pear trees, for example, are grafted in a prohibited fashion, and prohibited grafting is also common among plum and peach trees. The question, then, is whether one may plant a sapling that was produced through prohibited grafting in a nursery. If it has already been planted, may one continue caring for it, or must it be uprooted?

Some maintain that the prohibition is limited to grafting two different species together. Once the graft has been made, however, there is no further prohibition against maintaining the grafted tree. Therefore, if a sapling was grafted in a nursery, one may even plant it in the field and tend to its growth. According to this view, tree grafting is similar to animal kilayim: the prohibition is only to breed an animal of one species with an animal of another. Afterward, however, one may raise and use the offspring.

Many poskim, however, rule that one may not maintain a grafted tree (SA YD 295:7). They disagree, though, about the scope of this prohibition. According to one view, one who has a grafted tree in his field must uproot it. According to another, one may not weed around it or prune it to improve its growth but may leave it in the field untended. A third view maintains that the prohibition applies only until the rootstock and scion have fused. Once they have fused, one may maintain the grafted tree and even plant it.

In practice, some rely on the lenient view that the entire prohibition is limited to the act of grafting. According to this position, even before the graft has fully fused, one may plant the grafted sapling and tend to its growth. The halakha, however, follows most poskim: one may not maintain the grafted sapling until the graft has fused. After that point, one may plant it and care for it. Ideally, one follows the stringent view and refrains from maintaining a grafted tree even after the graft has fused. When there is uncertainty whether the grafting was prohibited – that is, whether it involved two distinct species, or was permitted because it involved two varieties of the same species – one may be lenient and maintain it.

When purchasing a fruit tree, one must verify that it was not grafted in a prohibited manner. Even according to the lenient authorities who permit planting a grafted sapling, buying from someone who performs prohibited grafting assists those who violate the prohibition. Since these laws are complex, one may rely only on a seller who is both knowledgeable in these laws and God-fearing, or on a nursery under reliable kilayim supervision.^[6]