Hybrid or Kil’ayim?

Parshas Kedoshim contains one of the two places in the Torah where the mitzvos of kil’ayim are taught.

Question #1: Kil’ei beheimah

May one attempt to crossbreed a mule with a stallion?

Question #2: Kil’ei zera’im

May I plant the vegetables in my garden close together?

Question #3: Kil’ei hakerem

Is there any way that I can plant vegetables near my vineyard?

Question #4: Harkavas ilan

Must I be careful before I purchase a fruit tree?

Many people assume that the halachic definition of the mitzvah of kil’ayim is the crossbreeding of different species of plants or animals, but, as we will soon see, not all of the laws of kil’ayim have to do with what a farmer or a scientist would call crossbreeding or hybridization.

My desktop dictionary defines hybrid as “the offspring produced by breeding plants or animals of different varieties, species, or races.” Thousands of years ago, mankind crossbred horses and donkeys to create mules and hinnies. This hybrid, called a pered (female pirdah) is already mentioned many times in Tanach. As a pack animal, the mule — produced from a male donkey, called a “jack” and a mare (female horse) — has many advantages over either of its parents. It is usually as strong as a horse, sturdy, sure-footed, and, notwithstanding its reputation for being “stubborn as a mule,” is more reliable for hauling than draft horses. (A hinney, which has less commercial value, is produced from a stallion (male horse) and a female donkey, called a “jenny.” The word “hinney” comes from its parents – a horse and a jenny.)

Other crossbred animals

Artificial insemination has been used to crossbreed camels and llamas with the goal of producing a larger quantity of quality llama wool. Mankind has created such interesting creatures as ligers, crossbreeds of male lions and tigresses, tiglons (sometimes called tigons) from male tigers and lionesses, leopons (male leopards and lionesses), wholpins (whales and dolphins) and geeps (goats and sheep). Most of these have resulted in limited, if any, commercial value, although it was thought by some that they might.

Crossbreeding animal species is one of the prohibitions of the Torah when it declares behemtecha lo sarbia kil’ayim (Vayikra 19:19). It is one of the unusual mitzvos that even a non-Jew is required to observe (Rambam, Hilchos Melachim 10:6).

The prohibition is only to create the crossbreed; one may use a mule or any other crossbred animal. However, not only is it prohibited to crossbreed a horse with a donkey, it is even forbidden to attempt to breed a mule or hinney with either a donkey or a horse (Mishnah, Kil’ayim 1:6). In fact, it is rare that such an attempt will produce offspring, although it is claimed anecdotally that this can happen upon occasion. Nevertheless, someone who attempts to crossbreed them violates a Torah prohibition.

Crossbreeding of plants

Crossbreeding of plants, or, as it is usually called, cross-fertilization or cross-pollination, is when one pollinates the flower of one species with pollen from a different species, to produce offspring with some characteristics of each. Many fruits have been developed this way, although I want to share that a nectarine is not a crossbreed of a peach and a plum, as often mistakenly thought. A nectarine is an ancient variety of peach (Prunus persica) that has a smooth skin. Botanists consider it to be the same species as peaches.

What is interesting is that, in the discussions about kil’ayim in the Torah, the Mishnah and the writings of Chazal, nowhere does it say that it is prohibited to cross-pollinate from one plant species to another. This does not mean to say that there is no prohibition of kil’ayim germane to trees or plants. Quite the contrary, there are three such prohibitions min haTorah. They are referred to as kil’ei zera’im, kil’ayim in plants; kil’ei hakerem, kil’ayim in vineyards; and kil’ayim in trees, usually referred to as harkavas ilan. But, as we will soon see, none of these three prohibitions has anything to do with crossbreeding.

Kil’ei zera’im

Kil’ei zera’im is planting two or more different species of grains, vegetables or other edible herbaceous plants in close proximity. Exactly what defines “close proximity” is a very complicated halachic topic, and depends on factors such as the shape and size of the vegetable patch, and what variety of produce one is planting. We should note that, from a botanical point of view, planting two species in close proximity will not cause hybridization because it does not affect the genetic makeup.

This mitzvah applies only in Eretz Yisroel. Thus, someone in chutz la’aretz may plant his backyard garden with a wide variety of vegetables without any halachic concern, whereas in Eretz Yisroel someone planting a garden patch must be very careful to keep the different species separate.

Kil’ayim in a vineyard, kil’ei hakerem

Kil’ei hakerem is the prohibition against planting grains or vegetables in, near, above or below a vineyard. Again, this forbidden planting will not affect the genetic makeup of any of the plants involved. It is also clear that this was not the concern in halacha as we see from many of the halachic details. Here is one example: Although it is prohibited to plant grains or vegetables near a vineyard, there is a way to permit it by separating the vegetable patch from the vineyard with a halachic wall between them. For example, if one places two poles and a wire across the top, a tzuras hapesach, between the vegetable patch and the vineyard, it is permitted to plant vegetables right next to the vineyard (Eruvin 11a). This is similar to what we do when we construct an eruv to permit carrying on Shabbos. It is quite clear that, botanically, the tzuras hapesach does not accomplish anything to prevent the mingling of the species. Yet, with the tzuras hapesach, it is permitted to plant the crop; without the tzuras hapesach, it is a Torah prohibition to do so! This certainly cannot be explained on a scientific basis.

Even one grapevine is problematic near a crop plant, so care must be taken even in the home garden. For example, a pot with herbs or a vegetable under a trellised grapevine could forbid the grapes and the produce of the pot!

Unlike other forms of kil’ayim, the produce of kil’ei hakerem is forbidden to use.

The prohibition of planting grains or vegetables in a vineyard applies in chutz la’aretz, but only miderabbanan (Kiddushin 39a).

Harkavas ilan – grafting trees

The laws of kil’ayim also prohibit grafting one species of tree or plant onto the wood stock, or lower trunk, of another species. Although a town dweller may feel that this is a rare occurrence, in fact, contemporary plant nurseries and tree farmers usually graft branches of a species that produces delicious fruit onto the hardier stock of a different species.

For example, most peach and nectarine trees are produced by grafting a peach or nectarine branch onto the stock of a hardier tree, such as an almond. Someone who performs this, either in Eretz Yisroel or in chutz la’aretz, violates a Torah prohibition whether he is Jewish or not (Rambam, Hilchos Melachim 10:6). Most authorities rule that one may not own, water or prune a kil’ayim tree, whether or not it is in Eretz Yisroel or in chutz la’aretz (Shulchan Aruch Yoreh Deah 295:7 and Piskei Teshuvah).

However, many observant Jews purchased agricultural properties that contained kil’ayim trees and did not cut down those trees. Was there any justification for their actions? Numerous halachic responsa discuss what was apparently a widespread practice in the eighteenth and nineteenth century. Whereas most poskim rule that these Jews violated the halacha, some authorities justify the practice of owning these trees, at least in chutz la’aretz (Shu’t Chasam Sofer, Yoreh Deah #288; Aruch Hashulchan, Yoreh Deah 295:17-18).

Because so many trees are grafted nowadays, someone who owns a fruit tree should have a horticultural expert check whether its stock is from the same species or not. Often, even a non-expert can detect if a tree was grafted onto a different species by simply scrutinizing the tree. If the bark somewhere near the bottom of the tree looks different from the upper part of the tree, this indicates that the upper part of the tree was grafted. Often one can see a line separating the grafted scion from the rootstock, or a difference in thickness between the top and bottom. Before purchasing a new tree at a nursery, examine the trunk carefully for signs of grafting.

The prohibition of planting vegetables and other edible crops together applies only in Eretz Yisroel, whereas grafting trees applies equally min hatorah in chutz la’aretz and in Eretz Yisroel.

Although planting and caring for a kil’ayim tree is forbidden, the fruit from such a tree is permitted. Thus, one may purchase fruit in a market without worrying about kil’ayim.


Although space does not allow us to discuss this fascinating topic, there is a huge amount of halachic literature discussing the very common instance of using an esrog from a tree that was grafted onto a non-esrog tree. Most authorities rule that this esrog may not be used to fulfill the mitzvah on Sukkos.


Rav Hirsch (Vayikra 19:19) explains that the root word ke’le means to keep or hold something back, and that the plural form kil’ayim is similar to yadayim or raglayim and means a pair. Therefore, the word kil’ayim means to pair together two items that should be kept apart. This is to teach us that although we are given the world to develop, we must follow the rules that Hashem established for us to do so.