The Torah commands in Parashat Kedoshim (Vayikra 19:19), “Sadecha Lo Tizra Kil’ayim,” forbidding planting different species of seeds together. For example, one is not permitted to plant wheat and barley seeds together.
The Sefer Ha’hinuch writes that this prohibition includes not just planting seeds of different species together, but also activities that promote the growth of different species that have been planted together. For example, it is forbidden to prune (remove weeds from) two plants that are growing together, as this facilitates their growth. It is likewise forbidden to take earth and cover seeds of different species that are already together in the ground, whether this is done by hand, by foot, with a utensil, or in some other manner.
Planting different species is forbidden not only in the ground, but also in an “Asitz Nakub” – a flowerpot with a hole on the bottom, such that the plants are connected to the ground.
This prohibition does not apply to planting together different species which are used not for eating, but for medicine. Since these species are not edible, they are not included in this prohibition.
The Torah forbids not only planting together different species of seeds, but also grafting different species of trees – meaning, taking a branch of one species and grafting it onto a tree of a different species.
If some seeds of one species happen to fall into seeds of a different species, they may be planted together if the minority species constitutes 1/42nd or less of the mixture. In such a proportion, the seeds are considered negated by the majority (“Bittul”), and thus the seeds may be planted without first separating them.
The Mishna teaches that on Rosh Hodesh Adar, the religious leaders would send officials to the fields to ensure that no Kil’ayim (combination of different species) was growing, and to uproot any Kil’ayim which they found.
The Torah forbids planting Kil’ayim in “Sadecha” – “your field,” which the Sages understood as a reference to specifically the Land of Israel. Thus, the prohibition against planting different species of seeds applies only in the Land of Israel, and not in other lands. Outside the Land of Israel, one may plant different species of seeds together. However, the prohibition against grafting different species together applies even outside the Land of Israel. In Israel, one may ask a gentile to plant different species of seeds. Both in Israel and in the Diaspora, one may not ask a gentile to graft a branch onto a different species of tree which is owned by a Jew.
This command applies in all times, and to both males and females. One who violates this prohibition, planting different species of seeds together, or grafting a branch onto a different species of tree, is liable to Malkut.