The Torah commands in Parashat Ki-Teseh (Debarim 22:9), “Lo Tizra Karmecha Kil’ayim” – that one may not plant seeds of a different species in one’s vineyard. The Gemara explains that this refers to planting grape seeds together with seeds of grain, such as wheat or barley, or with “Kanbus” or “Luf,” which are types of flax. Specifically, the Gemara teaches, one transgresses this prohibition if he plants at the same time, with the same seeder, a seed of grapes and two seeds of grain.
The Sefer Ha’hinuch refers us in this context to his comments regarding an earlier Misva (244) explaining the reason behind the prohibition against crossbreeding. The reason provided for that command is relevant also to the prohibition of “Kil’eh Ha’kerem” (planting grapes with other species).
As for why this prohibition is violated only when one plants two seeds of grain together with a single seed of grape, the Sefer Ha’hinuch suggests that this relates to the unique properties of grapes. The grape is an especially strong species, and its strength is neutralized only by grain (or the aforementioned two species of flax) in a quantity twice the amount of grape. Therefore, if only a single seed of grain is planted together with a seed of grape, this would not constitute a mixture, as the grape seed would overpower the other seed. Only when one plants a grape seed together with two seeds of grain can this be considered a combination of different species. The Sefer Ha’hinuch adds that this is also the reason why the Torah specified particular species for this prohibition. Other species – besides the principal grains, and “Kanbas” and “Luf” – are not strong enough to neutralize the power of grape even in larger amounts, and so the Torah forbade planting a grape seed specifically with these species.
This prohibition applies at all times, and is binding upon both men and women. On the level of Torah law, it applies only in Eretz Yisrael, but the Sages extended the prohibition to other areas.
One who plants these seeds together at the same time is liable to Malkut. (In Eretz Yisrael, a violator is liable to Malkut on the level of Torah law; outside Israel, he receives Malkut for violating the Sages’ enactment.) This command is violated the moment one plants the seeds together. The seeds become forbidden for benefit once they take root.