Way back in Mitzvah #244, we discussed the mitzvah of kilayim, that is, the prohibition against planting diverse species together in a field. Here, we have an additional prohibition called kilei hakerem, a prohibition against planting diverse species in a vineyard. If a person does so, he violates both Mitzvah #244 and this one.
In order to violate the prohibition of kilei hakerem, a person would need to sow two diverse species aside from the grapes that naturally grow in the vineyard. This prohibition also only applies to certain species, such as the five grains (wheat, rye, oats, barley and spelt) or hemp, which are considered more significant. Most vegetables, such as carrots, would not violate this prohibition under Biblical law, though they are prohibited by rabbinic enactment.
Way back in Mitzvah #244, we said that the main reason for prohibiting kilayim is that God created all the species of the world. For us to try to crossbreed, graft or otherwise mix species is like us saying that something is missing from God’s world, which is an affront to Him. But what’s the reason for a specific, additional prohibition when it comes to a vineyard? And why do we require two species besides the grapes to be considered a violation, not just one?
The reality is that wine and grapes are especially significant. Not only does a grape field have a special name (vineyard in English, kerem in Hebrew), but wine has its own, unique bracha. Because of its prominence, a single additional crop would be considered nullified by the far more significant grape vines. It takes two other crops to generate a kilayim situation in a vineyard.
This mitzvah applies to both men and women. Biblically, it only applies in Israel but it has been extended rabbinically to other lands. It is discussed in the Mishnah in tractate Kilayim and in the Talmud in tractate Pesachim on page 24b-25a. It is codified in the Shulchan Aruch in Yoreh Deah 296. This mitzvah is #216 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.