In the previous mitzvah, we explained the prohibition of diverse species grown together in a vineyard. If such things do grow together, we are not permitted to eat them. In fact, one may not benefit from such mixtures at all, so they must be burned (see Talmud Kiddushin 56b). (If this doesn’t seem like a negative mitzvah from our verse, the principle is that the Hebrew word “pen,” meaning “lest,” denotes a negative injunction – see Talmud Eiruvin 96a.)
This mitzvah applies whether the farmer actively planted the other species or whether they grew on their own but he permitted it. There is no ban if he was completely unaware of the other species, since the Torah says, “that you plant,” indicating that the farmer’s consent is necessary to activate the mechanism, as it were. If he tried unsuccessfully to uproot or fence off the encroaching growth, there is no ban on the produce since his actions indicate that he does not desire to cultivate mixed species in his vineyard.
The reason for this mitzvah is that wine is already kind of a gray area. On the one hand, it can be very useful in moderation. As such, we use it for everything from Kiddush and havdalah, and from the Seder to a wedding. On the other hand, wine in excess is a source of much evil in the world as we see in Scripture from the story of Noah’s drunkenness, that of Lot’s daughters plying him with wine, and elsewhere. Since wine is already “on probation,” if grapes are grown in a vineyard that is a source of sin, that’s considered the “third strike” and they may not be used.
This mitzvah applies to both men and women, Biblically in Israel and rabbinically elsewhere. It is discussed in the Mishnah in tractate Kilayim and in the Talmud in tractate Pesachim on pages 24b-25a. It is codified in the Mishneh Torah in the tenth chapter of Hilchos Maachalos Asuros. This mitzvah is #193 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.