Gemara

(Beraisa): If one sows Kilayim (a forbidden mixture of seeds) and Kilayim, he is lashed.

Suggestion: He gets 40 lashes.

Rejection #1: This is obvious. What is the Chidush of the Mishnah?!

Rejection #2: If so, why does it say 'Kilayim and Kilayim'?

Conclusion: Rather, he gets 80 lashes.

The Beraisa teaches that there are two ways to be liable for Kilayim, unlike R. Yoshiyah;

(R. Yoshiyah): One is liable for Kilai ha'Kerem (Kilayim with grapes) only if he sows a wheat seed, barley seed and grape seed b'Mapoles Yad (together).

The Beraisa obligates for wheat and grape seeds alone (i.e. without barley), and for barley and grape seeds alone.

136b (Rav Nachman bar Yitzchak): Nowadays, people conduct according to the lenient opinion of R. Yoshiyah regarding Kil'ayim;

Kidushin 38b (Mishnah): If Yerek (vegetables) are sold outside a vineyard in which Yerek is growing, in Eretz Yisrael, one may not buy them. In Surya one may buy. (Perhaps the Yerek is from elsewhere). In Chutz la'Aretz, one may buy, as long as he doesn't pick it himself.

39a: Rav Chanan and Rav Anan saw a man planting Kil'ayim in Chutz La'aretz.

Rav Anan: We should excommunicate him!

Rav Chanan: You do not know the Halachah.

1. They saw a man planting wheat and barley in a vineyard in Chutz La'aretz.

Rav Anan: We should excommunicate him!

Rav Chanan: You do not know the Halachah. The Halachah follows R. Yoshiyah.

Rav Yosef mixed diverse seeds and planted them together.

Question (Abaye - Mishnah): Kil'ayim applies in Chutz La'aretz mid'Rabanan.

Answer (Rav Yosef): That refers to Kil'ai ha'Kerem, but not Kil'ai Zera'im (diverse seeds not involving grapes or a vineyard).

Since one may not benefit from Kil'ai ha'Kerem in Eretz Yisrael, Chachamim forbade such Kil'ayim in Chutz La'aretz;

One may benefit from Kil'ai Zera'im in Eretz Yisrael, so Chachamim did not forbid it in Chutz La'aretz.

Rishonim

Rambam (Hilchos Kilayim 5:1): One who plants seeds of a vineyard together with two kinds of grain or two kinds of Yerek with is lashed twice, for "Sadcha Lo Sizra Kilayim" and "Lo Sizra Karmecha Kilayim."

Rambam (2): One is lashed for Kil'ai ha'Kerem only if he sows in Eretz Yisrael wheat, barley and a grape seed, or if he covered them with earth, or if he sowed two kinds of Yerek and a grape seed, or he sowed a seed of Yerek and a grain seed and a grape seed b'Mapoles Yad.

Rambam (13): If grain hardened as much as needed, or grapes fully ripened, they do not become forbidden due to Kil'ai ha'Kerem. If grain fully hardened and one planted a vine amidst it, or grapes fully ripened and one planted grain or Yerek next to it, even though it is forbidden, they do not become forbidden.

Rambam (14): Even though they do not become forbidden (mid'Oraisa), we fine him and forbid the seeds. However, the immature grapes are permitted.

Rambam (8:13): All the Shi'urim of separations between vines and grain or Yerek are only in Eretz Yisrael or Surya. In Chutz la'Aretz l'Chatchilah one may sow next to vines in a vineyard. It is forbidden only a grape seed with two kinds of Yerek or grain b'Mapoles Yad.

Rambam (14): Even though one may plant Yerek next to vines in Chutz la'Aretz, one may not eat what grows, even in Chutz la'Aretz. This is if he sees the seller pick it. A Safek is permitted.

Rosh (Hilchos Kilayim 4): In Chutz la'Aretz, the Rambam forbids only a grape seed with two other species b'Mapoles Yad. This implies that one may sow two species next to a vineyard. The case in Kidushin supports this. If it were forbidden to eat mid'Rabanan, they should have excommunicated the man, for presumably he sowed in order to eat what grows. We excommunicate to prevent transgression! In any case they should have informed him that he may not eat what grows. The Gemara connotes that they did not say anything to him. The Rambam means that even though one may sow Yerek next to a vineyard and eat what grows, if he did not sow it and perhaps it was sown b'Mapoles Yad, one may not eat what grows even in Chutz la'Aretz, like the Mishnah. It says a Safek is permitted to eat, if he did not pick it himself. This is the practice, to sow next to vines.

Rebuttal (Beis Yosef YD 296 DH Al): The Rosh says that if it were forbidden to eat them, they should have excommunicated the man. This is no proof. Perhaps he planted them in order to sell them to a Nochri! Since he did not transgress, they did not need to inform him. Perhaps he knows the Halachah! If you will say that they should have informed him, perhaps they did! The Gemara did not mention this, for it came to teach that the Halachah follows R. Yoshiyah. How can the Rosh say that the Rambam forbids Vadai? The Rambam says 'Oso' (it)! According to the Rosh, the Rambam should have said 'even though one may plant next to vines in Chutz la'Aretz, one may not eat Yerek found there.' 'Oso' shows that the Rambam discusses what he just mentioned, Yerek planted next to (but not b'Mapoles Yad with) the vineyard. The Rosh proves that the Rambam permits mid'Oraisa what was planted next to the vineyard, since it was not b'Mapoles Yad. One can prove oppositely! The Rambam said 'a Safek is permitted, like I wrote in Hilchos Ma'achalos Asuros.' There he discusses Vadai Kil'ayim mid'Oraisa! Also, in Halachah 8, the Rambam forbids planting next to vines, but it becomes forbidden, He cites the verse, i.e. the Torah forbids! This is only in Eretz Yisrael. In Chutz la'Aretz one may plant it, but it is forbidden. This is why the Tur did not explain the Rambam like the Rosh did.

Rambam (Hilchos Ma'achalos Asuros 10:8): The Torah forbids Kil'ai ha'Kerem when they were planted in Eretz Yisrael. In Chutz la'Aretz, it is mid'Rabanan.

Rosh (ibid.): Kil'ai ha'Kerem is forbidden in Chutz la'Aretz, but only if one sows a grape seed together with two kinds of grain or two kinds of Yerek b'Mapoles Yad. We hold like R. Yoshiyah. The Ri says that this is even though several Mishnayos and Beraisos and Sugyos are unlike R. Yoshiyah. E.g. one who passes a flowerpot with a hole through a vineyard forbids, and one who drapes his vine over another's grain forbids (Kil'ayim 6:4, 7:8), and hops in a vineyard is Kil'ayim (Shabbos 139a). Later generations returned to do like R. Yoshiyah. The Ra'avad says that R. Yoshiyah requires wheat, barley and a grape seed only for lashes, but to forbid, they need not be sown in Mapoles Yad, like the Mishnah of passing a flowerpot through a vineyard. It seems that the Ra'avad forbids mid'Rabanan, since the Torah forbids only all three together. The Ri holds that there is not even an Isur mid'Rabanan, since he says that the Mishnayos are unlike R. Yoshiyah. If there is an Isur mid'Rabanan, the Mishnayos could be like him! It seems that even the Ra'avad permits in Chutz la'Aretz, even though it is Asur b'Hana'ah in Eretz Yisrael, since this is only mid'Rabanan.

Rosh: The Ramah says that R. Yoshiyah admits that if one sowed even one species between vines in Eretz Yisrael, it is forbidden. In Chutz la'Aretz it is permitted. This is why they did not excommunicate the man who did so. In Chutz la'Aretz Chachamim decreed about sowing two species between vines. Rav Yosef forbids because it is Asur b'Hana'ah in Eretz Yisrael. He did not say that it is mid'Oraisa in Eretz Yisrael. This refers to sowing two species in a vineyard, for when it sprouts it resembles a Torah Isur. It is not clear whether it was sown b'Mapoles Yad. Sowing one species in a vineyard does not resemble a Torah Isur. Even though it is Asur b'Hana'ah mid'Rabanan in Eretz Yisrael, Chachamim did not decree in Chutz la'Aretz at all, like the episode with Rav Nachman.

Rebuttal (Rosh): What is the Ramah's source to distinguish between one and two species? In that episode it says 'he sowed Bizroni (seeds)', like it says about Rav Yosef! Also, since we say 'in Eretz Yisrael it is Asur b'Hana'ah' this connotes mid'Oraisa, for we do not find an explicit Isur Hana'ah mid'Rabanan regarding Kil'ayim. The Mishnayos unlike R. Yoshiyah forbid mid'Oraisa without Mapoles Yad. .We do not find that R. Yoshiyah forbids mid'Rabanan not b'Mapoles Yad. An Amora must explain himself clearly. If he forbids in Chutz la'Aretz what is forbid in Eretz Yisrael even mid'Rabanan, he should have said so, and he would teach also that there is Kil'ayim forbidden mid'Rabanan. Since he did not say so, this connotes that it is an Isur Torah.

Poskim

Shulchan Aruch (YD 296:4): One may not plant Yerek or grain next to vines, or vice-versa. If one did so, even though he is not lashed, he forbids, and both of them are Asur b'Hana'ah, the vines and the Yerek or grain. We burn both of them, for it says "Pen Tukdash ha'Mele'ah ha'Zera."

Beis Yosef (DH v'Lo): Why did the Tur omit what the Rosh brought from the Ra'avad? I.e. there is an Isur mid'Rabanan without Mapoles Yad only in Eretz Yisrael, but in Chutz la'Aretz even two species may be sown between vines.

Beis Yosef (DH v'Divrei): The Tur combined what the Rambam wrote in Hilchos Ma'achalos Asuros with his words in Hilchos Kil'ayim. He infers that even one species is forbidden with vines from the Rambam in Hilchos Ma'achalos Asuros, which connotes that he discusses one species. One could reject this and say that really, he discusses two species, like he says in Hilchos Kil'ayim, but it seems that he discusses one species.

Beis Yosef (DH v'Rabbeinu): R. Yerucham says that the Rambam is unclear. I explained that he holds that R. Yoshiyah requires three species (including the grape seed) to obligate lashes, but the vines and other produce are forbidden even without this, for presumably R. Yoshiyah agrees with all the Mishnayos. Since there are no lashes in Eretz Yisrael, one may plant it in Chutz la'Aretz. Since it is Asur b'Hana'ah in Eretz Yisrael, one may not eat it in Chutz la'Aretz.

Taz (2): The Ran holds that if he sowed only one species with a grape seed, there is no Isur at all.