Gemara

(Mishnah) Question: What is the source to permit planting five kinds of seeds in a patch six by six Tefachim, i.e. one on each other the four sides, and one in the middle? (

Answer: "Ki Cha'Aretz Totzi Tzimchah uch'Ginah Zeru'eha Satzmi'ach" - Zeru'eha is plural.

(Rav Yehudah): "Totzi" alludes to one, "Tzimchah" alludes to one, "Zeru'eha" alludes to two, and "Satzmi'ach" alludes to one;

We know that six by six Tefachim is the area needed in order that they will not nurture from each other.

Question: What is the source that Chachamim can rely on this knowledge?

Answer (R. Chiya bar Aba): "Lo Sasig Gevul Re'echa Asher Gavlu Rishonim" - one may not impinge on a Gevul (limit) that the early ones set.

85a (Rav): The Mishnah discusses a patch in a Churbah (surrounded by unseeded patches).

(Bei Rav): The case is, he filled the 'corners', therefore, the patch must be in a Churbah.

Question: It would be better to seed the surrounding patches, and leave the corners of this patch empty!

Answer: Chachamim decreed not to do so, lest one fill the corners.

Question: This should be permitted, like a patch of vegetables shaped like an ox' head (triangle)!

(Mishnah): If an ox' head of vegetables enters a field [seeded with something else] it is permitted, for it is clear that it is the end of another field.

Answer: The leniency of an ox' head does not apply to patches.

Kil'ayim 3:1 (Mishnah): If there is a wall a Tefach tall around the patch, one may seed 13 species - three on each wall, and one in the middle.

(Mishnah 2): If there was a wall a Tefach tall around the patch and it diminished, it is Kosher, since it was initially Kosher.

Rishonim

Rambam (Hilchos Kil'ayim 4:9): Kinds of vegetables that people seed only a small amount, one may sow even five kinds in one patch six by six Tefachim.

Kesef Mishneh: One may sow any vegetables, but not any Zera'im (species in which people eat the seeds, e.g. grains and legumes).

Rambam (ibid.): He sows one on each other the four sides, and one in the middle. He leaves about one and a half Tefachim between species, so they will not nurture from each other.

Ra'avad: We find that the Shi'ur of one and a half Tefachim only according to his Rav (the Ri mi'Gash, brought below). I do not know his source.

Rambam (ibid.): One may not seed more than five species, even if he distances between them, for many species in a patch like this is like a jumbled mixture.

Radvaz: Chachamim knew that five species within six Tefachim do not nurture from the other. Even so, if not for the verse, one might have thought that this is considered jumbled. The Rambam holds that the verse is an Asmachta. Even though the leniency of an ox' head does not apply to a patch, an ox' head on all four sides is very apparent, so it is permitted. Alternatively, the leniency does not apply between two patches, but it applies within a patch, for it is evident. Really, he holds that the leniency applies to a patch, like R. Yochanan.

Ri mi'Gash (Teshuvah 26): One must leave one and a half Tefachim between different species. One may plant one Tefach wide and three Tefachim long in the middle of each of the four sides, and leaves one and a half Tefachim empty towards each corner. He sows one Tefach in the middle of the patch, so there is one and a half Tefachim between it and the species on the sides. Rabanan knew that there is no nurturing from this distance. Really, one could sow nine species in the patch. However, then he sows only a [square] Tefach of each (in each corner, the middle of each side, and the middle of the patch), nine Tefachim in all. When he sows five, he sows 13 Tefachim.

Perush ha'Mishnayos (Kil'ayim 3:1): The Lav of Kil'ayim is only for Mar'is ha'Ayin. Therefore, we permit when it looks like the other species is separate, even if it is nearby. We say so about an ox' head. Therefore, we permit a corner of this species was next to and touched the side of another species, for it is an ox' head. All the more so we permit a corner of this species next to the corner of another species. If a side of this species was next to the side of another species, they may not touch, for they are jumbled and there would be Kil'ayim.

Rashi (84b DH v'Achas): A single seed is sown in the middle. It is three Tefachim from each of the sides. Even though there is no separation between species on adjacent sides, since it is Nikar (evident) that they are not growing together, it is permitted.

Poskim

Shulchan Aruch (YD 297:34): Kinds of vegetables that people seed only a small amount, one may sow even five kinds in one patch six by six Tefachim. He sows one on each other the four sides, and one in the middle. He leaves about one and a half Tefachim between species, lest they nurture from each other. One may not seed more than five species in such a patch, even if he distances between them, for this is a jumbled mixture.

Rebuttal (Gra 43, and Shenos Eliyahu on Kil'ayim 3:1): The Rambam's Perush is astonishing. Based on it, why did the Gemara mention Rabanan's knowledge? It is not relevant to nurturing, and not specifically to a patch six by six. Even in a quarter Tefach [one may sow five species as long as they do not appear to grow together]! Also, why does the Mishnah discuss a tall border? Why should it matter if the wall diminished? Also, one could seed one seed each of nine species in a patch. If the Gemara prefers to teach how one can seed the entire side, why is it not concerned for this when it discusses a patch with a border around it?!

Shenos Eliyahu (ibid.): I explain based on Mishnah 2:10, which says that vegetables must be separated by six Tefachim. The species on the sides have this separation. We are not concerned for the species in the middle, for it is evident that it is only one seed. Even though the species on adjacent sides are not distanced, it is permitted, just like an ox' head. If a species occupies six Tefachim, one may sow others species around it on all four sides, for they do not look jumbled. This is like R. Yonason in the Yerushalmi. If there was a wall around a Tefach tall, on each of the four sides one may sow one species within the wall, one species on the wall, and a third species outside the wall, and one in the middle of the patch, as long as it is six Tefachim.

Shenos Eliyahu (ibid., Bi'ur): There are four Perushim of the Rishonim. Rashi requires three Tefachim between different species. One may plant only one seed [widthwise] on each of the four sides, and one seed in the middle. He may plant along the length of each side, but not until the corner. He leaves a space so it will be like an ox' head. Tosfos requires one and a half Tefachim between different species. One may plant one Tefach wide on each side, and leaves an open square Tefach in each corner, so adjacent sides meet like an ox' head. He sows one square Tefach in the middle, so one and a half Tefachim separate it from the species on the sides.

Shenos Eliyahu (ibid.): The Rambam holds like Tosfos [regarding one and a half Tefachim separation], but he permits sowing eight [square] Tefachim in the middle, half of the entire patch excluding the Tefach on each side. He need not distance it from the other species, for the area sown in the middle [is rotated 45 degrees, so it] meets the sides like an ox' head. (See the first picture in the Bartenura.) Also R. Shimshon holds like Tosfos, but he does not apply the leniency of an ox' head to a patch, like the simple reading of the Gemara. One sows a single seed in the middle, and the sides up to a thickness of one and a half Tefachim towards the middle. Each side is tapered off to distance it one and a half Tefachim from the adjacent sides. I.e. one draws a line from 1 and 1/14 Tefachim from each corner, on both sides, at a 45 degree angle, towards the center, until it is one and a half Tefachim from the side, and sows within the lines. The distance between species is the diagonal of a square whose side is 1 and 1/14, i.e. one and a half. (See the second picture in the Bartenura.)

Tosfos Anshei Shem (Kil'ayim 3:1): According to this Perush of R. Shimshon, one sows less than 15 square Tefachim. Really, one could sow 15 full Tefachim! Starting in each corner, he sows a Tefach wide and three and a half Tefachim long, and a square Tefach in the middle. Perhaps the decree 'lest one fill the corners' forbids doing so.

Note: In the Hava Amina, one could sow 18 square Tefachim! I.e. starting in each corner, he sows a one and a half Tefachim wide and three Tefachim long, and a lone seed in the middle. Perhaps Tosfos Anshei Shem prefers to show how one could sow a greater area and have a significant amount of all of the five species.