Rambam - 1 Chapter a Day

But not in the Diaspora. See Halachah 3.

I.e., he transgresses a negative commandment, included as one of the 365 negative commandments by Sefer HaMitzvot (negative commandment 215) and Sefer HaChinuch (mitzvah 245).

The Radbaz states that the Rambam begins Sefer Zeraim with the laws of mixed substances - although the Mishnaic order of Zeraim begins with Pe‘ah because he concludes Sefer Hafla ‘ah (Hilchot Arachin 8:1) with the teaching that the High Court would concern itself with consecrated articles from the first of Adar onward. At that same time, they would also concern themselves with fields containing mixed substances (Shekalim 1:1). Hence, the Rambam mentions these subjects in sequence.
The Radbaz notes that this prooftext is also used to derive the prohibition against grafting two species of trees together and that this prohibition applies in the Diaspora as well as in Eretz Yisrael (see Halachah 5). He explains that although the prohibition against grafting trees is derived from this verse, since an equation is established between the prohibition against grafting and the prohibition against crossbreeding animals, there are certain dimensions of that prohibition that do not apply with regard to the prohibition against planting two species of produce together.

For by weeding a field, one stimulates the growth of crops. Although the Rambam considers weeding a derivative of plowing and not of sowing with regard to the forbidden Sabbath labors (Hilchot Shabbat 8:1), the Kessef Mishneh (to Hilchot Shabbat) explains that, in fact, it is a derivative of both forbidden labors, because one can weed for two intents: a) to improve the appearance of the field, in which instance, it is a derivative of plowing, and b) to cause the crops to grow better, in which instance, it is a derivative of sowing.

I.e., since the pot has a hole, the earth within it is considered to derive nurture from the earth and thus sowing within it is· considered equivalent to sowing in the earth itself (Kilayim 7:8). This principle applies, not only in this context, but in many other situations involving agricultural laws. The hole must be large enough to allow a small root to pass through (Kessef Mishneh).

The punishment given for transgressing Rabbinic prohibitions. Since the pot did not have a hole, the person is not liable for the violation of a Scriptural commandment. Nevertheless, our Sages prohibited sowing seeds in this manner (see Menachot 70a) and hence, doing so incurs this liability.

This applies to mixed species of seeds. With regard to mixed species in a vineyard, see Chapter 5, Halachah 5.

I.e., even in a field belonging to a gentile. For a gentile’s acquisition of property in Eretz Yisrael does not nullify the holiness of the land (see Hilchot Terumah 1:10). Hence, all of the mitzvot and prohibitions that apply previously continue to apply. See Turei Zahav 297:1.

We find several instances where a prohibition exists in telling a gentile to perform a forbidden act (Hilchot Shabbat 6:1; Hilchot Issurei Bi’ah 16:13; Hilchot Sechirut 13:3; see also Halachah 6). Thus the Ra’avad differs concerning this issue and maintains that making such statements to a gentile is forbidden. Both the Radbaz and the Kessef Mishneh explain that, we are speaking about an instance where the gentile is planting the mixed species in his own field. Hence, in this instance, since the gentile is acting for his own sake, there is no prohibition involved in the Jew telling the gentile to plant the mixed species. The Tur (Yoreh De’ah 296), however, interprets the Rambam as granting permission to tell a gentile to sow mixed species in the Jew’s field, but differs with that ruling himself.

If, however, he performs a deed that maintains the forbidden produce, he is liable for lashes (Jerusalem Talmud, Kilayim 8:1). This is the manner in which the Kessef Mishneh resolves the Ra’avad’s objections to the Rambam’s ruling.

There is not even a Rabbinic prohibition against doing so.

The Kessef Mishneh cites sources (including Chapter 5, Halachah 18) which indicate that the intent is not that the produce is not fit for human consumption, but rather that it is not fit for human use. Even if it is ordinarily used as animal fodder, it is still fit to be considered as a mixed substance. This interpretation is cited by the Turei Zahav 297:2.

For, as mentioned above, this restriction is derived from an association with the prohibition against cross-breeding different species of animals.

The Radbaz maintains that it is, however, permitted to graft a vegetable to a vegetable in the Diaspora, but not in Eretz Yisrael. The Tur and the Shulchan Aruch (Yoreh De’ah 295:3), however, also forbid grafting two species of vegetables together. Similarly, they forbid grafting a fruit-bearing tree onto a non-fruit-bearing tree.

Two explanations are given for this prohibition:
a) A gentile is forbidden to graft different species of trees together (Hilchot Melachim 10:6). Hence, having him perform the act is “placing a stumbling block in the path of the blind.”
b) Since it is forbidden to do anything to maintain a forbidden graft, it is also forbidden to give instructions for such a graft to be made.

The Beit Yosef (Yoreh De’ah 295) emphasizes that, according to the Rambam, this leniency applies even in Eretz Yisrael. This runs contrary to the view of the Tur who maintains that this is forbidden in Eretz Yisrael. The Tur ‘s view is supported by the Ra’avad who differs with the Rambam and forbids such a practice. The Kessef Mishneh and the Radbaz support the Rambam’s view.

With regard to the prohibitions against forbidden labor on the Sabbath, by contrast, greater stringencies apply to a person who performed a forbidden labor benefiting from his actions than to others (Hilchot Shabbat 6:23).

The Radbaz states that here, the Rambam is speaking about the status of the produce according to law. As stated in the conclusion of the following chapter, as a punishment for violating the law, our Sages decreed that the entire field should be designated as ownerless.

Therefore it is permitted to plant nectarines and other mixed species;

From here to the conclusion of the chapter, the Rambam is outlining general principles that will be relevant in the coming chapters (Kessef Mishneh).

The commentaries note that the Rambam’s statements here appear to contradict his statement in Chapter 3, Halachah 18, and Hilchot Matnot Aniyim 3:11, where the Rambam states that it is customary to sow mustard seed in individual rows.

I.e., it is not customary to grow an entire field of them.