Halacha

הלכה א
הַזּוֹרֵעַ שְׁנֵי מִינֵי זְרָעִים כְּאֶחָד בְּאֶרֶץ יִשְׂרָאֵל לוֹקֶה שֶׁנֶּאֱמַר (ויקרא יט יט) "שָׂדְךָ לֹא תִזְרַע כִּלְאָיִם":
כסף משנה
1.
A person who sows two species of seeds together in Eretz Yisrael1But not in the Diaspora. See Halachah 3. is liable for lashes,2I.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). as [Leviticus 19:19] states: "You shall not sow your field with mixed species."3The 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.

הלכה ב
וְאֶחָד הַזּוֹרֵעַ אוֹ הַמְנַכֵּשׁ אוֹ הַמְחַפֶּה כְּגוֹן שֶׁהָיְתָה חִטָּה אַחַת וּשְׂעוֹרָה אַחַת אוֹ פּוֹל אֶחָד וַעֲדָשָׁה אַחַת מֻנָּחִין עַל הָאָרֶץ וְחִפָּה אוֹתָן בֶּעָפָר בֵּין בְּיָדוֹ בֵּין בְּרַגְלוֹ בֵּין בִּכְלִי הֲרֵי זֶה לוֹקֶה. וְאֶחָד הַזּוֹרֵעַ בָּאָרֶץ אוֹ בֶּעָצִיץ נָקוּב. אֲבָל הַזּוֹרֵעַ בֶּעָצִיץ שֶׁאֵינוֹ נָקוּב מַכִּין אוֹתוֹ מַכַּת מַרְדּוּת:
כסף משנה
2.
[This prohibition, referred to as kilayim, applies whether one] sows, weeds,4For 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.
or covers seeds with earth, e.g., there was a kernel of wheat and a kernel of barley or a fava bean and a lentil, lying on the earth and one covered them with earth, whether with his hand, his foot, or with a utensil, he is liable for lashes. [This applies] whether he sows them in the earth or in a pot with a hole.5I.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 within the earth (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). When, by contrast, one sows them in a pot without a hole, he is liable only for stripes for rebellious conduct.6The 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.

הלכה ג
אָסוּר לִזְרֹעַ כִּלְאַיִם לְנָכְרִי. וּמֻתָּר לוֹמַר לְנָכְרִי לִזְרֹעַ לוֹ כִּלְאֵי זְרָעִים. וְאָסוּר לְאָדָם לְקַיֵּם כִּלְאֵי זְרָעִים בְּשָׂדֵהוּ אֶלָּא עוֹקְרָן וְאִם קִיְּמָן אֵינוֹ לוֹקֶה. וּמֻתָּר לְיִשְׂרָאֵל לִזְרֹעַ כִּלְאֵי זְרָעִים בְּיָדוֹ בְּחוּצָה לָאָרֶץ וַאֲפִלּוּ לְעָרֵב הַזְּרָעִים לְכַתְּחִלָּה וּלְזָרְעָן בְּחוּצָה לָאָרֶץ מֻתָּר וּדְבָרִים אֵלּוּ דִּבְרֵי קַבָּלָה:
כסף משנה
3.
It is forbidden to sow kilayim7This applies to mixed species of seeds. With regard to mixed species in a vineyard, see Chapter 5, Halachah 5. for a gentile.8I.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. It is, by contrast, permitted to tell a gentile to sow mixed species of seeds for his own sake.9We 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.
It is forbidden for a person to maintain mixed species of seeds in his field. Instead, he must uproot them. If he maintains them, he is not liable for lashes.10If, 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. It is permitted for a Jew to sow mixed species of seeds by hand in the Diaspora.11There is not even a Rabbinic prohibition against doing so. It is even permitted to mix seeds at the outset and then sow them. These concepts [have been communicated] by the Oral Tradition.

הלכה ד
אֵין אָסוּר מִשּׁוּם כִּלְאֵי זְרָעִים אֶלָּא הַזְּרָעִים הָרְאוּיִין לְמַאֲכַל אָדָם. אֲבָל עֲשָׂבִים הַמָּרִים וְכַיּוֹצֵא בָּהֶן מִן הָעִקָּרִין שֶׁאֵינָן רְאוּיִין אֶלָּא לִרְפוּאָה וְכַיּוֹצֵא בָּהֶן אֵין בָּהֶן מִשּׁוּם כִּלְאֵי זְרָעִים:
כסף משנה
4.
The prohibition against mixed species of seeds involves only plants that are fit for human consumption.12The Kessef Mishneh cites sources (including Chapter 5, Halachah 18) which indicate 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. Grasses that are bitter and similar roots that are fit only for medicinal purposes and the like are not included in the prohibition against mixtures of seeds.

הלכה ה
כִּלְאֵי הָאִילָנוֹת הֲרֵי הֵם בִּכְלַל מַה שֶּׁנֶּאֱמַר (ויקרא יט יט) "שָׂדְךָ לֹא תִזְרַע כִּלְאָיִם". כֵּיצַד. הַמַּרְכִּיב אִילָן בָּאִילָן כְּגוֹן שֶׁהִרְכִּיב יִחוּר שֶׁל תַּפּוּחַ בְּאֶתְרוֹג אוֹ אֶתְרוֹג בְּתַפּוּחַ הֲרֵי זֶה לוֹקֶה מִן הַתּוֹרָה בְּכָל מָקוֹם בֵּין בָּאָרֶץ בֵּין בְּחוּצָה לָאָרֶץ. וְכֵן הַמַּרְכִּיב יָרָק בְּאִילָן אוֹ אִילָן בְּיָרָק לוֹקֶה בְּכָל מָקוֹם:
כסף משנה
5.
[Grafting] a mixture of trees is included in the prohibition: "You shall not sow your field with mixed species." What is implied? When a person grafts a tree [of one species] unto a tree [of another species], e.g., he grafted a branch of an apple tree to an esrog tree or one from an esrog tree to an apple tree, he is liable for lashes according to Scriptural Law in any place, whether in Eretz Yisrael or in the Diaspora.13For, as mentioned above, this restriction is derived from an association with the prohibition against cross-breeding different species of animals. Similarly, if a person grafts a vegetable to a tree or a tree to a vegetable, he is liable for lashes in every place.14The 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.

הלכה ו
וְאָסוּר לְיִשְׂרָאֵל לְהָנִיחַ לְנָכְרִי שֶׁיַּרְכִּיב לוֹ אִילָנוֹת כִּלְאַיִם. וּמֻתָּר לִזְרֹעַ זְרָעִים וְזֶרַע אִילָן כְּאֶחָד וְכֵן מֻתָּר לְעָרֵב זַרְעֵי אִילָנוֹת וּלְזָרְעָן כְּאֶחָד שֶׁאֵין לְךָ כִּלְאַיִם בָּאִילָנוֹת אֶלָּא הַרְכָּבָה בִּלְבַד:
כסף משנה
6.
It is forbidden for a Jew to allow a gentile to graft different species of trees together for [the Jew].15Two 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.

It is permitted to sow the seeds of produce and the seeds of a tree together.16The 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. Similarly, it is permitted to mix seeds from different species of trees and sow them together. [The rationale is that] the only prohibition against mixed species that applies to trees is the prohibition against grafting [different species together].

הלכה ז
הַזּוֹרֵעַ זְרָעִים כִּלְאַיִם וְכֵן הַמַּרְכִּיב אִילָנוֹת כִּלְאַיִם אַף עַל פִּי שֶׁהוּא לוֹקֶה הֲרֵי אֵלּוּ מֻתָּרִין בַּאֲכִילָה וַאֲפִלּוּ לָזֶה שֶׁעָבַר וּזְרָעָן שֶׁלֹּא נֶאֱסַר אֶלָּא זְרִיעָתָן בִּלְבַד. וּמֻתָּר לִטַּע יִחוּר מִן הָאִילָן שֶׁהֻרְכַּב כִּלְאַיִם וְלִזְרֹעַ מִזֶּרַע הַיָּרָק שֶׁנִּזְרַע כִּלְאַיִם:
כסף משנה
7.
Although he is liable for lashes, when a person sows forbidden species together or grafts forbidden species of trees together, the produce that grows is permitted to be eaten, even by the person who transgressed and sowed it.17With 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). For it is only sowing that is forbidden.18The 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.
It is permitted to plant a branch from the grafted tree or plant the seeds from a vegetable that was planted together with mixed species.19Therefore it is permitted to plant nectarines and other mixed species.

הלכה ח
הַזֵּרְעוֹנִין נֶחֱלָקִין לִשְׁלֹשָׁה חֲלָקִים. הָאֶחָד מֵהֶם הוּא הַנִּקְרָא תְּבוּאָה וְהִיא חֲמִשָּׁה מִינִין הַחִטִּים וְהַכֻּסְּמִין וְהַשְּׂעוֹרִין וְשִׁבּלֶת שׁוּעָל וְהַשִּׁיפוֹן. וְהַשֵּׁנִי מֵהֶן הוּא הַנִּקְרָא קִטְנִית. וְהֵן כָּל זְרָעִים הַנֶּאֱכָל לְאָדָם חוּץ מִן הַתְּבוּאָה. כְּגוֹן הַפּוֹל וְהָאֲפוּנִים וְהָעֲדָשִׁים וְהַדֹּחַן וְהָאֹרֶז וְהַשֻּׁמְשְׁמִין וְהַפְּרָגִין וְהַסַּפִּיר וְכַיּוֹצֵא בָּהֶן. וְהַשְּׁלִישִׁי מֵהֶן הוּא הַנִּקְרָא זֵרְעוֹנֵי גִּנָּה. וְהֵן שְׁאָר זֵרְעוֹנִין שֶׁאֵינָן רְאוּיִין לְמַאֲכַל אָדָם. וְהַפְּרִי שֶׁל אוֹתוֹ הַזֶּרַע מַאֲכַל אָדָם. כְּגוֹן זֶרַע הַבְּצָלִים וְהַשּׁוּמִין וְזֶרַע הֶחָצִיר וְהַקֶּצַח וְזֶרַע לֶפֶת וְכַיּוֹצֵא בָּהֶן. וְזֶרַע פִּשְׁתָּן הֲרֵי הוּא בִּכְלַל זֵרְעוֹנֵי גִּנָּה. כְּשֶׁיִּזָרְעוּ כָּל מִינֵי זֵרְעוֹנִים אֵלּוּ וְיִצְמְחוּ נִקְרָא הַצֶּמַח כֻּלּוֹ כָּל זְמַן שֶׁלֹּא נִכָּר הַזֶּרַע דֶּשֶׁא וְנִקְרָא יֶרֶק:
כסף משנה
8.
Edible plants are divided into three categories:20From here to the conclusion of the chapter, the Rambam is outlining general principles that will be relevant in the coming chapters (Kessef Mishneh).
a) the first is called grain; it includes wheat, wild wheat, barley, oats, and spelt;
b) the second is called kitniot; it includes all the types of seeds eaten by men other than grain, e.g., beans, peas, lentils, millet, and rice, sesame seeds, poppy seed, white peas, and the like. c) the third is called garden seeds, i.e., seeds which are not fit for human consumption, but whose fruit is fit for human consumption, e.g., onion seeds, garlic [seeds], leek seeds, ketzach [seeds], cabbage seeds, and the like. Flax seed is considered in this category. When these seeds are sown and grow, before the seeds are recognizable, the entire plant is called herbage and it is called vegetables.

הלכה ט
וְיֵשׁ מִזֵּרְעוֹנֵי גִּנָּה זֵרְעוֹנִים שֶׁדַּרְכָּן לִזְרֹעַ מֵהֶן שָׂדוֹת כְּגוֹן הַפִּשְׁתָּן וְהַחַרְדָּל וְאֵלּוּ הֵן הַנִּקְרָאִים מִינֵי זְרָעִים. וְיֵשׁ מִזֵּרְעוֹנֵי גִּנָּה זֵרְעוֹנִים שֶׁאֵין דֶּרֶךְ בְּנֵי אָדָם לִזְרֹעַ מֵהֶם אֶלָּא עֲרוּגוֹת עֲרוּגוֹת קְטַנּוֹת כְּגוֹן הַלֶּפֶת וְהַצְּנוֹן וְהַתְּרָדִים וְהַבְּצָלִים וְהַכֻּסְבַּר וְהַכַּרְפַּס וְהַמָּרוֹר וְכַיּוֹצֵא בָּהֶן וְאֵלּוּ הֵן הַנִּקְרָאִים מִינֵי יְרָקוֹת:
כסף משנה
9.
There are some garden plants that that are sown in [large] fields, e.g., flax and mustard seed21The 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. and they are called "types of seeds." And there are other garden plants which people customarily sow only in small rows,22I.e., it is not customary to grow an entire field of them. e.g., turnips, radishes, spinach, onions, coriander, parsley, horse radish, and the like. These are called vegetables.

זרעים הלכות כלאים פרק א
Zeraim Kilayim Chapter 1