Halacha

הלכה א
יֵשׁ מִינִין בַּזְּרָעִין שֶׁיִּהְיֶה הַמִּין הָאֶחָד נִפְרָד לְצוּרוֹת הַרְבֵּה מִפְּנֵי שִׁנּוּי מְקוֹמוֹת וְהָעֲבוֹדָה שֶׁעוֹבְדִין הָאָרֶץ עַד שֶׁיֵּרָאֶה כִּשְׁנֵי מִינִין וְאַף עַל פִּי שֶׁאֵין דּוֹמִין זֶה לָזֶה הוֹאִיל וְהֵן מִין אֶחָד אֵינָן כִּלְאַיִם זֶה בָּזֶה:
כסף משנה
1.
There are certain species of plants which will divide into separate forms because of the difference in the place [where they grow] and the differences in the manner in which the earth is cultivated until they appear as two species. Nevertheless, since they are one species, they are not considered as kilayim with each other.

הלכה ב
וְיֵשׁ בַּזְּרָעִים שְׁנֵי מִינִין שֶׁהֵן דּוֹמִין זֶה לָזֶה וְצוּרַת שְׁנֵיהֶן קְרוֹבָה לִהְיוֹת צוּרָה אַחַת וְאַף עַל פִּי כֵן הוֹאִיל וְהֵן שְׁנֵי מִינִין הֲרֵי אֵלּוּ אֲסוּרִים זֶה עִם זֶה:
כסף משנה
2.
And there are species of plants that resemble each other and whose form is close to being the same. Nevertheless, because they are two species, it is forbidden [to grow] them together.

הלכה ג
כֵּיצַד. הַחֲזֶרֶת עִם חֲזֶרֶת גַּלִּים. וְהָעֵלְשִׁין עִם עֵלְשֵׁי שָׂדֶה. וְהַכְּרֵשִׁין עִם כְּרֵשֵׁי שָׂדֶה. הַכֻּסְבַּר עִם כֻּסְבַּר הָרִים. וְחַרְדָּל עִם חַרְדָּל מִצְרִי. וּדְלַעַת הַמִּצְרִית עִם דְּלַעַת הָרְמוּצָה אֵינָן כִּלְאַיִם זֶה בָּזֶה. וְכֵן הַחִטִּין עִם הַזּוּנִין. וְהַשְּׂעוֹרִים עִם שִׁבּלֶת שׁוּעָל. וְהַכֻּסְּמִין עִם הַשִּׁיפוֹן. וְהַפּוֹל עִם הַסַּפִּיר. וְהַפֻּרְקְדָּן עִם הַטֹּפַח. וּפוֹל לָבָן עִם הַשְּׁעוּעִים. וְהַקְּשׁוּת עִם הַמְּלָפְפוֹן. וְהַכְּרוּב עִם הַתְּרוֹבְתּוֹר. וְהַתְּרָדִין עִם הַלְּעוּנִין אֵינָן כִּלְאַיִם זֶה עִם זֶה. אֲבָל הַצְּנוֹן עִם הַנְּפוּסִין. וְהַחַרְדָּל עִם הַלַּפְסָן. וּדְלַעַת יְוָנִית עִם דְּלַעַת מִצְרִית אוֹ עִם דְּלַעַת הָרְמוּצָה אַף עַל פִּי שֶׁדּוֹמִים זֶה לָזֶה הֲרֵי אֵלּוּ כִּלְאַיִם זֶה בָּזֶה:
כסף משנה
3.
What is implied? Lettuce with wavy lettuce,1In his Commentary to the Mishnah (Kilayim 1:2), the Rambam defines this as "mountain lettuce." The examples cited by the Rambam in this and following halachot are all taken from Kilayim, ch. 1. Our translation is largely dependent on the Rambam's Commentary to the Mishnah and Rav Kappach's notes to that text. endives and wild endives, leek and wild leek, coriander and mountain coriander, mustard and Egyptian mustard, Egyptian squash and squash of ashes,2This squash has a bitter taste and is made edible only by putting it in a remetz, a pit filled with smoldering ashes (ibid.). are not considered as kilayim with each other. Similarly, wheat and undomesticated wheat, barley and oats, rye and spelt, beans and white peas, fabaceae and leguminosaea,3Two types of wild beans. white beans with azuki beans, zucchini and cucumbers, cabbage and cauliflower, beets and sorrel are not kilayim with each other. But radishes and Israeli radishes,4See Halachah 6. mustard and bitter cabbage, Greek squash and Egyptian squash, or and squash of ashes, although they resemble each other are kilayim with each other.

הלכה ד
וְכֵן בְּאִילָן שֶׁיֵּשׁ שְׁנֵי מִינִין שֶׁדּוֹמִין זֶה לָזֶה בָּעָלִין אוֹ בַּפֵּרוֹת הוֹאִיל וְהֵן שְׁנֵי מִינִין הֲרֵי אֵלּוּ כִּלְאַיִם. כֵּיצַד. הַתַּפּוּחַ עִם הַחִזְרָד. וְהַפַּרְסְקִין עִם הַשְּׁקֵדִים. וְהַשֵּׁזָפִין עִם הָרִימִין אַף עַל פִּי שֶׁדּוֹמִין זֶה לָזֶה הֲרֵי הֵן כִּלְאַיִם זֶה בָּזֶה. אֲבָל הָאֲגַסִּים עִם הַקְרוּסְטְמָלִים וְהַפְּרִישִׁים עִם הָעֵזְרָדִין אֵינָן כִּלְאַיִם זֶה בָּזֶה:
כסף משנה
4.
Similarly, with regard to trees, there are species which resemble each other with regard to their leaves or their fruit, but since they are separate species, they are kilayim. What is implied? Apples and crabapples, peaches and almonds, prunes and Arabian jujube, although they resemble each other are kilayim with each other. But apricots and white plums and quince and lavallei are not kilayim with each other.

הלכה ה
וְכֵן אִם יֵשׁ שָׁם זְרָעִים וְאִילָנוֹת אֲחֵרוֹת אַף עַל פִּי שֶׁהֵן שְׁנֵי מִינִין בְּטִבְעָן הוֹאִיל וְעָלִין שֶׁל זֶה דּוֹמִין לְעָלִין שֶׁל זֶה אוֹ פְּרִי שֶׁל זֶה דּוֹמֶה לִפְרִי שֶׁל זֶה דִּמְיוֹן גָּדוֹל עַד שֶׁיֵּרָאוּ כִּשְׁנֵי גְּוָנִין מִמִּין אֶחָד לֹא חָשְׁשׁוּ לָהֶן לְכִלְאַיִם זֶה עִם זֶה שֶׁאֵין הוֹלְכִין בְּכִלְאַיִם אֶלָּא אַחַר מַרְאִית הָעַיִן:
כסף משנה
5.
Similarly, there are other plants and trees which [our Sages] did not classify as kilayim although they are inherently two different species, because the leaves of one resemble the leaves of the other or the fruit of one resembles the fruit of the other very closely to the extent that they appear as different shades of the same species. [The rationale is that] with regard to kilayim we follow the appearance alone.5The Rambam also states this principle in his Commentary to the Mishnah (Kilayim 3:1). According to his understanding, it is our perception which determines whether one is mixing species or not. Their biological nature is a secondary factor.

הלכה ו
כֵּיצַד. הַלֶּפֶת עִם הַצְּנוֹן אֵינָן כִּלְאַיִם זֶה בָּזֶה מִפְּנֵי שֶׁפִּרְיֵיהֶן שָׁוִין. וְהַלֶּפֶת עִם הַנְּפוּס אֵינָן כִּלְאַיִם זֶה בָּזֶה מִפְּנֵי שֶׁהֶעָלִין שֶׁלָּהֶן שָׁוִין. אֲבָל צְנוֹן עִם הַנְּפוּס אַף עַל פִּי שֶׁהֶעָלִין דּוֹמִין זֶה לָזֶה וְהַפְּרִי דּוֹמֶה לַפְּרִי הֲרֵי אֵלּוּ כִּלְאַיִם הוֹאִיל וְטַעַם פְּרִי זֶה רָחוֹק מִטַּעַם פְּרִי זֶה בְּיוֹתֵר. וְכֵן כָּל כַּיּוֹצֵא בְּאֵלּוּ:
כסף משנה
6.
What is implied? Turnips and radishes are not kilayim with each other because their fruits are similar.6The Ra'avad differs on the principle explained by the Rambam and considers these species as kilayim. Turnips and Israeli radishes are not kilayim with each other, because their leaves resemble each other. But radishes and Israeli radishes are kilayim even though their fruits resembles each other and their leaves resemble each other, because the taste of the fruits are drastically different from each other.

הלכה ז
כַּמָּה מַרְחִיקִין בֵּין שְׁנֵי מִינֵי זְרָעִים שֶׁהֵן כִּלְאַיִם זֶה עִם זֶה. כְּדֵי שֶׁיִּהְיוּ נִרְאִים מֻבְדָּלִין זֶה מִזֶּה. אֲבָל אִם נִרְאִין שֶׁנִּזְרְעוּ בְּעִרְבּוּבְיָא הֲרֵי זֶה אָסוּר:
כסף משנה
7.
How much is it necessary to separate between two species of plants so that they will not be considered as kilayim [when planting them in the same field]? So that [the two species] will look distinct from each other. If, however, they appear as if they were sown together, this is forbidden.7The Rambam is stating a general principle with which he introduces the coming halachot.

הלכה ח
וְשִׁעוּרִין רַבִּים יֵשׁ בְּהַרְחָקָה הַזֹּאת הַכּל לְפִי גֹּדֶל הַשָּׂדֶה הַנִּזְרַעַת וּלְפִי רֹב הֶעָלִין וְשִׁלּוּחַ הַיּוֹנְקוֹת:
כסף משנה
8.
There are many different measures given with regard to the distance required to make this distinction. Everything depends on the size of the field that is being sown and the proliferation of leaves [the plants have] and the extent to which their branches spread out.

הלכה ט
כֵּיצַד. שָׂדֵהוּ שֶׁהָיְתָה זְרוּעָה מִין תְּבוּאָה וּבִקֵּשׁ לִזְרֹעַ בְּצִדָּהּ מִין תְּבוּאָה אַחֶרֶת בְּשָׂדֶה אַחַת מַרְחִיק בֵּינֵיהֶן בֵּית רֹבַע וְהוּא כְּעֶשֶׂר אַמּוֹת וְחֹמֶשׁ אַמָּה עַל עֶשֶׂר אַמּוֹת וְחֹמֶשׁ אַמָּה מְרֻבָּע בֵּין מִן הָאֶמְצַע בֵּין מִן הַצַּד. וְאִם לֹא הָיָה בֵּינֵיהֶן כַּשִּׁעוּר הַזֶּה אָסוּר וְאֵינוֹ לוֹקֶה עַד שֶׁיִּהְיוּ קְרוֹבִין בְּתוֹךְ שִׁשָּׁה טְפָחִים:
כסף משנה
9.
What is implied? If a person had sown a species of grain in his field and he sought to sow another species of grain in another field at its side, he must make a separation the size of the area in which one can sow a quarter of a kav between the two. This is approximately8I.e., slightly more than that sum (Kessef Mishneh). ten and one fifth cubits by ten and one fifth cubits. [This applies] whether [the space left empty ceases] in the midst [of these fields] or [continues along their entire] side.9Our translation is based on the gloss of the Kessef Mishneh who cites the Rambam's Commentary to the Mishnah (Kilayim 3:3) as a source.
The Radbaz and the Kessef Mishneh cite the Jerusalem Talmud (Kilayim 2:7) which states that if one species surrounds the other on all four sides, such a separation is not sufficient. They say that one could understand the Rambam as alluding to that view. The Aruch HaShulchan 297:32, however, explains that the Rambam differs.
If there is less than this space between them, sowing [these crops] are forbidden, but he is not liable for lashes unless they are six handbreadths [or less] close to each other.10I.e., unless it is less than the smaller figure, it is not forbidden by Scriptural Law. Nevertheless, as a safeguard, our Sages required an even greater separation.

הלכה י
הָיְתָה שָׂדֵהוּ זְרוּעָה יָרָק וּבִקֵּשׁ לִזְרֹעַ בְּצִדָּהּ שְׂדֵה יָרָק אַחֵר אֲפִלּוּ דְּלַעַת מַרְחִיק בֵּין שְׁתֵּי הַשָּׂדוֹת שִׁשָּׁה טְפָחִים עַל שִׁשָּׁה טְפָחִים מְרֻבָּע בֵּין מִן הַצַּד בֵּין מִן הָאֶמְצַע. וּפָחוֹת מִשִּׁעוּר זֶה אָסוּר וְאֵינוֹ לוֹקֶה עַד שֶׁיִּהְיוּ קְרוֹבִין בְּתוֹךְ טֶפַח:
כסף משנה
10.
If his field was sown with vegetables and he desired to sow another species of vegetables - even squash11Whose leaves spread very far (see Halachah 13). - in another field at its side, he must make a separation of a square six handbreadths by six handbreadths between the two whether [the space left empty ceases] in the midst [of these fields] or [continues along their entire] side. If there is less than this space between them, sowing [these crops] are forbidden, but he is not liable for lashes unless they are within a handbreadth of each other.12Although the Ra'avad rules stringently, stating that the measures mentioned in the previous halachah apply in this instance as well, the Radbaz and the Kessef Mishneh justify the Rambam's approach.

הלכה יא
הָיְתָה אַחַת מִשְּׁתֵּי הַשָּׂדוֹת זְרוּעָה תְּבוּאָה וְהַשְּׁנִיָּה שֶׁבְּצִדָּהּ יָרָק אוֹ דְּלַעַת מַרְחִיק בֵּינֵיהֶן בֵּית רֹבַע:
כסף משנה
11.
If there was grain sown in one of two fields and vegetables or squash sown in the other, one must make a separation the size of the area in which one can sow a quarter of a kav between the two.13Since there is grain involved, we rule stringently (Radbaz).

הלכה יב
בַּמֶּה דְּבָרִים אֲמוּרִים שֶׁצָּרִיךְ לְהַרְחָקָה בַּשִּׁעוּרִין הַלָּלוּ בֵּין שְׁתֵּי שָׂדוֹת. אֲבָל אִם הָיְתָה שָׂדֵהוּ זְרוּעָה יָרָק וְרָצָה לִזְרֹעַ בְּצִדָּהּ שׁוּרָה שֶׁל יָרָק מִמִּין אַחֵר דַּיּוֹ לַעֲשׂוֹת בֵּין הַשָּׂדֶה וּבֵין הַשּׁוּרָה תֶּלֶם אֶחָד אָרְכּוֹ שִׁשָּׁה טְפָחִים בִּלְבַד וְרָחְבּוֹ כְּעָמְקוֹ:
כסף משנה
12.
When does the need to make a distinction of the size mentioned above apply? Between two fields.14Here the Rambam does not define what constitutes a field. In Hilchot Matanot Aniyim 3:4, he states that a small field is 50 cubits by 2 cubits and in Hilchot Shechenim 1:4, he defines a field as a place that is fit to sow nine kabbim. In his Commentary to the Mishnah (Bava Basra 1:6), he defines that as being 3750 sq. cubits. If, however, he had sowed vegetables in his field and he desires to sow a row of another species of vegetables at their side,15If he wants to sow this row in the middle of the field, he must leave such a trench on either side (Radbaz). it is sufficient for him to leave a trench six handbreadths long16For a separation this size is sufficient to set the row apart as a distinct entity [the Rambam's Commentary to the Mishnah (Kilayim 3:3)]. with its width the same as its depth17The trench must be at least a handbreadth deep.
The Ra'avad differs with the Rambam's interpretation of this concept, but the Radbaz and the Kessef Mishneh substantiate the Rambam's understanding.
between the row and the field.

הלכה יג
הָיְתָה שָׂדֵהוּ זְרוּעָה תְּבוּאָה וְרָצָה לִזְרֹעַ בְּתוֹכָהּ שׁוּרָה שֶׁל יָרָק אֲפִלּוּ שׁוּרָה שֶׁל דְּלוּעִין שֶׁעָלִין שֶׁלָּהֶן אֲרֻכִּים וּמִסְתַּבְּכִין מַרְחִיק בֵּינֵיהֶן שִׁשָּׁה טְפָחִים. וְאִם נִמְשְׁכוּ הֶעָלִים שֶׁל דְּלַעַת וְנִכְנְסוּ לַתְּבוּאָה וְנִסְתַּבְּכוּ בָּהּ יַעֲקֹר מִן הַתְּבוּאָה שֶׁלִּפְנֵי הַדְּלַעַת עַד שֶׁלֹּא יִתְעָרְבוּ הֶעָלִין. וְאֵין צָרִיךְ לוֹמַר שֶׁאִם זָרַע שׁוּרָה מִמִּין זֶה וְשׁוּרָה מִמִּין אַחֵר שֶׁדַּיּוֹ לִהְיוֹת בֵּינֵיהֶן תֶּלֶם אֶחָד כְּמוֹ שֶׁיִּתְבָּאֵר:
כסף משנה
13.
When a field is sown with grain and a person desires to sow a row of vegetables - even squash whose leaves are long and become tangled - in its midst, he must leave a distance of six handbreadths between them.18See Chapter 4, Halachah 3, and notes. If the leaves of the squash became extended and entered into [the portion where] the grain [was sown] and became entangled with it, he should uproot enough of the grain in front of the squash so that the leaves will not become tangled.19The Radbaz explains that he could also cut off the extensions of the squash plants, but apparently, he would suffer a greater loss from doing that than from uprooting the grain. Hence, the Rambam is simply giving him good advice. Needless to say, if he sowed one row of one species and another row of another species, it is sufficient for there to be one trench between them, as will be explained.20The Radbaz mentions that this also refers to the concepts mentioned in the previous halachah. It is, nevertheless, placed here to teach that even though there is grain involved (see Halachah 9), since we are speaking only of one row, there is no need for additional stringency.

הלכה יד
הִרְחִיק בֵּין שְׁנֵי הַמִּינִין הַרְחָקָה הָרְאוּיָה לָהֶן וְהָיָה מִין זֶה נוֹטֶה עַל גַּבֵּי מִין זֶה בֵּין שֶׁנָּטְתָה תְּבוּאָה עַל הַתְּבוּאָה אוֹ יָרָק עַל גַּבֵּי יָרָק אוֹ יָרָק עַל הַתְּבוּאָה אוֹ תְּבוּאָה עַל הַיָּרָק הַכּל מֻתָּר. שֶׁהֲרֵי הִרְחִיק כַּשִּׁעוּר. חוּץ מִדְּלַעַת יְוָנִית שֶׁהִיא נִמְשֶׁכֶת הַרְבֵּה. לְפִיכָךְ אִם נָטְתָה יַעֲקֹר מִלְּפָנֶיהָ כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
14.
If he made an appropriate separation between the two species, but one of the species became draped upon the other one - whether the grain became draped upon other grain, a vegetable upon another vegetable, a vegetable upon grain, or grain upon a vegetable - everything is permitted, for he made a separation of the appropriate measure. [There is one] exception, Greek squash, because it becomes extended very far. Therefore if it becomes draped upon [another species], one should uproot the species in front of it,21Even though there is an appropriate separation, the species appear to be mixed [the Rambam's Commentary to the Mishnah (Kilayim 3:11)]. as explained [in the previous halachah].

הלכה טו
הָיָה בֵּין שְׁנֵי הַמִּינִים בּוֹר אוֹ נִיר אוֹ גַּפָּה אוֹ דֶּרֶךְ אוֹ גָּדֵר שֶׁהוּא גָּבוֹהַּ עֲשָׂרָה טְפָחִים אוֹ חָרִיץ שֶׁהוּא עָמֹק עֲשָׂרָה טְפָחִים וְרָחָב אַרְבָּעָה. אוֹ אִילָן שֶׁהוּא מֵסֵךְ עַל הָאָרֶץ. אוֹ סֶלַע שֶׁהוּא גָּבוֹהַּ עֲשָׂרָה טְפָחִים וְרָחָב אַרְבָּעָה. הֲרֵי זֶה מֻתָּר לִסְמֹךְ הַמִּין הָאֶחָד לְצַד אֶחָד מֵאֵלּוּ וְהַמִּין הָאַחֵר לַצַּד הַשֵּׁנִי הוֹאִיל וְאֶחָד מִכָּל אֵלּוּ מַבְדִּיל בֵּינֵיהֶן הֲרֵי הֵן נִרְאִים מֻבְדָּלִין זֶה מִזֶּה:
כסף משנה
15.
If between two species there were [any of the following] - a cistern,22Our translation follows the gloss of the Radbaz. Others interpret the term bor as referring to an unplowed field. a plowed field left fallow, a stone fence, a path,23Which is four cubits wide (see Hilchot Matanot Aniyim 3:3). a wooden fence that is ten handbreadths high, a trench that is ten handbreadths deep and four handbreadths wide, a tree whose branches hang to the earth,24And thus it appears as a tent [the Rambam's Commentary to the Mishnah (Kilayim 3:11)]. or a rock that is ten handbreadths high and four [handbreadths] wide - it is permissible to place the one species on one side of [the divider] and the other species on the other side [of the divider]. Since one of the above is separating between them, they appear distinct from each other.

הלכה טז
בַּמֶּה דְּבָרִים אֲמוּרִים שֶׁצָּרִיךְ הַרְחָקָה אוֹ דָּבָר הַמַּבְדִּיל בְּשֶׁזָּרַע בְּתוֹךְ שָׂדֵהוּ. אֲבָל אִם הָיְתָה שָׂדֵהוּ זְרוּעָה חִטִּים מֻתָּר לַחֲבֵרוֹ לִזְרֹעַ בְּצִדָּהּ שְׂעוֹרִים שֶׁנֶּאֱמַר (ויקרא יט יט) "שָׂדְךָ לֹא תִזְרַע כִּלְאָיִם". אֵין הָאִסּוּר אֶלָּא שֶׁיִּזְרַע שָׂדֵהוּ כִּלְאַיִם שֶׁלֹּא נֶאֱמַר הָאָרֶץ לֹא תִזְרַע כִּלְאָיִם. וְלֹא עוֹד אֶלָּא אֲפִלּוּ זָרַע בְּתוֹךְ שָׂדֵהוּ שְׂעוֹרִים סָמוּךְ לַחִטִּים וּמָשַׁךְ זֶרַע הַשְּׂעוֹרִים עַד שֶׁסְּמָכוֹ לִשְׂדֵה חֲבֵרוֹ שֶׁהִיא זְרוּעָה שְׂעוֹרִים הֲרֵי זֶה מֻתָּר. מִפְּנֵי שֶׁנִּרְאוּ הַשְּׂעוֹרִים שֶׁבְּתוֹךְ שָׂדֵהוּ שֶׁהֵן סוֹף שְׂדֵה חֲבֵרוֹ:
כסף משנה
16.
When is it necessary for there to be such a separation or a divider? When one is sowing [the two species] in his own field. If, however, one person sowed wheat in his field, his colleague is permitted to sow barley in a bordering [field], as [implied by] Leviticus 19:19]: "Do not sow mixed species in your field." The prohibition applies only to sowing mixed species in one's own field. For the Torah does not say: "Mixed species shall not be sown on the earth."25This ruling is an outgrowth of the rationale for the prohibition against mixed species: not to intermingle the different spiritual powers that give energy to the various crops (see the commentary of Rabbenu Bachaye to Leviticus 19:19 . If the properties belong to two different people - even though the land on which the different crops grow are adjacent - the two spiritual mediums of nurture will not be intermingled (Yayin Malchut).
The Ra'avad does not accept the Rambam's ruling. He accepts the following clause - for it is an explicit mishnah (Kilayim 2:7) - but maintains that the sowing of the two species next to each other are acceptable in that instance, because at the outset, there was a clear distinction. In this instance, by contrast, there is no distinction at the outset, and it appears that a prohibition was violated. The Radbaz justifies the Rambam's ruling, stating that if one is allowed to sow mixed species in his own field, because it appears to be an extension of his colleague's field, certainly, his colleague is allowed to plant a different species in his own field.

Moreover, even if one planted barley in his own field next to wheat and extended the barley until it was adjacent to the field of his colleague which was planted with barley, it is permitted. [The rationale is that] the barley in his own field appears to be the end of his colleague's field.26And thus they will not appear as mixed species to an onlooker.

הלכה יז
הָיְתָה שָׂדֵהוּ זְרוּעָה חִטִּים וּשְׂדֵה חֲבֵרוֹ בְּצִדָּהּ זְרוּעָה חִטִּים מֻתָּר לוֹ לִזְרֹעַ תֶּלֶם אֶחָד שֶׁל פִּשְׁתָּן בְּצַד חִטִּים שֶׁלּוֹ סָמוּךְ לִשְׂדֵה חֲבֵרוֹ. שֶׁהָרוֹאֶה יוֹדֵעַ שֶׁאֵין דֶּרֶךְ הָעָם לִזְרֹעַ תֶּלֶם אֶחָד שֶׁל פִּשְׁתָּן וְלֹא נִתְכַּוֵּן זֶה אֶלָּא לִבְדֹּק שָׂדֵהוּ אִם רְאוּיָה הִיא לִזְרֹעַ פִּשְׁתָּן אִם לֹא וְנִמְצָא כְּזוֹרֵעַ לְהַשְׁחָתָה. לְפִיכָךְ אָסוּר לִזְרֹעַ מִין אַחֵר בֵּין שְׁתֵּי שׁוּרוֹת אֵלּוּ שֶׁהֵן מִמִּין אֶחָד עַד שֶׁיַּרְחִיק בְּתוֹךְ שֶׁלּוֹ:
כסף משנה
17.
If his field was planted with wheat and a colleague's field adjoining it was planted with wheat, he is permitted to sow one row of flax at the side of his wheat adjoining his colleague's field.27Despite the fact that the two fields appear to be one continuous field of wheat, planting the one row of flax is permitted for the rationale explained by the Rambam (Radbaz). [The rationale is that] an observer knows that it is not the common practice to sow only one row of flax and this person is merely testing his field to see if it is fit to plant flax or not. Thus he is sowing the seed with the intent of destroying it.28I.e., he is not intending to reap any benefit from planting this flax. For this reason, the prohibition against kilayim does not apply. Therefore,29I.e., since the reason for the leniency is that everyone can see that he has no benefit from the row of flax, when that rationale does not apply, the leniency is not granted. it is forbidden to sow another species30I.e., a species other than flax that people might commonly sow only one row of. between these two fields31Our text is based on authoritative manuscripts of the Mishneh Torah. The version in the standard printed text is slightly different. that are planted with one species until he makes a distinction [between the two species] within his own property.

הלכה יח
הָיְתָה שָׂדֵהוּ וּשְׂדֵה חֲבֵרוֹ שֶׁבְּצִדָּהּ זְרוּעִין שְׁנֵי מִינֵי תְּבוּאָה לֹא יִזְרַע בֵּינֵיהֶם חַרְדָּל וַחֲרִיעַ אֲפִלּוּ תֶּלֶם אֶחָד מִפְּנֵי שֶׁהָעָם זוֹרְעִים מֵאֵלּוּ תֶּלֶם אֶחָד. אֲבָל אִם הָיוּ שְׁתֵּי הַשָּׂדוֹת זְרוּעִין שְׁנֵי מִינֵי יְרָקוֹת מֻתָּר לִזְרֹעַ בֵּינֵיהֶן חַרְדָּל אוֹ חֲרִיעַ. שֶׁמֻּתָּר לְהַקִּיף חַרְדָּל אוֹ חֲרִיעַ לְכָל מִין חוּץ מִן הַתְּבוּאָה מִפְּנֵי שֶׁאֵינָן מַזִּיקִין אוֹתָהּ. וְכֵן אִם הָיְתָה זָוִית שֶׁל זֶרַע זֶה נוֹגַעַת בְּצֶלַע שֶׁל זֶרַע הָאַחֵר בְּתוֹךְ שָׂדֵהוּ הֲרֵי זֶה מֻתָּר מִפְּנֵי שֶׁהֵן נִרְאִין מֻבְדָּלִין זֶה מִזֶּה. וְאֵין צָרִיךְ לוֹמַר אִם הָיְתָה זָוִית שֶׁל זֶרַע זֶה נוֹגַעַת בְּזָוִית שֶׁל זֶרַע הָאַחֵר מִפְּנֵי שֶׁהֵן נִרְאִין כְּסוֹף שָׂדֶה שֶׁנָּגַע בְּסוֹף שָׂדֶה שֶׁהוּא מֻתָּר בְּלֹא הַרְחָקָה וְלֹא הַבְדָּלָה כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
18.
When a person's field and a colleague's field were sown with two [different] types of grain,32Needless to say, this applies when the fields are sown with the same species (Radbaz). he should not sow even one row of mustard seed or safflower seed, because it is customary to sow one row of these.33I.e., these are crops for which there is a benefit in sowing a single row. Hence, in contrast to the flax mentioned in the previous halachah, a prohibition exists.
The commentaries note that the Rambam's statements here appear to contradict his statement in Chapter 1, Halachah 9, in which he states that it is customary to sow entire fields of mustard seed. They are, however, reinforced by his statements in Hilchot Matnot Aniyim 3:11.
If, however, there were two fields planted with different species of vegetables, it is permitted to sow mustard seed or safflower seed between them. For it is permitted to bring any species close to mustard seed or safflower seed with the exception of grain.34Since mustard seed and safflower seed have a harmful effect on other crops, there is no prohibition. Significantly, in his Commentary on the Mishnah (Kilayim 2:8), the Rambam writes the direct opposite that mustard seed and safflower seed have a harmful effect on grain, but not on other crops.
Rav Yosef Corcus notes that from the previous halachah, one might think that it is permitted to plant only a row of flax, but not of other species, because only flax is planted as an experiment. And from this halachah, one might think only mustard seed and safflower seed are forbidden, for otherwise, there would be no reason to single them out individually, but other species are permitted. In resolution, he explains that here the emphasis is on mustard seed and flax seed individually. Since they are permitted next to other species, one might think that they are also permitted next to grain. Hence, it is necessary to emphasize that this is not so.
[There is a stringency in the latter instance, because the mustard seed or safflower seed] do not have a harmful effect on [grain].
Similarly, if a corner [of a portion] of one's field [sown] with one species touches a block [of the field sown] with another species, it is permitted, because they appear separate from each other.35Even though they are adjacent, the manner in which they are planted makes a clear distinction. Needless to say, if the corner [of a portion of one's field sown] with one species touches a corner [of the field sown] with another species, it is permitted without setting them apart or making a distinction as we explained,36In Halachah 16. because it appears that it is the end of one field that touched the end of another field.

זרעים הלכות כלאים פרק ג
Zeraim Kilayim Chapter 3