כִּלְאֵי אִילָנוֹת, הֲרֵי הֵם בִּכְלַל מַה שֶׁנֶּאֱמַר, שָׂדְךָ לֹא תִזְרַע כִּלְאָיִם. וְעַל כֵּן אָסוּר לְהַרְכִּיב מִין בְּשֶׁאֵינוֹ מִינוֹ, כְּגוֹן עָנָף שֶׁל תַּפּוּחַ בְּאֶתְרוֹג, אוֹ אֶתְרוֹג בְּתַפּוּחַ. וַאֲפִלּוּ מִינִים הַדּוֹמִים זֶה לָזֶה, כְּגוֹן תַּפּוּחַ בְּתַפּוּחַ יַעֲרִי וְכַיוֹצֵא, כֵּיוָן שֶׁהֵם שְׁנֵי מִינִים, אֲסוּרִים זֶה בָזֶה. וְאָסוּר לְיִשְֹרָאֵל לְהַנִּיחַ לַגּוֹי שֶׁיַרְכִּיב לוֹ אִילָנוֹ כִּלְאָיִם. The prohibition against the grafting of diverse tree species is implied in the verse: "You shall not sow your field with diverse seeds."1Leviticus 19:19. Therefore it is forbidden to graft a branch of one kind of tree upon another,2Even outside of Eretz Yisroel—Shulchan Aruch 295:1. such as the branch of an apple tree upon a citron tree,3Ramah 295:6 (and Shach 295:3) permit the grafting of a fruitless tree onto a different kind of a fruitless tree. Chasam Sofer Responsa 288 says it is best to refrain from this as well. However, you may not graft a fruit tree with a fruitless tree. (Shulchan Aruch 295:3) or a branch of citron upon an apple tree. Even between similar species, like a branch of a [cultivated] apple tree upon a wild apple tree,4Levushei Sarad Chapter 106 says these are different species because their taste is very different. However, the Mishkanos Yaakov Responsa 66 reinforces the prevailing custom that permits grafting an apple tree with a wild apple tree because they are one species and this does not depend on their taste. since they are two varieties of apples,5If one variety is not edible, Levushei Sarad Chapter 10 says it is considered like a fruitless tree and it is forbidden to graft it with an edible variety. Mishkanos Yaakov Responsa 66 disagrees, and considers them one species. they are forbidden to be grafted with one another.6Ramah 295:6 says it is best to refrain from grafting any two trees that are not exactly the same kind because not everyone is knowledgeable if they are the same species or not. A Jew is forbidden to allow a non-Jew to graft two diverse kinds of trees for him.
אָסוּר לְקַיֵם הַמֻּרְכָּב כִּלְאָיִם. אֲבָל הַפְּרִי הַגָּדֵל מִמֶּנּוּ, מֻתָּר. וּמֻתָּר לִקַּח עָנָף מִן הַמֻּרְכָּב וּלְנָטְעוֹ בְּמָקוֹם אַחֵר. It is forbidden to maintain a tree upon which another kind has been grafted, but it is permitted to eat its fruit. It is permitted to take a branch of a grafted tree and plant it elsewhere.
כִּלְאֵי הַכֶּרֶם וְכִלְאֵי זְרָעִים אֵינָם אֲסוּרִים בְּחוּץ לָאָרֶץ, אֶלָּא אִם כֵּן זָרַע שְׁנֵי מִינֵי תְבוּאָה אוֹ שְׁנֵי מִינֵי יֶרֶק עִם זֶרַע הַכֶּרֶם בְּיָחַד. Sowing a vineyard with two kinds of seeds, or sowing a field with two kinds of seeds, is not prohibited outside of Eretz Yisroel7This refers to both sowing and eating the produce. (See Shulchan Aruch 296:69) unless you have sown two kinds of grain or two kinds of vegetables together with the seeds of the vineyard.