Pesachim 39B

ערוגה שהיא ששה על ששה טפחים ⁠— זורעין בתוכה חמשה זרעונין, ארבע ⁠— על ארבע רוחות הערוגה, ואחת באמצע. מהו דתימא: הני מילי בזרעין, אבל בירקות ⁠— לא, קא משמע לן.

With regard to a garden bed that is 6 by 6 handbreadths in area, one may plant in it 5 different types of seeds, 4 types on the 4 sides of the bed and one type in the middle. This Mishnah teaches that it is permitted to plant different types of seeds in one garden bed, provided that one maintains the appropriate distance between them. The Talmud explains: Rav’s statement is nevertheless necessary. Lest you say that that applies only to seeds, but with regard to vegetables, no, it is prohibited to plant them in a single garden bed even if this separation is maintained, Rava teaches us that it is permitted to plant in this manner with vegetables as well.

למימרא דירקות אלימא מזרעים? והתנן: כל מיני זרעים אין זורעין בערוגה אחת, כל מיני ירקות זורעין בערוגה אחת! מהו דתימא הני מרור מין זרעים נינהו, קא משמע לן.

The Talmud asks: Is that to say that vegetables are stronger in the level of their prohibition than seeds? But didn’t we learn in a Mishnah in Kilayim: With regard to multiple types of seeds, one may not plant them together in one garden bed, even if there is space between them; however, with regard to multiple types of vegetables, one may plant them together in one bed? This Mishnah apparently indicates that the halakha is stricter concerning seeds. The Talmud answers: Rav’s statement is necessary. Lest you say that these bitter herbs are a type of seed, not a vegetable, and therefore it is prohibited to plant them in a bed with other seeds, Rav teaches us that all the various bitter herbs are classified as types of vegetables.