הלכה על קידושין 77:13

And the principle that comes out in our hands from their words, may their memory be blessed, regarding forbidden mixtures of seeds is that anytime that there is a proper distance between the two types - and that is one and a half handbreadths, as we said - even if the leaves are mixed up, we do not concern ourselves about them; and likewise, anytime they appear separated one from the other, such that the leaves of one garden bed lean to one side and the leaves of the garden bed adjacent to it [lean] to another side - even though they are feeding one from the other - we do not concern ourselves with their feeding. As the Torah [only] paid heed to both of them together - that they feed one from the other and that their feeding be seen clearly by the eyes of the onlookers. And about what are these words speaking, that it needs distancing, or something that separates? When he sowed in his [own] field. But if his field is planted with wheat, it is permitted for his fellow to sow barley adjacent to it; as it is stated, "your field shall you not sow [with] a forbidden mixture" - meaning to say, specifically, your field - as it was not written, "the land shall you not sow [with] a forbidden mixture." And they, may their memory be blessed, also learned (Kiddushin 39a) [from] "your field," to say that it is only in the Land that the prohibition of forbidden mixtures of seeds is practiced, but not outside of the Land. [This is the case,] even though they did not say thus regarding the grafting of trees, which is also derived from "your field," but which is rather practiced in every place - and as Shmuel said [there] in the first chapter of Kiddushin, since he compares the grafting of trees to the mating of beasts which is practiced in every place. They, may their memory be blessed have already resolved [this difficulty] there in Kiddushin. And if you desire, my son, to know [their resolution], see it there. And the rest of its details are in Tractate Kilayim.