Kilayim, Chapter 4, Mishnah 9. In the previous Mishnah we discussed the law of one who plants when there is a certain distance between the plantings, and our Mishnah deals with that very same idea on a larger scale: a person who plants an entire vineyard, many rows, with a distance of sixteen amos between one row and the next.
One who plants his vineyard in rows of sixteen amos:
One who plants a vineyard whose rows are "shesh esreh amos" - sixteen amos apart from one another - may sow seeds between them, and not only that, but he may sow the seed a mere six tefachim away from the vine. The reason: the area between the rows does not have the status of a 'clearing in the vineyard' (karachas hakerem).
A 'clearing in the vineyard' refers to an existing vineyard that was destroyed in its middle: there we require that the empty space be sixteen amos, and even then one must keep four amos away on every side. But here, since the vineyard was planted from the outset with gaps of sixteen amos, the area between the rows never acquired the status of a vineyard at all.
An incident that occurred in a place called Salmon:
Rabbi Yehudah testifies about an incident that occurred: a certain man planted his vineyard with the rows sixteen amos apart from one another, and this was his practice:
He would bend the branches of two adjacent rows toward one side, so that they would not cover the area between them.
The plowed ground between them he would sow with a different seed that is not vines, such as grain.
The following year he would bend the branches toward the other side, and sow the ground that had lain fallow the previous year - and in this way he gave each of the fields its turn to rest each year.
This case came before the Sages, and they permitted it - in accordance with the law stated in the first clause, that since there are sixteen amos between the rows, one may sow between them.
And the reason for bending the branches to the other side: so that the branches would not grow over the grain sown there. Even though there are sixteen amos between the rows, we do not want the branches covering the grain growing between them.
The view of Rabbi Meir and Rabbi Shimon:
Rabbi Meir and Rabbi Shimon go further and say that even one who plants a vineyard of many rows with gaps of eight amos - it is permitted. True, in a vineyard that was destroyed in its middle, in a 'clearing in the vineyard', eight amos are not enough and sixteen amos are required; but since he planted with gaps of eight amos from the outset, that is enough for the area not to acquire the status of a vineyard, and therefore he may keep six tefachim away from the vines and sow in the area between them.
In summary: In this Mishnah we learned that a vineyard planted from the outset in rows sixteen amos apart does not acquire the status of a 'clearing in the vineyard', and therefore one may sow between the rows even at a distance of only six tefachim from the vine. Rabbi Yehudah brought proof from the incident in Salmon, where they bent the branches to one side and sowed grain between the rows, and the following year bent them to the other side - and the Sages permitted it. Rabbi Meir and Rabbi Shimon extended the permission even to gaps of eight amos, since such planting from the outset prevents the area from acquiring the status of a vineyard.