Kilayim, Chapter Four, Mishnah Five
In todays mishnah Bet Hillel and Bet Shammai dispute how many rows of vines are necessary for the vines to be considered a vineyard and not simply a row of vines.
1) One who has planted a row of [at least] five vines:
a) Bet Shammai says: these constitute a vineyard.
b) But Bet Hillel says: they do not constitute a vineyard unless there are two rows.
2) Consequently, one who says four cubits within the vineyard:
a) Beth Shammai says: he has sanctified one row [of the vineyard, and created kilayim].
b) But Bet Hillel says: he has sanctified two rows.
Section one: According to Bet Shammai, a row of five vines counts as a vineyard. Hence, one who wants to plant seeds near such a row of vines must leave a gap of four cubits in order to tend to the vines and then he may plant. In contrast, Bet Hillel says that there must be two rows in order for vines to count as a vineyard. The Tosefta explains that each row must have at least three vines.
Section two: In this section we learn that the previous definition can cause Bet Shammai to be more lenient than Bet Hillel. The mishnah deals with a person who planted seeds within four cubits of a vineyard, meaning within the space needed to work the vineyard. According to Bet Shammai, this causes the first row of the vineyard to become kilayim. To signify that something has become kilayim the mishnah uses the word sanctify (kidesh), which is the word used in Deuteronomy 22:9. Sometimes when things become sanctified they can no longer be usedit is prohibited to derive benefit from them. Since a vineyard can consist of only one row, when the Torah states that the vineyard becomes sanctified, Bet Shammai interprets this to mean that the minimum area which can be considered a vineyard becomes sanctified.
Since Bet Hillel holds that two rows are necessary for vines to be considered a vineyard they correspondingly hold in this situation that two rows of the vineyard have been sanctified and are forbidden from use.