Sheviis, Chapter Two, Mishnah 9. Our mishnah deals with two types of produce - onions that do not produce seed ("betzalim hasrisim") and "pul haMitzri" (the Egyptian bean) - and with the change in their status when water was withheld from them before Rosh Hashanah.
The background: Egyptian bean grown for its greens:
We are speaking here about the Egyptian bean grown for its greens, and not for its seed. In the previous mishnah we learned that a bean grown for its seed is determined by the time it takes root; but our mishnah deals with one grown for its greens, whose ordinary law is to be determined by the time it is harvested. That is the rule we would normally apply to them.
Withholding water thirty days before Rosh Hashanah:
Our mishnah, however, speaks of one who withheld irrigation from them for thirty days before Rosh Hashanah, and this withholding changes their status. Ordinarily, since they are destined to be picked after Rosh Hashanah, they would belong to the coming year; but because water was withheld from them for thirty days, the maaser taken from them belongs to the previous year - "mit'asrin l'she'avar" - if this is a year that is determined by the previous year. And if the previous year was the sixth year, they do not belong to the year that follows, in which they were picked. Withholding the water gives them the status of produce grown in a field that relies on rainwater, and therefore they belong to the previous year.
But if he does not withhold water from them for thirty days, they will be "asurin b'shevi'is" - they will be considered shemittah produce, since they retain their ordinary status as vegetables that are determined by the time of their picking, which falls in the coming year, and their law is set according to whatever that year happens to be.
A rain-fed field from which water was withheld:
The mishnah continues: grain from a field that ordinarily relies on rainwater, except that because of the types of vegetables growing in it he would irrigate it, and he withheld water from it for two seasons. These seasons are far longer than thirty days, for we are speaking of a field based on rainwater, and even when he did irrigate it, he did so only on rare occasions. Here too the law is the same: the produce retains the status of a rain-fed field, and it is not determined by the coming year but by the previous year.
The words of Rabbi Meir: withholding water for two seasons is enough for the produce to remain in the status of rain-fed and be determined by the previous year.
"V'chachamim omrim: shalosh" - the water must be withheld for three of those periods of time, a far longer span, in order to ensure that it has the status of the previous year rather than that of the coming year of picking, by which vegetables are ordinarily determined.
In summary: in this mishnah we learned that seedless onions and the Egyptian bean grown for its greens, whose ordinary law is to be determined by the time of their picking, change their status when water was withheld from them for thirty days before Rosh Hashanah - and they are tithed according to the previous year; and if water was not withheld from them, they remain in their status as vegetables and are forbidden as shemittah produce. We also learned the law of a rain-fed field from which water was withheld: according to Rabbi Meir two seasons suffice, and according to the Sages three are required.