Shevi'is, Chapter 2, Mishnah 7. The question of which year a crop grew in is not a theoretical one; it carries many halachic consequences. If the produce belongs to the Shemittah year, it is subject to the laws of Shevi'is; and if it belongs to another year, we must determine whether the laws of Maaser Sheni or the laws of Maaser Ani apply to it. Furthermore, one may not separate maaser from produce that grew in one year on behalf of produce that grew in another year.
What determines the year of a fruit?
The determination is not necessarily made according to growth alone. With vegetables, the year is fixed by the picking - the year in which the vegetable was harvested - and the New Year for this purpose, the point at which we decide which year we are in, is the first of Tishrei. There are, however, crops that are not all harvested together at one time, and so the Sages established a different criterion for them, based on taking root - the year in which they took root. These are the crops our Mishnah discusses.
The words of the Mishnah:
"Ha'orez, veha'dochan, veha'pragin, veha'shumshmin, she'hishrishu lifnei Rosh Hashanah - mis'asrin l'she'avar, u'mutarin ba'shevi'is" - for these crops, the year is fixed according to when and where they took root:
"Mis'asrin l'she'avar" - the maaser relates to the previous year, the one before Rosh Hashanah, in which they took root. If that was a year of Maaser Sheni or a year of Maaser Ani, one separates from them in accordance with the law of that year.
"U'mutarin ba'shevi'is" - if they took root in the sixth year, before Shemittah, they are permitted even during the Shemittah year, because the year to which they belong is fixed by the taking of root, which occurred before Rosh Hashanah.
"V'im lo - asurin ba'shevi'is, u'mis'asrin la'shanah haba'ah" - if they took root only after Rosh Hashanah:
Regarding Shevi'is - if the coming year is the Shemittah year, they are forbidden during it.
Regarding maasros - the maaser is fixed according to the coming year, since the taking of root occurred after the eve of Tishrei, after Rosh Hashanah.
In summary: in this Mishnah we learned that while vegetables are fixed by the year of picking, with their New Year on the first of Tishrei, rice, millet, poppy and sesame - which are not all harvested at once - have their year fixed by the taking of root: if they took root before Rosh Hashanah, they are tithed for the previous year and are permitted during Shevi'is; if they took root after Rosh Hashanah, they are forbidden during Shevi'is and are tithed for the coming year.