Terumot, Chapter 9, Mishnah 1. This chapter deals with the laws of planting Terumah. Wheat kernels - the typical foodstuff the Mishnah is discussing - can be used in two ways: they can be ground and eaten as flour and bread, or they can be returned to the ground, where they will grow into a new stalk from which a further crop will be harvested, yielding a far greater return on the investment.
Planting Terumah according to Torah law:
By Torah law, if a person plants kernels of Terumah in the ground, whatever grows from the ground is ordinary chullin and is not Terumah.
It is worth clarifying the term 'chullin' here, since it is used in two senses that can easily be confused with one another:
Chullin as opposed to Terumah - ordinary produce that is not sanctified and is not restricted to kohanim.
Chullin as opposed to tevel - produce from which all the gifts have already been separated (Terumah, maaser and the like), and which is therefore permitted to be eaten.
Accordingly, when we say that what grows from planted Terumah is chullin, the intention is the first sense: the crop is not restricted to kohanim. It is, however, still tevel, and maaseros must be separated from it.
The concern of the Sages:
As long as the Terumah in the kohen's possession is pure, he keeps it as wheat kernels in his kitchen, grinding and processing it as needed and eating it. But once the Terumah becomes tamei, the kohen can no longer eat it, and he has two options: to burn it, or to set it aside for planting.
If what grows from the planting is considered chullin, the kohen has a clear financial incentive: instead of buying fresh seed at planting season, he will keep the kernels of tamei Terumah on the shelf until planting time arrives and save a great deal of money. This is precisely what the Sages were concerned about: the kernels will sit in his house for months, and since they are fit to eat, he is liable to forget that this is tamei Terumah and eat it - a full Torah prohibition.
The Sages therefore removed the financial incentive entirely and ruled: if one plants Terumah, whatever grows from the ground remains Terumah. Given that, the kohen will not cultivate an entire field of Terumah, since it is not worth much - only a pure kohen may eat it, and nobody prefers Terumah over chullin. And consequently he will not go to the trouble of storing the seeds of tamei Terumah, but will destroy them instead.
This rabbinic law is among the best known of the eighteen decrees enacted by Beis Shammai at the time when they were the majority in the upper chamber, as related in the Gemara in the second chapter of Maseches Shabbos (daf 17).
It emerges, then, that what grows from planted Terumah is chullin by Torah law and Terumah by rabbinic law. This dual status gives rise to many complexities, since one must be stringent in both directions, and this is the subject of the opening mishnayos of the chapter.
The words of the Mishnah:
"HaZorea Terumah shogeg - yofoch" - one who plants seeds of Terumah unintentionally, having forgotten that they were Terumah, is permitted to work the ground and destroy the sprouts, so that he not be stuck with a field full of Terumah. This permission applies only until the wheat has reached a third of its ripening, since from that point on it is considered food and has the status of Terumah by rabbinic law, and it is forbidden to destroy Terumah.
"U'meizid - yekayem" - one who planted seeds of Terumah knowingly committed a transgression, and therefore we are stringent with him and he is not given permission to work the field. He is forced to grow the Terumah and to lose a year of yield in his field on produce that is not worth very much. The Bartenura explains: once he planted Terumah intentionally, the field looks like and is regarded as a field of Terumah, and working it looks like deliberately destroying Terumah, even before the crop reached a third of its ripening and became actual food. Therefore, even though under ordinary circumstances it would have been permitted to work such a field, here the Sages were stringent and did not permit it.
"Im higia le'shlish - bein shogeg bein meizid yekayem" - once the crop has reached a third of its ripening, there is no longer any difference between unintentional and intentional. What is growing is already considered food, the laws of Terumah apply to it, and it is forbidden to destroy Terumah. He is therefore obligated to maintain the rabbinic Terumah growing in his field, and he may no longer work it.
The exception - flax:
This framework applies to every type of crop, with one exception: "U'vapishtan, meizid - yofoch" - and with flax, one who acted intentionally must turn the soil over. Even though he planted the flax seeds deliberately, not only is he permitted to plow, he is obligated to plow and destroy the plants before they grow.
Flax is planted mainly for its stalks: the long fibers are extracted from them and made into cloth and textiles. Yet it also has an edible component - flaxseeds, which are eaten in our own day as well and are rich in omega 3, along with the flaxseed oil produced from them. Flaxseeds were used in the time of Chazal too, but this was only a secondary use.
When someone plants flaxseeds of Terumah, the plant that grows from them is considered Terumah by rabbinic law. Since the primary purpose of flax is its fibers, the Sages gave the entire plant the status of Terumah. Now a person might mistakenly think that only the seeds are Terumah, while the stalks are permitted without restriction. In order to prevent this confusion entirely, along with a mistaken attitude toward the rabbinic Terumah of the flax plants, it was established that he should quickly run a plow over them and not let them grow and ripen into food, and in this way the whole issue is bypassed from the outset.
In summary: for every type of crop the rule is that one who planted Terumah unintentionally is permitted to plow and destroy the plant, provided that the crop has not yet reached a third of its ripening and become considered food. But with one who planted intentionally we are stringent and do not allow him to plow, and what grows is rabbinic Terumah which he must treat according to the laws of Terumah. Once the crop has reached a third, whether unintentionally or intentionally, he must maintain it. Flax is the exception to this rule, and with it even one who acted intentionally is obligated to turn the field over.