Demai, Chapter 5, Mishnah 10. This Mishnah deals with separating Terumah from two different types of pots - not flower pots, but pots in which vegetables are planted. The Mishnah distinguishes between two kinds of earthenware pots:
A perforated pot: a pot that has a hole in it, through which the soil inside it is connected to the soil of the ground.
An unperforated pot: a pot that has no hole, so that the soil inside it remains detached from the ground.
The halachah establishes that whatever grows in a perforated pot is considered attached to the ground and is obligated in tithes by Torah law, whereas whatever grows in a pot that is not attached to the ground is not obligated in tithes by Torah law.
The words of the Mishnah:
"Atzitz nakuv harei hu ka'aretz" - a perforated pot has the status of ground itself, and therefore what grows in it is obligated in tithes by Torah law. "Torem min ha'aretz al atzitz nakuv, ume'atzitz nakuv al ha'aretz - terumaso terumah": whether he separated Terumah from produce that grew in ordinary ground on behalf of produce that grew in a perforated pot, or from what grew in a perforated pot on behalf of what grew in the ground, the Terumah takes effect.
Although the Mishnah uses language that suggests only after the fact - "terumaso terumah" - in practice one may do this even at the outset, since both types of produce, whether they grew in a pot or in the ground, are obligated by Torah law. This wording was chosen only because of what follows in the latter part of the Mishnah.
From the exempt on behalf of the obligated:
"Mishe'eino nakuv al hanakuv - terumah, veyachzor veyitrom" - when he separated Terumah from what grew in an unperforated pot, which is not obligated by Torah law, on behalf of what grew in a perforated pot, which is obligated, he has separated from the exempt on behalf of the obligated. Since he designated that produce as Terumah, it has the status of Terumah; but he must go back and separate Terumah from what grew in the perforated pot, which is what is obligated by Torah law.
From the obligated on behalf of the exempt:
"Min hanakuv al she'eino nakuv - terumah, velo tei'achel ad sheyotzi aleha terumos uma'asros" - in the reverse case, where he separated from what grew in a perforated pot, which is obligated by Torah law, on behalf of what grew in an unperforated pot, which is exempt by Torah law, this too has the status of Terumah.
However, the kohen may not eat it until he separates Terumah and tithes from it. The reason: since he called it Terumah, he is obligated to give it to a kohen, but it is not genuine Terumah by Torah law, because it was separated on behalf of produce that is not obligated in Terumah at all, and therefore the separation is not valid. The result is that this very portion remains tevel by Torah law, and Terumah and tithes have not yet been separated from it. He must therefore give it to a kohen, and the kohen may not eat it until Terumah and tithes are separated from that very item that was called Terumah.
In summary: in this Mishnah we learned that a perforated pot has the status of ground, and what grows in it is obligated in tithes by Torah law, whereas what grows in an unperforated pot is exempt by Torah law. Separating from the ground on behalf of a perforated pot and the reverse is valid even at the outset. Separating from the exempt on behalf of the obligated: the Terumah takes effect, but he must go back and separate from the obligated produce. Separating from the obligated on behalf of the exempt: the Terumah takes effect and is given to a kohen, but the kohen may not eat it until Terumah and tithes are separated from it.