Peah, Chapter 2, Mishnah 5. This Mishnah teaches us further criteria for when a field is divided into two separate obligations of peah.
One Who Sows His Field with a Single Species:
The Mishnah opens: "HaZorea es sadehu min echad, af al pi shehu osehu shtei goranos - nosen peah achas". What is a goren? A threshing floor, to which the grain is brought after harvesting, and therefore it serves here as a term for a single harvesting. In other words: one who sows a single species of grain in his field, even if he harvested it at two separate times, gives only one peah, since it is a single species.
One Who Sows Two Species:
"V'im zara shnei minim, af al pi shehu asa'an goren achas - nosen shtei peos" - if he sowed two different species in his field, even if he harvested them all together as one goren, he gives two separate peos, because they are two species.
One Who Sows Two Kinds of Wheat:
What is the law when one sows two kinds of wheat in his field? It is all wheat, only that these are two different varieties. Here the goren is what determines the matter:
"Asa'an goren achas - nosen peah achas" - if he harvested everything at once, the two kinds of wheat are considered a single species, and one peah suffices.
"Shtei goranos - nosen shtei peos" - if he harvested at two separate times, they are considered two species, and he gives two peos.
In summary: with a single species, the species determines the matter and not the goren, and therefore even two goranos do not divide it into two obligations; with two species, the species determines the matter, and therefore even a single goren does not unite them into one obligation; and with two kinds of wheat, the goren determines the matter - one goren unites them into a single peah, and two goranos divide them into two peos.