Kilayim, Chapter Two, Mishnah Five
This mishnah deals with a field in which other species grew on their own and whether or not this is a case of kilayim.
1) His field was sown cumin or with arum, he must not sow on top of them, since they produce crops only after three years.
2) [A field of] grain among which sprang up some aftergrowth woad, alternatively an area of threshing-floors in which many species sprang up, alternatively [a field of] clover among which grew up a number of species of herbs, he is not obliged to weed them out.
3) But if he weeded or cut down the weeds, they say to him: Uproot it all, except for one species.
Section one: Cumin and arum (a type of onion) only sprout up after three years. Therefore if he has already sown his field with one of these types of seed, he shouldnt plant something else there because eventually the cumin or arum will sprout up.
Section two: This section mentions a situation in which weeds have sprouted up among an area where he planted with a certain species or on a threshing floor, where they thresh grain. He is not obligated to uproot the weeds because he didnt want them to be there. We learn here that these are not treated like cases of kilayim because he didnt plant two types of seeds together, nor was he even desirous of their coexistence.
Section three: If he weeded or cut down some of the weeds, the court now must force him to uproot the rest of the weeds because by getting rid of some of them, he has revealed that he wishes to keep there the ones that he didnt remove. Since he wants to keep them, they cause a case of kilayim in the field.