Kilayim, chapter 7, mishnah 6. The mishnah deals with a "gazlan shezara es hakerem" - a person who seized by force a vineyard that was not his, sowed seeds in it (not vine seeds, but other seeds that create kilei hakerem), and afterward left the field and abandoned what he had stolen.
The state of the field when the owner returns:
When the original owner comes back to his field, nothing that took place until that point creates a prohibition of kilei hakerem, since the robber cannot render another person's field forbidden, as we saw in the previous mishnayos. But from the moment he returns, if the owner maintains the kilayim growing in the field, the mixture becomes forbidden on his account, and therefore he is obligated to cut down and remove the foreign growth as soon as possible.
The mishnah therefore says that he cuts down the seeds even on Chol HaMoed. Ordinarily such work is forbidden on Chol HaMoed, but here it was permitted because of the prohibition of kilayim - and because the aspect of kilayim is a matter of how things appear to the eye, and so we want the forbidden appearance removed as quickly as possible.
"Ad shelish":
The owner is even obligated to hire workers at a wage higher than the going rate. Up to what amount must he add to the standard wage? The mishnah says: "ad shelish" - up to a third. In the Talmud Yerushalmi the Amoraim disagree about how to measure this:
Up to an addition of a third above the standard wage customarily paid for this type of work.
Up to a third of the value of the produce, that is, of the crop being harvested itself.
If the cost of hiring workers goes beyond this third, the owner may cut the growth himself and take as long as he needs, and he is not required to hurry - and the mishnah says that he cuts it in his usual manner even after Chol HaMoed.
From when is the field considered the robber's?
The mishnah closes with this question. We said at the outset that the robber cannot create the prohibition of kilei hakerem, because the field is not his. But at what stage does the field become his, to the point that the sowing is considered kilei hakerem and becomes forbidden? The mishnah says: "mishenishtakach" - once the name of the original owner has been forgotten, and people view the field as the robber's field and relate to it as his. At that stage the field becomes his, and even though he acquired it unlawfully, it is considered his with regard to the ability to create kilei hakerem and render the produce forbidden.
In summary: in the case of a robber who sowed kilayim in a vineyard - as long as the field is not his, his actions do not render anything forbidden, but once the owner returns he is obligated to cut the growth down immediately, even on Chol HaMoed, and to add to the workers' wages up to a third (whether a third of the wage or a third of the value of the produce); and if the cost is higher than that, he cuts it in his usual manner even after the festival. And once the owner's name has been forgotten and the field is called by the robber's name, the field becomes his for this purpose, and he himself can render it forbidden through kilei hakerem.