Bava Metzia, Chapter 9, Mishnah 4. Here too, we are discussing a person who undertook to cultivate a field. In the current case, it is a fixed tenancy (chachirus) - a tenant who agreed in advance to pay the field owner ten kor of wheat. After the harvest is finished, this tenant refuses to go and uproot the weeds that grew in the field where he harvested the wheat.
The tenant's claim:
"I agreed to pay you ten kor of wheat in any case, so what do you care whether I take care of the weeds or not? I said I would give you ten kor - and I am giving it. And if there are weeds causing damage to future crops, that is my fault and my loss, not yours."
The Mishnah's answer:
The Mishnah rules that beis din rejects this claim and it is not accepted, since the field owner can reply to the tenant: "Tomorrow you might get up and leave, and I will be left to deal with the weeds - and this is not fair." Since the tenant is expected to do this, as it is the logical thing to do, he is obligated to do so.
The scenario of the Mishnah - in a place with no clear custom:
The very fact that this is defined based on what is "logical" teaches us that we are dealing with a situation where the custom is not clearly defined, meaning there is no clear custom regarding whether the tenant is obligated to weed or not. In such a case, the Mishnah rules that he is obligated to weed, for otherwise long-term damage will be caused. But in the other two scenarios, there is no need for the Mishnah's ruling:
A clear custom to weed: If there was a clear precedent, and the custom was explicitly to weed, the Mishnah would not have needed to say anything, since the law is simple and self-evident.
A clear custom not to weed: If, for whatever reason, the custom is not to weed, but rather to wait for the next plowing season, when the field is plowed and the weeds are turned over and uprooted at that stage - the field owner cannot force the tenant to weed now, at this early stage.
In conclusion: Since there is no clear custom on the matter, the owner has the right to demand proper work in the field and to have it weeded after the harvest is completed.