Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.035.

Section : When Zakah is Due on Plants and Fruits.

Zakah is due on plants when the grains mature and are ready to be rubbed off and on the fruit when it is ripened. In the case of dates, for example, the indication will be their brightness or red colour, and with grapes their sweetness. Zakah becomes due only after grains are husked or the fruit becomes dried. If the farmer sold his grain after it had matured, and the fruit after it had ripened, then its zakah will be paid by him and not the buyer. This is because the obligation to pay zakah became due when the produce was still in the owner’s possession.