Parshas Kedoshim mentions the prohibition against orlah, which forbids benefitting from the fruit of a tree for the first three years after it is planted. This prohibition applies even if such a tree becomes confused with ordinary trees. Thus the Mishnah states:
When a plant which is orlah... becomes mixed with other plants, one should not harvest [any of the fruit]. If one did harvest, the prohibited [fruit] becomes batul (nullified) when it is mixed with a permitted substance 201 times greater.
The Mishnah teaches that when a plant which is orlah becomes mixed with other plants, one should not harvest any of the fruit, for we follow the principle: “At the outset, one should not nullify a forbidden substance.”
But when the forbidden plant is mixed with 201 permitted plants (for unlike other substances prohibited by the Torah, which become nullified when mixed with 60 times more of a permitted substance, orlah requires 201 times its size) and the fruit has already been harvested, it can be eaten.
The implication is that orlah cannot become nullified until after it is harvested. While the fruit is on the tree, neither it nor the tree itself can be nullified, reflecting the principle: Whenever a plant is mechubar, connected to its source, it cannot become nullified.
