Related על תרומות 1:5

How is this done [i.e, what rules apply to the taking of terumah from the same species]? We do not take terumah from the new on behalf of the old (Ter. 1:5), we do not take terumah from this year's produce on behalf of last year's produce (see Ter. 6:6), but in a field that grows two crops (i.e., early and late fruits) in [the same] year, and so too an irrigated field, we [may] take terumah and tithes from one on behalf of the other. [With respect to] vegetables picked on the evening preceding Rosh Hashanah [but which remain in the field], until the sun rises, he may return [and take terumah and tithes from] those picked [vegetables], but once the sun rises, we do not take terumah and tithes from one on behalf of the other because [once the sun rises on Rosh Hashanah, vegetables collected thereafter] are "new " and [vegetables collected previously] are "old" (i.e., from the previous year). [Note that Rosh Hashanah is the new year for tithing vegetables, see Rosh Hashanah 1:1.]

How [else] is this done? We do not take terumah from detached [produce] on behalf of [produce that is still] attached [to the ground] (Ter. 1:5). [If] he declared, "This detached produce shall be [considered] terumah and tithes on behalf of this attached [produce]," or "This attached produce shall be [considered] terumah and tithes on behalf of this detached produce," [it is as though] he has not said anything. But [if he declared], "This detached produce shall be [considered] terumah and tithes on behalf of this attached produce once it becomes detached," and behold, he ate from it by accident, he may take terumah and tithes from another place, until the time that it detaches [and he would not incur liability for consuming terumah or tithes]. Once it detaches, his words become permanent [and he would be liable]