One may not separate from detached produce on behalf of produce that is still attached to the ground, nor from attached produce on behalf of detached produce, because they are not at the same stage of obligation: detached produce is already subject to terumot and ma’asrot, while attached produce is not (Mishna, Terumot 1:5).
When necessary, however, one may separate terumot and ma’asrot from detached produce on behalf of attached produce, provided that he stipulates that the terumot and ma’asrot will take effect only after the attached produce is picked. This is permitted only if the attached produce is already fit to eat, at least in pressing circumstances, as explained in the note.[3]
[3]. One should recite the entire nusach while the produce is still attached to the ground, specifying exactly which part is to be the terumot and which part is to be the ma’asrot. He should then state that they will take effect only when all the produce is picked. Since the produce can already be picked and eaten, it is not considered a davar she-lo ba la-olam – something not yet halakhically present – and one may therefore separate terumot and ma’asrot from it on condition that the separation take effect when it is picked (Kiddushin 62a–b; MT, Laws of Terumot 5:9).
This is permitted only if the detached produce set aside for terumot and ma’asrot remains fit to eat until the attached produce for which it was separated is picked, and only if there is enough of it to constitute the required amount of terumot and ma’asrot for the produce that will ultimately be picked. Each time he picks produce, he should say that the terumot and ma’asrot are to take effect in accordance with what he specified previously (Mishpat Kohen §§ 35, 149). R. Akiva Eger adds that, based on the same principle, one may also separate terumot and ma’asrot from attached produce on behalf of attached produce, with the understanding that the separation will take effect after the produce is picked (Gilyon Ha-shas to Kiddushin 62a).
Ḥazon Ish, Demai, end of § 17, adds a qualification: this method works only if the attached produce does not continue growing afterward. If it does continue to grow, one must separate terumot and ma’asrot again for the additional growth. This position, however, is very difficult to accept. Not only would it leave the entire method with almost no practical use, but the Gemara itself allows this arrangement once the produce has reached one-third of its growth – that is, once it is fit to eat in pressing circumstances, but before it is fully ripe. This implies that the produce may continue growing until it reaches full maturity, and that the produce separated at the outset is sufficient to exempt it from terumot and ma’asrot. For this reason, the amount initially separated must be sufficient for the quantity of produce as it will be at the time of picking. This is also the position of Rav Kook (Mishpat Kohen §§ 35, 149), R. Tzvi Pesaḥ Frank (Har Tzvi, Zera’im 1:75:2), and R. Shlomo Zalman Auerbach (Minḥat Shlomo 1:55).
