ציטירנדיקער פּירוש אויף במדבר 18:27

As it is its way to grow by [being watered by] all water - because it is watered by drawn water like a vegetable, which is the reason for [the rule for] vegetables [too], as is stated in Rosh Hashanah: [the terms] "threshing floor" and "winepress" are used regarding tithes, [as in] "as with the new grain from the threshing floor", etc. [Numbers 18:27]. "Just as [the grain that is brought to] the threshing floor and [the wine that is brought to] the winepress are special in that they grow on most water" -- that is, rainwater -- "and they are tithed in accordance with the outgoing year" [Rosh Hashanah 14a:7]. As we have stated in Rosh Hashanah, we reckon [the produce's year for the purpose of its tithes] according to the year in which it reach one-third of its growth [Rosh Hashanah 12b:12], and not according to when it was picked. We derive this from scripture: "so too, anything that grows on most water" -- namely, trees -- "is tithed in accordance with the outgoing year." (Even though their tithes are Rabbinic in origin, we ascribe this rule to scriptural verses.) "This comes to exclude vegetables, which grow on all water. Consequently, they are tithed in accordance with the incoming year." [Rosh Hashanah 14a:7]

ובה"א אין תורמין (see Tractate Terumot, Chapter 1, Mishnah 4) - whomever has [olive] oil that he is obligated to separate from it the priest’s due and olives that is obligated to separate from it priest’s due, he is not allowed to separate the priestly gift from the olives according to the measure that he has to separate the priest’s gift from the olives and from the oil, and to exempt the oil through the priest’s gifts of the olives, for it is written (Numbers 18:27): “as with new grain from the threshing floor [or the flow from the vat],” from that which is completed on that which is completed, and not from what is not completed on that which is completed.

ומעשר ראשון שלא נטלה תרומתו – and even he is [punished] with death, as it is written (Numbers 18:27): “This shall be accounted to you as your gift. As with the new grain from the threshing floor [or the flow from the vat].”