Tosefta על ראש השנה 1:1

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.]

[If] it was the second [year of the agricultural cycle] and the third [year's crops] came in, the earlier [harvest] is tithed as second tithe, and the later [harvest] is tithed as poor man's tithe. [If] he picked an etrog in the evening before Tu B'shvat (the 15th of Shevat, the new year for trees, see Rosh Hashanah 1:1), until the sun rises, he may return [and take terumah and tithes from] those picked [etrogs], 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 Tu B'shvat, fruit collected thereafter] are "new " and [fruit collected previously] are "old" (i.e., from the previous year). If it was the third year and the fourth year's crop came in, the earlier [harvest] is tithed as poor man's tithe, and the fourth year's crop is tithed as second tithe.