Tosefta do Maaserot 1:17

The "granary" for tithes [of produce] (i.e., the point at which produce must be tithed) rendering one liable on account of the laws of tevel (i.e., the prohibited consumption of or benefit from untithed produce), is once their work is completed -- [meaning] the work of being brought in [from the field]. (See Bava Metzia 88b:6).)

Siah (a hyssop-like plant) and Syrian oregano and thyme [are liable] from when they blossom, and all the red herbs, from when they blossom. If it was the second [year of the agricultural cycle] and the third began [as they were blossoming], behold, they are [considered to] belong to the third. [If] it was the eve of shevi'it (the seventh year of the agricultural cycle, when fields lie fallow), and [the blossoms formed when] the shevi'it year started, behold, they [are considered to] belong to the shevi'it year. Rabban Shimon ben Gamaliel says, Beit Shammai and Beit Hillel did not disagree about the [produce that already became] ripe [for purposes of tithing before the onset of the new year], that it [belongs] to the previous [year even if it was not picked by the time the new year started], or about that which has not yet blossomed, that it [belongs] to the coming [year]. On what did they disagree? On fruit that blossomed in the previous year and ripened in this year -- per Beit Shammai, we go according to when it blossomed, and per Beit Hillel, we go according to when it became ripe (following the girsah of Minchat Bikkurim).

They separate Terumah from gourds and pumpkins, even though the watermelons and cucumbers did not yet have their fuzz removed [in hot water, following MSS Erfurt (see Maasr. 1:5)]. Even though he did not remove the fuzz from the cucumbers that he intends to remove from the kettle, he is not liable [for tithes] until he [does in fact] remove them from the kettle.

They separate Terumah from gourds and pumpkins, even though the watermelons and cucumbers did not yet have their fuzz removed [in hot water, following MSS Erfurt (see Maasr. 1:5)]. Even though he did not remove the fuzz from the cucumbers that he intends to remove from the kettle, he is not liable [for tithes] until he [does in fact] remove them from the kettle.