Reference for Maasrot 1:1
כְּלָל אָמְרוּ בַּמַּעַשְׂרוֹת, כָּל שֶׁהוּא אֹכֶל, וְנִשְׁמָר, וְגִדּוּלָיו מִן הָאָרֶץ, חַיָּב בַּמַּעַשְׂרוֹת. וְעוֹד כְּלָל אַחֵר אָמְרוּ, כָּל שֶׁתְּחִלָּתוֹ אֹכֶל וְסוֹפוֹ אֹכֶל, אַף עַל פִּי שֶׁהוּא שׁוֹמְרוֹ לְהוֹסִיף אֹכֶל, חַיָּב קָטָן וְגָדוֹל. וְכָל שֶׁאֵין תְּחִלָּתוֹ אֹכֶל אֲבָל סוֹפוֹ אֹכֶל, אֵינוֹ חַיָּב עַד שֶׁיֵּעָשֶׂה אֹכֶל:
They stated a rule with regard to tithes: Anything which is food, and is guarded, and grows from the earth, requires tithes. And they stated another rule: Anything that is in its beginning is food, and in its end is food, even if one guards it [and does not pick it when it is small] in order to add food, requires tithes whether it is small or large. And anything that is not in its beginning food, but is in its end is food, does not require tithes until it becomes food.