Tu BiShvat

מתני׳ ארבעה ראשי שנים הם באחד בניסן ר"ה למלכים ולרגלים באחד באלול ראש השנה למעשר בהמה ר' אלעזר ור"ש אומרים באחד בתשרי באחד בתשרי ראש השנה לשנים ולשמיטין וליובלות לנטיעה ולירקות באחד בשבט ראש השנה לאילן כדברי בית שמאי בית הלל אומרים בחמשה עשר בו: גמ׳ ...

Rosh Hashanah 2a:1-5

MISHNA: They are four days in the year that serve as the New Year, each for a different purpose: On the first of Nisan is the New Year for kings; it is from this date that the years of a king’s rule are counted. And the first of Nisan is also the New Year for the order of the Festivals, as it determines which is considered the first Festival of the year and which the last. On the first of Elul is the New Year for animal tithes; all the animals born prior to that date belong to the previous tithe year and are tithed as a single unit, whereas those born after that date belong to the next tithe year. Rabbi Elazar and Rabbi Shimon say: The New Year for animal tithes is on the first of Tishrei. On the first of Tishrei is the New Year for counting years, as will be explained in the Gemara; for calculating Sabbatical Years and Jubilee Years, i.e., from the first of Tishrei there is a biblical prohibition to work the land during these years; for planting, for determining the years of orla, the three-year period from when a tree has been planted during which time its fruit is forbidden; and for tithing vegetables, as vegetables picked prior to that date cannot be tithed together with vegetables picked after that date. On the first of Shevat is the New Year for the tree; the fruit of a tree that was formed prior to that date belong to the previous tithe year and cannot be tithed together with fruit that was formed after that date; this ruling is in accordance with the statement of Beit Shammai. But Beit Hillel say: The New Year for trees is on the fifteenth of Shevat. GEMARA: ...

ראש השנה ט׳ ב:י״ג

ת"ר אחד הנוטע אחד המבריך ואחד המרכיב ערב שביעית שלשים יום לפני ראש השנה עלתה לו שנה ומותר לקיימן בשביעית פחות משלשים יום לפני ראש השנה לא עלתה לו שנה ואסור לקיימן בשביעית

Rosh Hashanah 9b:13

The Sages taught in a baraita: If one plants a tree, or layers a vine shoot into the ground so that it may take root, or grafts a branch onto a tree on the eve of the Sabbatical Year thirty days before Rosh HaShana, as soon as Rosh HaShana arrives, a year is counted for him. The thirty days count as a full year with regard to the prohibition of orla, and it is permitted to preserve the plant during the Sabbatical Year, as this is not considered new growth. However, if one performed these actions less than thirty days before Rosh HaShana, then when Rosh HaShana arrives, a year is not counted for him for orla, and it is prohibited to preserve the new growth during the Sabbatical Year.

ראש השנה י׳ א:א׳

ופירות נטיעה זו אסורין עד ט"ו בשבט אם לערלה ערלה ואם לרבעי רבעי

Rosh Hashanah 10a:1

And if the planting, layering, or grafting took place more than thirty days before Rosh HaShana, the fruit of this planting is prohibited until the fifteenth of Shevatof the fourth year since the tree’s planting, even though the three years were already completed the previous Rosh HaShana. This principle applies both for orla during the year of orla, when it is prohibited to eat the fruit, and for fourth-year produce during the year of fourth-year produce, which must be eaten in Jerusalem or redeemed before it is eaten outside Jerusalem.

ראש השנה י׳ א:ג׳

פעמים שברביעית ועדיין אסורה משום ערלה ופעמי' שבחמישית ועדיין אסורה משום רבעי

Rosh Hashanah 10a:3

The baraita explains: The repetition of the word “and,” indicated by the conjunctive vav that joins these verses, teaches that there are times that the tree is already in its fourth year and yet the fruit is forbidden as orla, from the verses “three years…and in the fourth year”; and there are times that the tree is already in its fifth year and yet the fruit is forbidden as fourth-year produce, from the verses “in the fourth year…and in the fifth year.”

ויקרא י״ט:כ״ג-כ״ה

(כג) וְכִי־תָבֹ֣אוּ אֶל־הָאָ֗רֶץ וּנְטַעְתֶּם֙ כָּל־עֵ֣ץ מַאֲכָ֔ל וַעֲרַלְתֶּ֥ם עָרְלָת֖וֹ אֶת־פִּרְי֑וֹ שָׁלֹ֣שׁ שָׁנִ֗ים יִהְיֶ֥ה לָכֶ֛ם עֲרֵלִ֖ים לֹ֥א יֵאָכֵֽל׃ (כד) וּבַשָּׁנָה֙ הָרְבִיעִ֔ת יִהְיֶ֖ה כָּל־פִּרְי֑וֹ קֹ֥דֶשׁ הִלּוּלִ֖ים לַיהוָֽה׃ (כה) וּבַשָּׁנָ֣ה הַחֲמִישִׁ֗ת תֹּֽאכְלוּ֙ אֶת־פִּרְי֔וֹ לְהוֹסִ֥יף לָכֶ֖ם תְּבוּאָת֑וֹ אֲנִ֖י יְהוָ֥ה אֱלֹהֵיכֶֽם׃

Leviticus 19:23-25

(23) When you enter the land and plant any tree for food, you shall regard its fruit as forbidden. Three years it shall be forbidden for you, not to be eaten. (24) In the fourth year all its fruit shall be set aside for jubilation before the LORD; (25) and only in the fifth year may you use its fruit—that its yield to you may be increased: I the LORD am your God.

[Tuesday] night is Tu Bishvat, New Year for fruit trees. A fruit which has already begun its formation on [Tuesday] belongs to [5777; i.e., last year]; one which is formed only at night will belong to [5778; i.e., this year]. The particular year a fruit belongs to is relevant for various agricultural laws. For tithes, since it is forbidden to tithe from the fruits of one year on those of another, and also because the second tithe varies between maaser sheni and maaser ani depending on the year.

https://www.ou.org/torah/mitzvot/meaning-in-mitzvot/orla/

תענית כ״ג א:ט״ו-י״ז

יומא חד הוה אזל באורחא חזייה לההוא גברא דהוה נטע חרובא אמר ליה האי עד כמה שנין טעין אמר ליה עד שבעין שנין אמר ליה פשיטא לך דחיית שבעין שנין אמר ליה האי [גברא] עלמא בחרובא אשכחתיה כי היכי דשתלי לי אבהתי שתלי נמי לבראי יתיב קא כריך ריפתא אתא ליה שינתא נים אהדרא ליה משוניתא איכסי מעינא ונים שבעין שנין כי קם חזייה לההוא גברא דהוה קא מלקט מינייהו אמר ליה את הוא דשתלתיה א"ל בר בריה אנא אמר ליה שמע מינה דניימי שבעין שנין חזא לחמריה דאתיילידא ליה רמכי רמכי אזל לביתיה אמר להו בריה דחוני המעגל מי קיים אמרו ליה בריה ליתא בר בריה איתא אמר להו אנא חוני המעגל לא הימנוהו אזל לבית המדרש שמעינהו לרבנן דקאמרי נהירן שמעתתין כבשני חוני המעגל דכי הוי עייל לבית מדרשא כל קושיא דהוו להו לרבנן הוה מפרק להו אמר להו אנא ניהו לא הימנוהו ולא עבדי ליה יקרא כדמבעי ליה חלש דעתיה בעי רחמי ומית אמר רבא היינו דאמרי אינשי או חברותא או מיתותא

Taanit 23a:15-17

One day, he was walking along the road when he saw a certain man planting a carob tree. Ḥoni said to him: This tree, after how many years will it bear fruit? The man said to him: It will not produce fruit until seventy years have passed. Ḥoni said to him: Is it obvious to you that you will live seventy years, that you expect to benefit from this tree? He said to him: That man himself found a world full of carob trees. Just as my ancestors planted for me, I too am planting for my descendants. Ḥoni sat and ate bread. Sleep overcame him and he slept. A cliff formed around him, and he disappeared from sight and slept for seventy years. When he awoke, he saw a certain man gathering carobs from that tree. Ḥoni said to him: Are you the one who planted this tree? The man said to him: I am his son’s son. Ḥoni said to him: I can learn from this that I have slept for seventy years, and indeed he saw that his donkey had sired several herds during those many years. Ḥoni went home and said to the members of the household: Is the son of Ḥoni HaMe’aggel alive? They said to him: His son is no longer with us, but his son’s son is alive. He said to them: I am Ḥoni HaMe’aggel. They did not believe him. He went to the study hall, where he heard the Sages say about one scholar: His halakhot are as enlightening and as clear as in the years of Ḥoni HaMe’aggel, for when Ḥoni HaMe’aggel would enter the study hall he would resolve for the Sages any difficulty they had. Ḥoni said to them: I am he, but they did not believe him and did not pay him proper respect. Ḥoni became very upset, prayed for mercy, and died. Rava said: This explains the folk saying that people say: Either friendship or death, as one who has no friends is better off dead.

דברים כ׳:י״ט-כ׳

(יט) כִּֽי־תָצ֣וּר אֶל־עִיר֩ יָמִ֨ים רַבִּ֜ים לְֽהִלָּחֵ֧ם עָלֶ֣יהָ לְתָפְשָׂ֗הּ לֹֽא־תַשְׁחִ֤ית אֶת־עֵצָהּ֙ לִנְדֹּ֤חַ עָלָיו֙ גַּרְזֶ֔ן כִּ֚י מִמֶּ֣נּוּ תֹאכֵ֔ל וְאֹת֖וֹ לֹ֣א תִכְרֹ֑ת כִּ֤י הָֽאָדָם֙ עֵ֣ץ הַשָּׂדֶ֔ה לָבֹ֥א מִפָּנֶ֖יךָ בַּמָּצֽוֹר׃ (כ) רַ֞ק עֵ֣ץ אֲשֶׁר־תֵּדַ֗ע כִּֽי־לֹא־עֵ֤ץ מַאֲכָל֙ ה֔וּא אֹת֥וֹ תַשְׁחִ֖ית וְכָרָ֑תָּ וּבָנִ֣יתָ מָצ֗וֹר עַל־הָעִיר֙ אֲשֶׁר־הִ֨וא עֹשָׂ֧ה עִמְּךָ֛ מִלְחָמָ֖ה עַ֥ד רִדְתָּֽהּ׃ (פ)

Deuteronomy 20:19-20

(19) When in your war against a city you have to besiege it a long time in order to capture it, you must not destroy its trees, wielding the ax against them. You may eat of them, but you must not cut them down. Are trees of the field human to withdraw before you into the besieged city? (20) Only trees that you know do not yield food may be destroyed; you may cut them down for constructing siegeworks against the city that is waging war on you, until it has been reduced.