The first day of [the Hebrew month] Shevat is the New Year for the tree according to the House of Shamai. The House of Hillel says it is on the 15th of Shevat. (Talmud, Rosh Hashanah 2a)
Question: Why is it necessary for us to know the date of the "New Year for Trees"?
Answer: The Hebrew calendar is set up according to a seven year cycle. The seventh year is known as the year of shemittah, during which the land is left idle and no work is done in the fields. In the first, second, fourth and fifth year of the six year cycle, the farmers have to set aside "first tithe" (ma'aseir rishon) for the Levite tribe, and "second tithe" (ma'aseir sheini) is brought to Jerusalem to be eaten. On the third and sixth year, "tithe for the poor" (ma'aseir ani) is given in lieu of the second tithe.
Tithes must be given from the fruits which grow on the tree each year. One cannot give from produce of the current year for another year. For tithing, the new year is calculated from the time when the fruits of the trees begin to blossom. Tu B'Shevat is the cut-off date between one year and the next. In the year which follows the shemittah year, fruits which blossomed before Tu B'Shevat belong to the first year of the cycle, and fruits which blossom after Tu B'Shevat belong to the second year of the cycle.
Question: What is the basis of the dispute between the House of Shamai and the House of Hillel regarding the date of the New Year for Trees?
Answer: The Mishnah (Rosh Hashanah 16a) states that on the festival of Sukkot (celebrated on the 15th of the Hebrew month of Tishrei) the world is judged in regard to water. This does not contradict to the opinion in the Talmud that on Rosh Hashanah -- i.e., the 1st of Tishrei -- the entire world is judged, because the judgment on Rosh Hashanah is general judgment that only creates the potential for the water to be given. The detailed practical determination concerning the water takes place on Sukkot.
According to the Jerusalem Talmud (Rosh Hashanah 1:2), the significance of the New Year for trees is that until then all trees live on the water of the previous year. After the first day of the month of Shevat the trees derive their life source from the water of the new year. Thus, the effect of the new water occurs four months after the period of judgment.
Hence, the dispute between the schools of Shamai and Hillel concerns the significance of the potential (b'koach) and the actual (b'poel). The House of Shamai is of the opinion that the potential is of primary significance. Consequently, according to Shamai, since in potential the judgment of water took place on the first of the month of Tishrei, four months later we celebrate the New Year for the trees, when the potential begins for them to derive nurture from the waters which were included in the judgment of the entire world four months ago.
However, according to the House of Hillel, priority is given to that which is actual. Thus, the actual decision on water takes place on the fifteenth of Tishrei. Therefore, four months later, on the fifteenth of Shevat, the trees start living from the new waters.
Question: The Torah states "Man is like the tree of the field" (Deuteronomy 20:19). The Jewish people have often been compared to different trees. What lesson can man learn from trees?
Answer: Trees teach us the following:
A tree is planted by first putting a seed in the ground. Afterwards, it is necessary to frequently water the ground and remove the weeds. In each and every Jew, G-d planted a Divine seed -- his soul. It is man's obligation to water it with Torah study and protect it by weeding out bad friends and influences.
A healthy tree continues to grow and grow. A healthy Jew must continuously grow spiritually. This is accomplished through studying Torah and performing its precepts.
To ensure that a young tree will grow straight, it is tied to two supports, one on each side. To ensure that a young child grows beautifully, the parents must always be at his side and constantly supervise him.
The strength of the tree depends on how well it is rooted in the ground. The root of the Jew is his belief.
The beauty of a tree is the fruit it produces. mitzvot and good deeds are man's fruits.