Hashem left the celebrating of Tu B'Shvat up to the Jewish people. Rav Kook said regarding the month of Shvat, that the desire to plant trees comes from the longing to provide life for future generations. The mazal (power) of the month of Shvat is a dli (bucket), which is Moshe's month, as Haman saw. Moshe is likened to a bucket that received the water of the Torah and gave the Jewish people to drink. By the time of Tu B'Shvat, the rains have finally been absorbed enough that the sap of the tree begins to rise. Avodah zara (idolatry) is metaphorically worshipping the fruit instead of the tree (the source). We are fragile and at risk of being knocked down by strange winds, just like trees. The charuv (carob) tree takes seventy years to bear fruit.