The Mazel of Shevat is the דלי. This hints to water as in יזל מים מדליו, water shall flow from his wells.
With this we can find a hint to Tu B’shevat in והיה כעץ שתול על פלגי מים אשר פריו יתן בעתו, as מים refers to Shevat. פלגי is a reference to the 15th day since פלג means half (as in פלגא דגופא, a half of a body). As a result it refers to halfway through the month, the 15th day. So, on the 15th of Shevat it is the time of the tree (עץ שתול) which yields fruit (אשר פריו יתן בעתו).
On this day the Mishna Brura writes ילנותלהרבות אז במיני פרות של א, we increase in the types of fruits from trees. Another hint to Tu B’shevat is found in דות לשם ה' וה עדות לישראל לה-שבטי י. שבטי alludes to the month of Shevat. ה-י has a Gematria of 15, thereby making reference to the 15th of Shevat. On this day we thank Hashem by saying a Bracha on fruits (להודות לשם ה').
