It was an issue disputed by Beis Hillel and Beis Shammai.
They agreed that the simcha increase happens 2 weeks after the new year for trees, as on that day you gather the fruits to start making the wine and liquor.
However they dispute as to which date the new year for trees is. See the first mishnah for Rosh Hashanah.
Beis Shammai hold it is the first of Shvat, so they also follow that you increase the simcha from Tu Bishvat.
So if you did it from this date you would be following the practice of Beis Shammai.
The reason for the celebration 2 weeks after is of course the G'zeira Shava with 15th / 15th. Sukkot is the celebration on the 15th day i.e. 2 weeks after Rosh Hashanah. Pesach is the one on the 15th day from the 1st Nisan new year. So there is also one for the new year for trees..