(Based on Reb Tzadok, of course)
Sefer Yetzira states that in Shvat himlich os tes b'le'itah.
Le'itah is the middah of Esav ("Hal'iteni...), and an achilah gasah.
The tikkun is Tu b'Shvat, the eating of peiros l'shem Shomayim.
"Asid adam liten din v'cheshbon al kol ma she'ra'asah eino v'lo achal" (Yerushalmi) - the eating of exotci peiros is a tool to recognize Ma Rabbu Ma'asecha Hashem.
This is the tikkun for the first chet, which was a chet of achilah - ma'achalos asuros.
And that tikkun is with a tes - the first of which appears in the word Tov in Bereishis. Why is Tes good? The Maharal says it is the highest integer - ten is a unit of shleymus, not a number. This is because it represents the eight points of the cube and the inner point that unites them (ten transcends - ten is mei'ein Olam HaBo, Tes is shleymus ba'Olam Hazeh).
Moreover, the ninth sefirah is yesod - tzaddik ochel l'sova nafsho = tzaddik yesod olam ("Kol ha'olam kulo nizon b'shvil Chanina beni...").
The Bnei Yissaschar says Tu b'Shvat is the Arbaim yom kodem yetziras ha'vlad of the Beriah b'rachamim of Nisan (Day 1= 25 Adar). A world of growth, a world of chesed. A world of sheva yipol tzaddik v'kam, and "Ein Adam omed al Divrei Torah elah im nichshal bahem." The tree "fell" and "died" during the winter, but yesh tikvah l'acharisech - the sap starts to rise -"Kum Besulas Yisroel." And the sap is called saraf - the fire and chamimus of techilas ha'avodah ha'mechudeshes...
V'yehi ratzon she; yiyeh zeh techilas zman ha'geulah!