The mishnah in Rosh Hashana 1:3 lists the months when they would send messengers to notify people which day Rosh Chodesh was:
- Nisan because of Pesach
- Av because of Tisha Be'av
- Elul because of Rosh Hashana
- Tishrei because of Yom Kippur and Sukkos
- Kislev because of Chanuka
- Adar because of Purim
- When the Beis Hamikdash was standing: Iyar because of Pesach Sheini
Why weren't messengers also sent for Rosh Chodesh Shevat so that people knew the boundary between the maaser crops for fruit trees? If they didn't know the correct date by Tu Bishvat, I assume they had to maintain three separate piles of fruit until the Adar messengers came: definitely last year, definitely this year, and a small pile of doubtful fruits that ripened on the day that might be Tu Bishvat.