There are four days in the year that serve as the New Year: On the first of Nisan is the New Year for kings; …On the first of Elul is the New Year for animal tithes …On the first of Tishrei is the New Year for counting years, for calculating Shmita Years and Jubilee Years, for planting, and for vegetables. …On the first of Shevat is the New Year for the tree …this ruling is in accordance to Beit Shammai, But Beit Hillel say: The New Year for trees is on the fifteenth of Shevat.

Mishna, Rosh Hashana, 1, 1