What does the Gemara mean? It is true that since most of the rain fell, it
is now time now for the trees to blossom. But how does that make Shevat the
Rosh Hashanah for trees? What does blossoming have to do with picking a date
with which to begin the new year for trees?
(a) RASHI holds that separating Ma'aser from the fruits of such trees is
only mid'Rabanan, and it is the Rabanan who determine the dates for the
start of the new year of Ma'aser of trees (Rashi 13b, DH Mitoch; 23b DH Mena
Hani Mili). The Rabanan decided that it is appropriate for the Rosh Hashanah
for trees to be at the time that trees usually begin to blossom.
(b) TOSFOS (DH b'Echad) explains that the only time that is fit to be
considered a new year is the first of Tishrei. The reason the Rabanan said
that Shevat is the Rosh Hashanah for trees is because trees that blossom
after Shevat grow exclusively from rainwater that fell since Rosh Hashanah.
Only after they absorb an amount of water equivalent to most of the year's
rainfall, do trees have enough water to blossom. Consequently, since most of
the rain of the year falls by the fifteenth of Shevat, any fruits that
blossomed after that date blossom entirely from water that fell after
Tishrei, and thus their Chanatah is attributed to the new year and the
fruits they bear belong to the new year (which began with Tishrei). If a
tree blossoms before Shevat, it is merely an indication that it grew from
the rain that came before Tishrei and belongs to the previous year's fruits.
This means that the Rosh Hashanah for trees is actually the first of
Tishrei, and blossoming after Shevat is merely an indication that the
blossoms grew from rain that fell after Tishrei. (This explanation has a
source in the Yerushalmi, Rosh Hashanah 1:2.)
In this respect, the Ma'aser of fruit differs from vegetables (Yerek) and
grains (Tevu'ah). These items belong to the year in which they were picked
or grew a third of their growth, even though the water that nourished them
came from *last year's* rains (see TOSFOS and TOSFOS YESHANIM).
(Tosfos is probably following his opinion that the new year for trees which
starts in Shevat affects not only Halachos which are mid'Rabanan, but even
Halachos which are mid'Oraisa, such as the laws of Shevi'is for trees -- see
TOSFOS DH Mah Goren.)
(c) RABEINU CHANANEL understands the Gemara's question differently. The
Gemara is not only asking why Shevat was established as the Rosh Hashanah
for trees. Rather, the Gemara is asking specifically on Beis Hillel, why the
new year for trees is the *fifteenth* of Shevat. Why did Beis Hillel insist
on making this Rosh Hashanah in the middle of the month, when every other
Rosh Hashanah is at the beginning of a month!
The Gemara answers that Beis Hillel maintains that the Rosh Hashanah for
trees cannot occur until most of the rainy season has passed. That season
begins on the seventeenth of Cheshvan (see Mishnah, Ta'anis 10a) and
continues until the end of Nisan, for a total of five and a half months. The
beginning of Shevat cannot be the Rosh Hashanah for trees, because by that
time most of the rainy season has still not passed (because Rosh Chodesh
Shevat is only two and a half months from the beginning of the season) and
only a minority of the rain has fallen by that time. The Rosh Hashanah for
trees must be the fifteenth of Shevat, which is after *most* of the rainy
season has passed.