אָמַר רַב יְהוּדָה: הַאי מַאן דְּנָפֵיק בְּיוֹמֵי נִיסָן וְחָזֵי אִילָנֵי דְּקָא מְלַבְלְבִי, אוֹמֵר: ״בָּרוּךְ שֶׁלֹּא חִיסֵּר בְּעוֹלָמוֹ כְּלוּם וּבָרָא בּוֹ בְּרִיּוֹת טוֹבוֹת וְאִילָנוֹת טוֹבוֹת לְהִתְנָאוֹת בָּהֶן בְּנֵי אָדָם״.
On a related topic, the Gemara cites that Rav Yehuda said: One who goes out during Nisan and sees trees that are blossoming recites: Blessed…who has withheld nothing from His world, and has created in it beautiful creatures and trees for human beings to enjoy. Rav Zutra bar Toviya said that Rav said: From where is it derived that one recites a blessing over scent? As it is stated: “Let every soul praise the Lord” (Psalms 150:6). He explains the verse: What is it from which the soul derives benefit and the body does not derive benefit from it? You must say: That is scent. Even over items from which only the soul derives benefit, one must recite a blessing and praise God.
Q1: What is the idea behind this Mitzvah?
A: Shitah Mekubetzet explains: To praise Hashem for the ongoing recreation of the Universe. This seems to be the way the Rambam understands the sugyah. It's a praise Hashem in wonder when one sees the marvellous trees in their blooming state. That's why, according to his Shitah, it would be better to do it in the fields, and not on a random tree.
Q2: What types of trees are included in the Mitzvah?
A: 3: Birkas ha-Ilanos is said only on fruit-bearing trees[Mishnah Berurah 226:2.]. If one mistakenly said the blessing on a barren tree, he need not repeat the blessing on a fruit-bearing tree[Shevet ha-Levi 6:53].
1: Fruit trees which are blooming, which does not yet have fruits on it.
Mishnah Berurah 226:4.
A2: The blessing is said upon seeing the actual flowering of the tree. The growth of leaves alone is not sufficient to allow one to recite the blessing Mishnah Berurah 226:2..
Q3) Are two trees necessary?
Some Poskim, (like Chazon Ovadia) require 2 trees, and no Bracha is recited in one tree, most Poskim don't require two trees.
Many Poskim, like the Chida write that Lchatchillah two trees are better, so ideally bracha should't be recited on 1 tree. Rav Shalom Zalman story.
Q4) Must it be two different types of trees?
A: No. (Kaf ha-Chayim 226:2.) But Some Poskim say it is a good hiddur Mitzvah.
Teshuvos Halachos Ketanos 2:28.
Q5) Can the trees be within the city?
A) Some Poskim say it is a Hiddur Mitzvah to find trees outside of the city. Kaf Hachaim
Q6) When is the best time to fulfil this Mitzvah?
Chodesh Nissan, since that is the time most trees renew themselves. (Mishnah Berurah)
Aruch Hashulchan allows a person to bless on a tree during the month of Sivan, if one sees it blooming then. In a country where tree bloom in Cheshvan, some passkey, like the Minchat Yitzchak allow one to bless then.
Some Kabbalistic Poskim like the Sdey Chemmed and Kaf Hachayim say the Bracha should only be recited during Nissan.
Some Poskim write the Bracha should be recited as soon as possible, so ideally on Rosh Chodesh Nissan. - Beis Levi)
Q7) Can one recite the Bracha at night?
A: Yes. Tzitz Eliezer 12:20-6.
Q8) Can one make the Bracha on Shabbat?
A: Some Poskim write it's better not to recite the Bracha on Shabbat. Kaf Hachaim
We are concerned about breaking twigs, or borer.
But the Aderet would Davka do the Bracha on Shabbat, to get 100 blessings. And R Ovadia agrees.
Q9) Is a Minyan required?
A) No. But Kaf Hachaim brings down that there is an Inyan to have a minyan followed by Kaddish. Before the blessing is recited, V’yehi noam followed by Hallelukah hallelu Keil min ha-shamayim is said.
Q10) Are women obligated in this Mitzvah?
Yes, despite this being time bound. This is a one time mitzvah so women can participate just like men. Har Tzvi 0.C. 118
Q11) Are blind people chayav?
Chazon Ovadia, says they can't, though its proper to listen.
Q12) Why Nissan?
Zohar says Nissan is the Month we free the souls of the tzaddikim to Gan Eden.
And some cabalistic sefarim say we free the souls trapped in the tree. so Nissan is the best time to do it!