The Blessing of the Trees during Chodesh Ha-aviv
As the month of Nisan, Chodesh Ha-aviv, begins, we are presented with a unique and beloved mitzva: the blessing on the trees (birkat ha-ilatnot). Let us examine the characteristics of this mitzva and its connection specifically to Chodesh Ha-aviv. The Talmud mentions this blessing:
- R. Yehuda says: “If one goes out during Nisan and sees the trees blossoming, he should say, 'Blessed be He Who has not left His world lacking in anything, Who has created in it good creations and good trees for the enjoyment of mankind.’”
Rambam copies the Talmud’s words, with some minor changes:
A person who goes out to the fields or gardens in the month of Nisan and sees flowering trees sprouting branches should recite the blessing: “Blessed are You, God, our Lord, King of the universe, Who has not left His world lacking in anything, Who has created in it good creations and good and fine trees for the enjoyment of mankind.”
This blessing expresses our gratitude to God for the universe which He created for us, in order to sustain us.  Based on these sources, halakhic authorities have inferred that the blessing on the blossoming is specifically on fruit-trees, not on non-fruit-bearing trees, because we make the blessing over the benefit we receive from the fruits. Similarly, one may see this as a source for the halakhic dispute as to the question of whether one may make the blessing over blossoming hybrid trees, which result from sin and are contrary to God’s will; therefore most halakhic authorities rule that one should not make a blessing over a crossbred tree. 
In light of the words of the Talmud and Rambam, we may ask: should we conclude that we make this blessing specifically in the month of Nisan? Alternatively, perhaps one can make it also before Nisan if one sees blossoming trees (for example, if one sees a blossoming almond tree in Shevat or Adar), and so too, if one happens not to have made the blessing in Nisan, one may do so in Iyar.
From the words of some Rishonim, we may infer that one may also make the blessing at another time, as R. Avraham bar Yitzchak writes: 
It is not specifically in the days of Nisan, but at the time that one sees blossoming trees for the first time that year.
Similarly, Ritva writes: 
The days of Nisan are not specific, but in each and every place, whenever the trees blossom.
This is also how Roke’ach rules, as do other halakhic authorities. This indicates that the essence of the blessing is the encounter with the initial bloom, not the season of the year. This has practical halakhic ramification, because in the southern hemisphere, trees blossom in Elul and Tishrei. There, one should make this blessing at the time that the local trees bloom, even though it is not the month of Nisan.
However, Chida writes that, according to the kabbalists, one should make the blessing specifically in Nisan (Birkei Yosef 226:2):
But I have heard that, truth be told, this blessing is not applicable except during the days of Nisan. Therefore, even on almond blossoms, one must make the blessing in the month of Nisan. Indeed, we have observed that the senior rabbis made the blessing on almond blossoms in the days of Nisan. Thus, this is the essence of the law.
According to the kabbalists, even upon almond trees, which bloom before Nisan, the blessing can only be made in the month of Nisan. This indicates that according to Kabbala, the blessing is not only for the fact of blooming, but the timing of the blooming in Nisan, during the land of Israel’s Chodesh Ha-aviv.
It is well-established that the Jerusalemites and the members of the Old Yishuv throughout Israel would go out in the month of Nisan to fields, gardens, and orchards to recite the blessing in a large group; specifically, they require a quorum of ten. In this assembly, they add to the blessing the recital of a chapter of Tehillim and Kaddish as well.
In fact, the halakhic authorities determine that it is best to make the blessing in Nisan, in order to fulfill all views. However, one may make the blessing also after Nisan on the condition that one does not wait “until after the fruits grow, for then he may no longer make the blessing.” 
Similarly, the authorities emphasize that we are talking about seeing blooming flowers, not leaves; and that one must recite the blessing over the blossoming of fruit-trees specifically, not non-fruit-bearing trees: 
It is specifically flowers, because leaves alone are not enough, and even the flowers must be on a tree which provides food — because from this blossom a fruit will grow — but not a non-fruit-bearing tree.
What can we learn from the fact that this blessing should be made on the blossoming of flowering fruit-trees — not mature fruits and not crossbred trees (which were made in defiance of the biblical prohibition)? What can we learn from the kabbalists’ view that one should make it in Nisan specifically? It appears that we may answer this in the light of Rav Kook’s point, which we mentioned above: 
“Today you are going out, in the month of spring”… Today has its unique spiritual character, prepared for your Exodus, and it is in Chodesh Ha-aviv, a time in which blossoming and the renewal of life are realized throughout nature. This occupies a unifying position through the quintessence of creation: everything participates in your wondrous Exodus.
In other words, there is a link between the Exodus from Egypt, which initiates the redemption of the entire world in which reality starts to bloom, and the season of the year in which the Exodus takes place, aviv.
This is intimately connected to the land of Israel. In this context, we have mentioned the famous dictum of HaRav Kook about the month of Nisan:
Israel’s exodus from Egypt remains forever the springtime of the entire world.
There is a certain correspondence between spiritual processes and natural processes which happen in this season in the land of Israel (op. cit. below):
[The Torah states:] “And when Lord brings you into the land of the Canaanites… which He swore to your fathers to give you, a land flowing with milk and honey, you shall keep this service in this month” (Shemot 13:1-5).
Specific incidents in the land of Israel are connected to great events, to the appearance of the sanctity of the light of God from which they came. Thus, this mark is etched upon the spring season, in its perfection in the land of holiness, like a focusing lens that shows us the light and the glow, the renewal of life and liberty which has been wrought for us in the days of our great spring, at the season of our departure from Egypt by the strong hand of the Master of all actions, blessed be He. This principle is expressed in the unique land, which is flowing with milk and honey, the land of the five nations which are specified here, whose produce and yield emerge from its land and relate to the specific and wondrous attributes of its atmosphere. The bounty of aviv shines in it, in all of its light and the splendor of its glory, and from it the glory of its bounty branches out over all of the land, because aviv is the mark of resurrection, paralleling and reflecting the words “You shall keep this service in this month.”
The blossoming of the fruit-trees marks a transition from a period of defoliation and dormancy to a period of growing and renewal. Therefore, we recite this blessing focusing on the blossom, not that which has already passed over the threshold to become a fruit. Similarly, we do not make a blessing on a crossbred tree, because it does not represent the blossoming of God’s act. In this transition, there is a certain renewed resuscitation, from a wintry state, in which it appears that reality has shriveled and that there is no chance, to a state of renewal, with the sprouting of new fruits and new sustenance, the future and its hope.
In the land of Israel, there is a certain correspondence between the time of blossoming of fruit-trees and the time which is fitting for redemption, in which there is a certain blooming of spiritual and physical reality. Therefore, despite the fact that in terms of the specific phenomenon of a fruit-tree blooming, one may still make this blessing at another time, in terms of the general phenomenon and all it implies, it is better to recite the blessing specifically at this time — in Nisan, when we were redeemed; in Nisan, when we will be redeemed.
Thus, there is no better setting for this blessing than a great assembly, many people gathering to recognize the divine phenomenon of blossoming and renewal in nature, in the month of springtime. To this, we add also the verses of praise and redemption from Tehillim (148:13-14):
Let them praise the name of Lord,
For His name alone is exalted;
His majesty is above earth and heaven.
He has raised up a horn for his people,
Praise for all his saints,
For the people of Israel who are near to him.
Praise the Lord!
 Berakhot 43b
 Hilkhot Berakhot 10:13.
 See Penei Moshe, end of Yerushalmi Kiddushin.
 See Responsa Minchat Yitzchak vol. III, 25:3 and Responsa Yabbia Omer OC, ch. 20.
 In Sefer Ha-eshkol, p. 68.
 Chiddushim, Rosh Hashana 11a.
 Ch. 342.
 Tehillim 148 was chosen for this. We should note that one should not be overly punctilious about this if one may miss his opportunity; one should be meticulous and take the first opportunity to recite the blessing, even if it means reciting it alone.
 See Responsa Yechaveh Daat, vol. I, ch. 1.
 Shulchan Arukh, OC 226:1, Mishna Berura 1 ad loc.
 Mishna Berura 226:2.
 Olat Reiya vol. I, pp. 37-38.