Rosh Chodesh Nissan (this year : April 1) officially ushers in the Spring season when we may recite Bircat HaIlanot, the blessing upon seeing fruit trees in bloom. This special blessing is recited only once a year. In it, we praise G‑d's ongoing renewal of Creation. The blessing is as follows:
"Baruch Ata Ado-nai, E-loheinu, Melech Ha'olam,
shelo hasair b'olamo kloom,
ubara bo briyot tovot v'ilanot tovot,
leihanot bahem b'nai adam."
"Blessed are You, G‑d, our Lord,
King of the Universe,
that His world is not lacking anything,
and He created in it
good creations and good trees
for the pleasure of mankind."
(Readings for before and after the blessing are found in some prayer books.)
It is desirable to seek the opportunity to recite Bircat HaIlanot as soon as possible in the month of Nissan (unless Rosh Chodesh Nissan falls on Shabbat). It is said only upon one's seeing a tree in bloom for the first time. The growth of leaves alone is not sufficient to allow one to say the blessing; there must be flowers (or buds, according to some authorities).
According to Kabbala, this blessing has special significance….
One who saw fruit trees in bloom during Nissan, but forgot or neglected to recite the blessing, may then recite it only until the time that the fruit of the tree has (begun to have) grown.
Ideally, Bircat HaIlanot should be made outside of the city limits, and on at least two trees, and the more the better (as in an orchard). There are those who hold that these trees should be of at least two different species. Also, it is desirable that it be recited with a minyan of 10 males if possible, so that it can be followed by Kaddish.
According to Kabbala, this blessing has special significance. When Jews make this blessing they are able to redeem souls who have been re-incarnated in the vegetal realm, making it possible for them to continue or complete their soul rectification. It should therefore, be said with intense concentration.
In addition, it is desirable to follow the blessing with a donation of 3 coins to tzedakah (charity), corresponding to the three soul levels of nefesh-ruach-neshama. Also, the Ben Ish Hai recommends giving 4 coins for the elevation of the soul of one's departed parents, based on the four letters of the Name.
"See [said Isaac of Jacob], the fragrance of my son is like the fragrance of a field that G‑d has blessed." (Gen. 27:27) It is interpreted that this verse applies to anyone who recites Bircat HaIlanot with the proper intention.
[Based on Shulchan HaShalem, Ohalei Tsaddikim, and Kaf HaHayim]