There is a Misva to recite "Birkat Ha'ilanot" ("the blessing over trees") upon seeing blossoming trees during the month of Nissan. The recitation of the Beracha and the accompanying text is followed by a "Kaddish Yehe Shelama," and for this reason it is preferable to recite Birkat Ha'ilanot in the presence of a Minyan.
However, as Rav Pinhas Zevihi (contemporary) rules in his work Birkat Yosef (p. 83), if a person chances upon blossoming trees in private, he should recite the Beracha at that point, rather than wait for the opportunity to recite the Beracha with a Minyan. Rav Zevihi tells that thirty years ago, when Hacham Ovadia Yosef served as Chief Rabbi of Tel-Aviv, Hacham Ovadia came to visit him towards the beginning of Nissan and they noticed two blossoming trees. Despite the fact that they were not in the presence of a Minyan, the two Rabbis immediately recited Birkat Ha'ilanot with great joy and fervor. Rav Zevihi tells that the trees dried very quickly that year, such that many people who delayed the recitation of Birkat Ha'ilanot lost the opportunity to fulfill this Misva. This incident demonstrates the importance of reciting Birkat Ha'ilanot at the earliest opportunity, even without a Minyan.
Hacham Ovadia Yosef (Yabia Omer vol. 2, Yoreh De'a 18:3) issues a similar ruling with regard to the monthly recitation of Birkat Ha'lebana (the Beracha over the new moon). This Beracha, too, should ideally be recited together with a Minyan, but Hacham Ovadia rules that it is nevertheless preferable to recite it at the first opportunity. Otherwise, cloud cover might prevent one from reciting it afterward, and he will then miss the opportunity of fulfilling this Misva.
If a person saw blossoming trees during Nissan and did not, for whatever reason, recite Birkat Ha'ilanot, he recites the Beracha upon his second sighting of the trees, provided that the fruits have not emerged and the trees are still in their blossoming stage. The Mishna Berura (commentary to the Shulhan Aruch by Rabbi Yisrael Kagan, 1839-1933), in Siman 226 (5), cites this ruling from the Mahasit Ha'shekel (Rabbi Shemuel Klein, Germany, 1724-1806), and this is also the position taken by Rav Zevihi (ibid., p. 86) and Hacham Ovadia Yosef (in his new work on the laws of Berachot, p. 466).
Summary: There is a Misva to recite Birkat Ha'ilanot upon seeing blossoming trees during the month of Nissan. Although ideally one should recite this Beracha with a Minyan, one who has the opportunity to recite the Beracha should do so immediately, even without a Minyan, rather than risk missing the opportunity to fulfill the Misva. If one did not recite the Beracha upon his first sighting of blossoming trees, he may do so at the second sighting, provided that the fruits have not emerged and the tree is still in its blossoming stage.
