Birkat Hailanot, or the Blessing on the Trees, is a special blessing that is recited once a year during the Jewish month of Nisan, which began Thursday. Many sages were careful to recite the blessing immediately at the beginning of Nisan with many people. In the blessing, we thank G-d for turning dry trees into flowering trees.
The 12th-century sage Maimonides wrote that the blessing is to be recited "out in the fields". Some wanted to learn from this that one must recite the blessing on trees outside the city. But many explained that this is where the trees were outside the city, and you may recite the blessing wherever there are fruit trees.
The blessing should be preferably recited during the month of Nisan, and the best is to do so as soon as possible. Those who didn't recite the blessing during Nisan may do so during the following month of Iyar without mentioning G-d's name.
One can recite the blessing at night if it is possible to see the flowers of the tree. It is permissible to recite the blessing on Shabbat, but halachic authorities were worried that one would take a prayer book out of the area where it is permissible to carry it or that someone would pick something from the tree.
In the southern hemisphere (South America, South Africa, Australia, and more), Birkat Hailanot is recited during the month of Tishrei, the beginning of spring there. If a person made the blessing in the month of Nisan and travels in the month of Iyar to these countries – he does not make the Brachah again during Tishrei because the blessing is only recited once a year.
In principle, one tree is enough to recite the blessing, but the Kabbalists said there should be at least two trees.
One may only recite the blessing on fruit trees.
Birkat Hailanot is considered a blessing that is not bound to a specific time since it is possible to make the Brachah at any time of the year, like the blessing on lightning and thunder, but in practice, in the Land of Israel, one makes the Brachah in the month of Nisan.
If you see trees while traveling by bus or car and you can recite the blessing immediately (within two to three seconds), you may recite the blessing.