What are the principles of Birkat ha'ilanot?
Birkat ha'ilanos is a special Brachah that is made once a year and is given a separate chapter in the Shulchan Aruch. It is told of Gdolei Yisrael who were careful to make the Brachah immediately at the beginning of Nissan with a big amount of people. A. The idea behind Birkat ha'ilanos: In the Blessing of the Trees we thank G-D for turning dry trees into flowering trees. B. Where to make the Brachah: Maimonides wrote "Going out into the fields", and some wanted to learn from his word that one must make the Brachah on trees outside the city. But many explained that this is in a situation where the trees were outside the city, and you can make the Brachah wherever there are fruit trees. C. Until when can one make the Brachah: Preferably in the month of Nisan, and the best is to make the Brachah as soon as possible. Those who haven't made the Brachah in Nissan, make the Brachah in the month of Iyar without mentioning G-D's name. D. Is it possible to make the Brachah at night or on Saturday: At night you can make the Brachah if you see the flowers of the tree. On Shabbat it is possible to make the Brachah, but the Poskim were worried that one would take a Siddur out of the Eruv and that someone would pick something from the tree. E. In the southern hemisphere of the earth (South America, South Africa, Australia and more) Birkat ha'ilanos is made on Tishrei, which is the beginning of the spring there. If a person made the Brachah in the month of Nisan and travels in the month of Iyar to these countries – he does not make the Brachah again on Tishrei, because this Brachah is made once a year. F. On how many trees does one make the Brachah: in principle, one tree is enough, but the Kabbalists said that there should be at least two trees. G. One makes the Brachah on fruit trees only. H. Women: Birkat ha'ilanos is considered a Brachah that is not bound to a specific time, because potentially it is possible to make the Brachah at any time of the year like the Brachah of lightning and thunder, but in practice in the Land of Israel one makes the Brachah in the month of Nisan. I. making the Brachah while traveling: If you see trees while traveling by bus or car and you can make the Brachah immediately (within two to three seconds) you can make the Brachah. Rabbi Yitzchak Ben Yosef (translated from Hebrew)