Birkat Ha-ilanot is only said once a year. The Gemara says that it is recited in Nisan, (which many say refers to all of tekufat Nisan). But poskim say that if you live in the southern hemisphere where trees bloom in, say, November, then you would say the bracha in November. My question is, if a person is in Australia and sees trees blooming in November, should he say the bracha then (since you are not supposed to pass over a mitzvah), or is it better to wait and say it in Nisan when he will be back in Israel or the U.S.? Thank you.

Being that you can only say it once a year, it seems that the "year" would be figured, surely on this issue, from Nissan, for the Jewish calendar revolves around the Land of Israel (that Pesach must come out in the spring here, and there are even rishonim who say that this bracha cannot even be made outside of Nissan). Accordingly, if you made the bracha in Israel in Nissan, you can't make it again a half a year later in Australia, but say you weren't in Israel in Nissan, and you said it in Cheshvan in Australia, then you can say it again in Nissan in Israel, for it's considered a new year.