In Israel, Tu B'Shevat is one of the successfully secularlized Jewish holidays that the Israeli people have developed from minor religious festivals. In Israel, even in January, spring is coming and it makes sense to think about planting trees -- unlike here, when the ground is deeply frozen and tree planting is impossible. The ancient tree-planting customs of the holiday transferred perfectly to the agricultural and reforestation commitments of the Israeli pioneers.
Besides planting trees, the Israelis celebrate by eating a wide variety of tree fruit, such as dates and raisins -- a practice introduced in the seventeenth century by one of the mystic rabbis who lived in Safed, Israel. Much earlier, the holiday had been established to set a date for taxation of trees by age -- there was a prohibition against eating fruit from trees under three years old, and this set the date when a tree's aging began. Therefore this was the perfect moment to plant a tree so it would be the maximum age allowed.
When I attended a Tu B'Shevat party during my one long stay in Israel, I realized how many things separate secular American and secular Israeli Jews. I'm aware that American religious Jews have tried to link the traditions of this holiday to environmental issues and responsibility, but that seems forced to me. And American environmentalism contrasts deeply to Israeli interest in agricultural development. Despite having made the desert bloom (and planted all those forests) the Israelis aren't really that committed to the types of environmental preservation that Americans maintain. For example, recently I read about how they are mining their sand dunes for building materials and destroying a beautiful desert environment at Arava.
Some of this history comes from "The Lesson of Tu B’Shvat: A Judaism for Every Season"