On Sunday evening, a group of around 20 people in Highgate, north London will sit down for a ceremony involving a haggadah and an etrog. It may sound a bit of a religious mash-up but then Tu Bishvat, the New Year for Trees, is a time for creative experiment.

The idea for holding a Seder for the festival is traced back to the circle of mystics who settled in Safed in the 16th century. Whereas the way to observe much of Jewish ritual is laid down in Jewish law, that is not the case with the Tu Bishvat Seder and participants can choose how to format it.

Like the Pesach Seder, it is traditionally built around four cups of wine, which symbolise the four seasons but also the four spiritual realms envisioned by the Kabbalah. On Tu Bishvat, however, it is customary to use both red and white wines.

The Highgate group have not only compiled their own haggadah but published it. “Over the years we were collecting a lot of material and trying out different haggadot,” says its editor Joe Berke. “I had the idea to somehow bring them all together.”

The first Tu Bishvat text, Pri Etz Hadar, was printed in Europe in the 18th century, taking its name from the commandment in the Bible to take a beautiful fruit on Succot — hence the etrog. Some used to express the hope on Tu Bishvat they might find a good etrog for Succot later in the year.

In the pecking order of the Jewish calendar, Tu Bishvat may come below the five major festivals of the Bible, and then Purim and Chanukah, but it has enjoyed increasing popularity in modern times. Zionism breathed new life into it, reclaiming the Land of Israel by planting new trees. And more lately, tree planting has taken on a new ecological significance in the battle against climate change.

The 84-page Highgate Haggadah, which is produced in English, blends the different elements in its collection of pictures, prayers, reflections and songs. It also records some of the historic customs from communities around the world.

In southern Morocco, the wealthy would invite the townsfolk to their homes and fill their hats with fruit. In Afghanistan, Jews would buy material to make new clothes for Pesach. In some places, women who longed for a child would embrace a tree at night.

“As we got more and more material, it occurred to me it’s a beautiful holiday,” Dr Berke says. “It’s not based on suffering — no one is trying to kill us. It’s a very joyful occasion that celebrates the renewal of life.”

The festival is rich in symbolism. The Torah is likened to a tree of life. The kabbalistic Tree of Life diagram illustrates the divine energies that sustain creation.

“The mystical sages of Safe in the 16th century understood God as a tree of life, whose roots were in heaven and whose branches extended toward earth, bringing all of us life and blessing,” the introduction to the Highgate Haggadah explains.

While the cups of wine usually go from white to red — with mixtures of the two in-between — in Highgate it goes from red to white, which “completes the journey from materiality to spirituality”.

Also, the Seder is punctuated by the eating of three different kinds of fruit (ten each time — 30 in all, although some eat 15) which symbolise different layers of spirituality. First come fruits with a hard layer but edible inside such as nuts, next fruits with an edible exterior but inside stone such as a plum: third, fruits you can eat almost whole such as berries.

But what about a fourth set of fruits? Since the final level of being is beyond the physical, no fruit is actually eaten. The fourth spiritual realm can be symbolised by the fragrance of the etrog. “It is said that the prophet Ezekiel saw a chariot carrying the mysteries of creation, like the smell of the etrog carries the aroma of Divinity throughout the worlds,” explains the Highgate Haggadah.

A few years ago, Dr Berke received a request from a BBC producer to film the Seder as part of a documentary about Spitalfields fruit and veg market. “I said no way, it’s too disruptive,” he recalls., “Then I said, I tell you what, if you can find an etrog, you can do it, thinking he would never find an etrog in February — but they did!”

For this year, he has carefully preserved a pair of etrogim from Succot in his fridge.

Dr Berke, a psychotherapist whose books include a study of Freud and Chasidism, first began attending a Tu Bishvat Seder at Highgate Synagogue some 25 years ago before the group moved to his home. The haggadah started with a four-page sheet and branched out each year.

Four pounds of its £10 cover price will go towards new trees in Israel. It is published by Teva, the name of Dr Berke’s now sadly departed dog (who is memorialised in a photo in the haggadah).

The Highgate Haggadah, Teva Publications, is available from Amazon at £10