One of Zurich's most glitzy and up-and-coming neighbourhoods, Europaallee, has a rather strange and fragrant issue to contend with. 12 years after they were first planted, many of the trees that line the street will have to be replaced, because they are starting to smell "like vomit and rancid butter". Here’s why:

Europaallee ginkgo trees kick up a stink

Our story starts way back when Europaallee - a new development in the heart of Zurich created by the city and Swiss Federal Railways that features expensive apartments to rent and glitzy bars, cafes restaurants and offices - was first being designed over 12 years ago. At the time, Zurich landscape architect Stefan Rotzler suggested that ginkgo trees should be planted along the pedestrianised street.

At the time, the architect was faced with fierce opposition, with local officials noting that female ginkgo trees are famous for producing incredibly pungent seeds. Nevertheless, after swearing that he would only plant male trees and promising that they would be easier to maintain, Rotzler’s vision won out.

15 percent of Europaallee trees produce a vomit-like smell

After 12 years of maturing, it’s clear that Rotzler and his team failed to keep their promise, with the Tages-Anzieger noting that 15 percent of the 76 trees planted on Europaallee are clearly female. Unlike male ginkgos, which produce elongated flowers in spring, the females produce small seed-like capsules on their stems which are only visible once the plants mature. One example of a female ginkgo is the tree nearest the Credit Suisse ATM on the street. 

In China, where the plant is originally from, the female seeds grow into fruit which are a delicacy if picked early enough. However, if left on the vine for too long, the beads and fruit start to emit a horrific smell. While hard to describe, the aroma has been likened to rancid butter and vomit. By the autumn, the remaining fruit and seeds fall to the ground, producing a stink bomb-like effect for anyone unlucky enough to be passing by.

How did the guffing gingkos come to be planted in the first place?

Speaking to the Tages-Anzeiger, Rotzler said there are two theories as to how the pongy trees came to be planted on Europaallee. The first was demand: because so many trees were needed at once, an international call was made to anyone who had a ginkgo tree to hand. 

This led officials to go to a tree nursery in the south of the Netherlands, where 80 yet to mature trees were chosen - meaning it was very hard to tell which were male and which female. Therefore, some female trees may have slipped through the net.

The second is more groundbreaking: according to research from universities in Japan, published in 2016, the ginkgo can change its gender. This means that the trees may have switched genders as they were maturing in Zurich, leading to the pungent mixture.

Luckily, the contract that the city signed with the tree nursery allows Rotzler to save his blushes: all of the female trees on Europaallee will now be replaced by male trees from the same nursery.