The Ticino palm falls victim to its atypical reproduction

WSL

For decades, the Ticino palm has given southern Switzerland a tropical feel. But in a bid to halt the spread of this invasive tree, the authorities have banned its sale. The measure could have been limited to female plants, but the atypical reproduction of the males led to its complete ban.

Adopted by the Federal Council last March, the Ordinance on Release into the Environment (ODE) came into force on September 1. It prohibits the sale, import or even gifting of 31 invasive alien plants that endanger local flora and fauna.

The ban affects plants that have until now been popular with garden centre regulars, such as the butterfly tree and cherry laurel. But the measure also affects a real star among the exotic plants found on Swiss soil: the Ticino palm.

A champion invasive plant

“But there’s nothing Ticino about it,” protests Antoine Jousson, who has written his master’s thesis on this species. Speaking on Swiss television RTS, Jousson points out that its real name is the hemp palm and that it originated in China. Found from the end of the 19th century on large middle-class estates, the tree acclimatised very well in Ticino, even becoming one of the symbols of this canton south of the Alps.

“The actual dispersal began in the mid-1970s,” explains Jousson. “Studies showed that there was a link with rising temperatures.”

The hemp palm can be described as a champion invasive species. Its roots don’t take up much space, so it can easily establish itself in undergrowth. Its height – up to 15 metres – allows it to outstrip all other trees, and it doesn’t shed its leaves in winter, giving it a competitive edge over deciduous trees. There’s not too the Swiss winter can do stop it spreading, as it can withstand temperatures as low as -18 degrees Celsius.

WSL, Boris Pezzatti

The presence of hemp palm poses a threat to other plants and certain animals, which it deprives of food and habitat. Palm trees also burn easily, which is an added danger in these times of global warming.

Given all these drawbacks, the Ticino palm has been described as ‘an inconvenient beautyExternal link’, by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).

A popular outsider

In Ticino, the battle seems already lost. The aim now is to prevent it spreading north of the Alps, even though some regions have already been affected, notably the shores of Lake Geneva and Lake Zurich.

But the palm tree also has its supporters. A survey carried out last year showed that almost 60% of those questioned had a positive opinion of them. As a result, palms are popular and sell well in garden centres. Keen to maintain this lucrative trade, garden centres proposed limiting the sales ban to female plants only. This proposal, which was put forward during the consultation procedure on the amendment to the ODE, was also supported by the canton of Zurich.

On paper, the solution seemed implacably logical. Since each female plant can disperse up to 100,000 seeds a year in the wild, banning them could have stopped this tree from reproducing.

Evolving reproduction

However, the work of Jousson and other members of the team at the Geneva Botanic Garden showed that male plants do not necessarily need female plants to reproduce.

“Hemp palms are generally presented as male or female, but we discovered that between 15% and 20% of individuals can change sex,” explains Jousson. Male at the beginning of their life, they are able to produce fruit at the end of their life. This phenomenon was already known in other families of palms, but had never before been studied in the hemp palm.

“We are probably in an evolutionary phase between a monoecious system, where there can be males and females on the same plant, and a dioeceous system where the two sexes are separate,” adds Yamama Naciri, curator of the Geneva Botanical Garden. This is not insignificant for the invasion phenomenon. It means that the males are autonomous and can colonise new areas on their own. This survival trait has led to a complete ban on the hemp palm from garden centre shelves, much to the dismay of those who wanted to give their garden an exotic touch.

Adapted from French by DeepL/ac