Thieves are thought to be stealing colourful plants in northern France because smoking some varieties can get you high

Some gardeners prefer the spiky Hydrangea paniculata; others opt for the smoother mop-head varieties such as Hydrangea macrophylla. Others, often those with yellow as opposed to green fingers, do not much care. They just like to rip off the petals of any old hydrangea – also known as hortensia – and smoke them, police in France have claimed.

Gendarme in northern France are on the trail of the "Hortensia Gang", acting on reports that dozens of the wintering plants have been stolen or severely pruned in the past few weeks.

Initially sceptical that youngsters were roaming private and public gardens chopping off hydrangea heads and leaves to dry, mix with tobacco, and smoke as a cheap alternative to cannabis, police say they are now investigating.

As part of their inquiries, officers spoke to local pharmacists who confirmed the hallucinogenic and euphoria-inducing effects of the hydrangea bloom, of which there are thousands of varieties. Experts say the effect is similar to that of Tetrahydrocannabinol (THC) found in the cannabis plant.

According to the local newspaper La Voix du Nord, police are looking into around 20 complaints from local gardeners as well as town halls and village mayors about disappearing flowers over the past few weeks.

Captain Frédéric Evrard, spokesperson for the Nord-Pas-de-Calais regional gendarmerie, blamed economic hard times.

"With the crisis we have the impression people are now turning towards natural products, because synthetic ones are more expensive. If these thefts are linked to drug use, then it's the same sort of wave as the hallucinogenic mushrooms collected in the wild," Evrard told Le Figaro.

The French authorities believe the hortensia habit has been imported from Germany. Bavarian public gardens are regularly pilfered for their hydrangea flowers.

Before anyone rushes out to the nearest garden centre, hydrangea-smokers risk poisoning themselves, said Kurt Hostettmann, honorary professor of pharmacology at the University of Lausanne and Geneva in Switzerland.

"The secondary effects of it are very bad for the health," Hostettmann told Le Matin newspaper last year.

He said the flowers could provoke stomach and respiratory problems, speed up the heart, cause dizzy spells and, if consumed in large quantities, produce hydrogen cyanide (also known as prussic acid), the base of Zyklon B, the poison gas used in the Nazi death chambers, causing a slow and painful death.

The Royal Horticultural Society says hydrangeas "may occasionally be affected by pests such as scale insects, hydrangea scale, capsid bugs, aphids and vine weevil". It can now add French fumeurs to the list.