Police in northern France are on the hunt for a gang of thieves who are believed to be stealing hydrangea plants and using the flower's leaves to smoke.

Detectives were hunting the so-called "Hortensia gang" after reports of missing and over-pruned plants.

It is thought young people are using the plant as an alternative to cannabis, as they can be easily identified and picked from people's gardens. When the leaves are left to dry and mixed with tobacco, they can be smoked in the same way as marijuana.

What effect do hydrangeas have?

Local pharmacists in the region said the flower could give a euphoric feeling when smoked. They also told police hydrangeas could have a hallucinogenic effect, similar to that of Tetrahydrocannabinol (THC) found in the marijuana plant.

It is thought the young shoots of the plant can produce an intense high. Although it is not illegal to smoke hydrangeas, stealing them is.

Capt Frédéric Evrard, a spokesperson for the Nord-Pas-de-Calais regional gendarmerie, said the phenomenon was similar to those who pick wild mushrooms for recreational drug use.

He said: "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 its the same sort of wave as the hallucinogenic mushrooms collected in the wild."

Where did the phenomenon come from?

Smoking hydrangeas for a hallucinogenic effect has previously been reported in German news sources. In 2011 Frank Federau, a spokesperson for the Lower Saxony state criminal police, told Pharmazeutische Zeitung: "We've recognised the phenomenon of hydrangea theft for at least ten years. The numbers usually rise in spring."

According to reports, smoking the plant has its own "scene" among young people, similar to that of cannabis.

What are the risks?

Hydrangea plants contain cyanogenic glycosides, which are toxic if ingested. Smokers risk poisoning themselves if excessive quantities are inhaled, as the toxins can build up quickly.

Dangers include stomach and respiratory problems, as well as increasing heart rate and dizzy spells. There may be a build up of hydrogen cyanide, otherwise known as prussic acid, an extremely poisonous inorganic compound. Under the brand name Zyklon B, prussic acid was used in the gas chambers of Nazi concentration camps during the Second World War.