“Scratch and sniff” cannabis cards are being handed out by police in England to help members of the public detect drug farms.
In 2011-12 the number of cannabis factories rose by 15 per cent and they are increasingly being set up in residential areas.
Hot spots targeted in the Crimestoppers campaign include West and South Yorkshire, London, Greater Manchester and Avon and Somerset.
The cards being handed out contain a chemical that replicates the smell of the plants when they are growing.
A report last year found that cannabis farms are increasingly being set up in residential areas.