Thousand of houses in the United Kingdom are being sent marijuana-scented "scratch and sniff" cards in a new bid to track down marijuana growers.

In a press release, the charity group Crimestoppers explained the cards were designed to educate residents about the smell of marijuana. They hope once people can recognize the smell of cannabis plants, they will be able to help law enforcement officials identify illegal growing operations.

"The Crimestoppers campaign will help members of the public to recognize the signs and smell of a cannabis farm. The police will use the intelligence generated by the campaign to help build on recent successes in tackling this issue," said Andy Bliss of the Association of Chief Police Officers.

"We also know that many people don’t realize that the empty, run down house or flat on their street with people coming and going late at night may actually be a commercial cannabis farm. It’s not just the stereotype of the remote rural set or disused industrial estate unit."

The green-and-black cards also provide other signs to look out for, including homes that have "visitors at unsociable hours" or buildings that generate an excessive amount of heat. Homes with closed blinds should be under suspicion as well, according to the leaflet.

BBC News noted that the Netherlands was the first to use marijuana-scented cards to track down growers.

Not surprisingly, the Crimestoppers' new campaign was met with hostility from advocates of marijuana legalization.

"As for the cost of the campaign in product development, design, distribution and publicity…you would think that there were more serious crimes being carried out that needed public focus, and ones that more members of the public actually come to trouble because of. How to catch a Banker perhaps? That won’t help the police keep their budget though – catching peaceful and non violent cannabis consumers is a fair bit easier," Greg de Hoedt of NORML UK wrote in a blog post.

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[Woman holding nose via Shutterstock]