A scratch-and-sniff experiment launched three years ago to combat illegal weed grows has gone nationwide in the Netherlands.

That's right, thousands of weed-scented cards will be distributed in four Dutch cities -- including Amstelveen near Amsterdam -- this week. The initiative is in place to help people recognize the smell of marijuana cultivation, and turn their neighbors in if necessary.

Since the cards were introduced in The Hague and Rotterdam, the number of busted pot plantations has gone up exponentially, with one call to police a day in each city. In fact, the proportion of people able to sniff out pot increased from 40 to 60 percent. Last year police busted 5,800 nurseries, according to police statistics.

There are an estimated 30,000 illegal pot grows in the Netherlands, with ops often set up in attics, cellars, garages and even entire houses. Police estimate the bulk cultivation and sale of cannabis was worth some 2.2 billion euros in 2012.

Though it remains technically illegal, the Netherlands decriminalized the consumption and possession of less than five grams of pot in 1976 under a "tolerance" policy. Cops generally ignore citizens growing no more than five plants for personal use, though that too is illegal.