CANNABOOST plant food is one of the best selling products at the Hydroexpress hydroponics store in Stirchley, a working-class part of Birmingham. The small shop, its windows filled with graffiti-style posters, also sells fertilisers with names like “Nirvana” and “Bud Candy”, alongside strong lights and giant rolls of tin foil to line greenhouses. In one corner, a couple of juicy-looking tomato plants grow in a demonstration set-up. But the youth behind the counter guesses that his customers are “not all growing tomatoes”.

Birmingham now has 58 hydroponics shops, up from 42 just a year ago. Whether aided by the latest plant-growing technology or not, cannabis production is soaring. According to the Association of Chief Police Officers, the number of cannabis factories detected each year increased from around 800 in 2004 to 7,000 in 2010. Birmingham is one of the most fertile areas; West Midlands Police, which set up a Cannabis Disposal Unit in 2010 to tackle the problem, dismantled more than 500 factories last year.

Your correspondent visited one recently closed by police; the gardener was a cocaine-addicted woman growing a few plants in a spare room in the hope of earning a cut. Other set-ups have been found in tents in the bedrooms of high-rise council flats and in the lofts of terraced family houses. Many growers are simply feeding their own habits. As one officer on the West Midlands Police drugs team says, “It's becoming the most popular cottage industry in the country.”

Small growers are squeezing out both importers and the well-connected, often Vietnamese, gangs that once dominated domestic production. The big cannabis factories set up by the latter, with their telltale heat hazes, are fairly easy to spot. Smaller operations are often uncovered only when the electric lights start fires, or when local teenagers mount a burglary.

The police and the courts can neither keep up with the surge in small-scale production, nor are they desperately keen to do so. Last month the government published new sentencing guidelines that advised judges to treat small cultivators less strictly. Attitudes to smokers are softening, too. The reclassification of cannabis in 2009, from class C to the more stringent class B, was oddly accompanied by a more liberal approach to policing consumption. Users caught on the street are rarely arrested; rather, they are issued “cannabis cautions” (a reprimand which doesn't appear on a criminal record) or fined.

In Brixton, a south London neighbourhood, an open-air cannabis market exists within ten minutes' walk of the underground station. The dealers are frequently moved on but they soon regroup elsewhere. As one dealer admits, his competitors are a bigger hassle than the police. “They get to fightin', over money and things,” he says in a deep Caribbean drawl. Violence is far more likely to get a dealer into legal trouble than business.

Strangely, this lackadaisical approach is not encouraging people to take up the reefer habit. According to the European Monitoring Centre for Drugs and Drug Addiction, the proportion of people who admit to having used cannabis in Britain has fallen more quickly than in any other European country over the past few years. Just 6.8% of adults told another survey that they used cannabis in 2010, down from 10.9% eight years earlier. The herb is now ubiquitous and effectively tolerated—and, perhaps as a result, not all that alluring.