Green green grass of home: police crack down on cottage industry of cannabis

If making big money from cannabis cultivation was the aim, there is no evidence of success in this small, rented, two-storey terrace home, raided by Manchester police.

Downstairs, the middle-aged male tenant had just sat down in a bare sitting room, in front of afternoon television, to enjoy a piece of buttered white toast and a porn magazine when the police arrived. Of the three rooms upstairs, only the bedroom is furnished, but sparsely – a bed, a dirty duvet, no sheets on the mattress, and a ratty pair of slippers half-tucked under the bed. "He's not living in affluence," a police officer remarks, glancing in.

The other two rooms have been stripped down to bare floorboards; one contains six black plastic bin bags, which on inspection contain old plant pots and the moulding stumps of cannabis plants. The back bedroom is filled with an enormous black tent, almost ceiling height, which unzips to reveal a silver reflective lining, high intensity strip lights, fans, extractor flues and a sophisticated odour filtration system. Outside the tent, blue curtains have been taped to the window with black masking tape so no light seeps through.

There are no live plants and the tenant, a tired-looking, pale man with stubble, questioned at his kitchen table by the police, wearily admits that he grew one crop "to pay off a debt", but is not currently cultivating new plants.

Across the UK, 7,865 cannabis farms were discovered in 2011-12, an increase of 15% on the previous year's figures and over double the number for 2007-8 when police found just 3,032. Previously in Manchester, cannabis cultivation was done on a larger scale by gangs, sometimes from China and Vietnam, who would fully convert terrace houses, knocking down walls to make larger growing areas, taking electricity direct from the mains, to avoid triggering the suspicion of the energy companies over unusually high consumption.

Recently, there has been a shift towards smaller-scale farms, in line with a national trend, identified by the Association of Chief Police Officers' 2012 report into the commercial cultivation of cannabis, which noted a "diversion into multi-occupancy premises to reduce risk". It said: "There is an emergence of the 'multiple site model' whereby a large number of gardeners are employed to manage small-scale factories across multiple residential areas".

Fire services have reported rising numbers of cannabis farm-related house fires, triggered by the re-routing of electricity supplies. The Acpo report noted an increase in robberies, burglaries and violence (including the use of firearms) linked to cannabis farms and highlighted the use of "debt bondage" – where individuals are forced to grow the drug, to pay off loans.

Crimestoppers recently sent out scratch-and-sniff stickers to help people identify the distinctive smell of cannabis and encourage them to alert the police. Forces in 12 areas of the country with a high incidence of cannabis farms have this month stepped up the number of raids – sometimes tipped off by members of the public who have noticed the strong smell from neighbouring houses.

Finishing off at the Manchester raid, the police officers shred the tent with knives to make it unusable, and stamp on and break tent poles. They remove the lights and electrical equipment to a van parked outside the house at the end of a quiet cul-de-sac, watched by neighbours, and about a dozen local children who came running to see police in body armour file into the building.

Ten minutes' drive away another Manchester force has just raided a printing studio, where in addition to producing signs and business cards, workers appear to have had a sideline growing cannabis plants. The smell as you walk up metal steps to the office is a curiously thick mix of ink and cannabis.

The first room in the workshop is given over to layout surfaces and plan chests, but a large side room has been emptied of anything to do with the core business and converted into a well-nurtured cannabis farm, where 50 plants have reached around a metre tall, and look ready for harvesting. Lights have been strung up from the ceiling and attached to a Plug & Grow timer socket, so that for 12 hours they shine brightly on the plants, and for 12 hours there is darkness.

The investigating officer shines a torch into the blackness, casting ferny shadows of cannabis plants on to the ceiling. He thinks they might be 12 weeks old and estimates that the street value from the room's contents would be in the region of £35,000. A sign marked "strictly staff only" is stuck to the main entrance, and the plantation room was secured with a bicycle lock.

In the neighbouring building, an architectural supplies business, two vast black-and-silver tents have been erected beneath the rafters in the loft, and supplies for a second plantation have been delivered – silver hydroponics lights, aluminium flexi-duct, odour filters, and black plastic plant pots. One of the tents is about the size of a high-street changing room and has just one plant inside, with a small desk fan directing cool air at its leaves. The other is double the size, and still empty.

Detective Chief Inspector Michael Mangan, head of crime operations at Oldham police station, examines the tents with interest. "This is the new style," he says. "You could just put it up in your bedroom. It is … find a room, put some sheeting down, put some lights up and grow the plants. I do actually think I could do it myself." Six people were arrested when police raided the two workshops.

In the past year, Manchester police have discovered about one cannabis farm a day, even without a dedicated campaign to hunt for them. Halfway down the stairs to the evidence storeroom in the basement of Oldham police station the smell of cannabis plants hits you, a heavy tropical smell reminiscent of cut grass and rotting nettles. An aisle of shelves in the corner is full from floor to ceiling with dozens of large brown paper bags, filled with rotting cannabis plants. Fresher bags of wilting plants have been piled onto stolen bicycles in the corridor. The smell is overpowering, but staff no longer notice it. The bags are due to be taken for incineration in the next few days.

"I think people have realised that growing cannabis is profitable, with little perceived risk, and not very difficult to do. There is a market there to be exploited," Mangan says. "We think it is mostly done by local criminal gangs who have recognised that this is a commodity that will make money for them."

A mile from the terrace house raided by police on Thursday, staff at the newly opened hydroponics shop – which specialises in water-based plant cultivation – say their products are not designed for cannabis cultivation.

Their stock, displayed on shelves in an unwelcoming dark basement in a business park unit by the motorway, is identical to the equipment in the raided buildings, and many of the products have very knowing names that hint at their purpose without spelling it out. Canna Start costs £9 and the shop assistant explains that it is a good product to encourage young shoots – perhaps, he says, young tomato plants.

The shop also sells unconventional gardening items — such as head torches to wear when you water plants in the dark so you don't tread on them. Many of the products are designed to stimulate big buds, which is the most saleable part of a cannabis plant. The shop assistant explains that this might be useful for growing tulips – "bigger buds would mean bigger flowers". But he is unable to explain why people might want to be growing tulips or indeed tomatoes inside the house or in the dark, or in tents that are on sale for £400 a shot, lit with specialist lamps that cost up to £350 each, or why they would spend money on expensive odour filtration systems.

The hydroponics magazines on display have women in bikinis advertising fertilisers, tents and plugs, and plant nutrients are promoted with pictures of skulls on sticks. There is no mention of cannabis, but curiously for a gardening magazine there are articles advising readers what they should do if they are arrested. Mangan notes that there are more hydroponic shops springing up, but points out there is nothing illegal about them, and in any case any attempt to control them would simply divert the business online.

Police visit another terrace house, ring the bell, but since the owner is out they decide to batter down his front door. Neighbours pop out to say that he is probably at work. Half a dozen police officers make their way into the house, which although it smells strongly of cannabis, turns out to contain only four plants, growing in the boiler cupboard.

Andy Bliss, Acpo's lead officer for drugs, conceded that tipoffs are not always accurate. "Intelligence can be mercurial," he said. But he stressed the raids were important. "These farms are a magnet for criminality, and are associated with significant violence right up to firearms involvement."