In countless spare rooms, attics and garages, hundreds of thousands of leafy plants are being lovingly tended, their roots fed by nutrient-rich water, their leaves bathed by hot lamps 24 hours a day.
And if the neighbours have a room with the windows blacked out from which emanates a rich, earthy smell, they are probably growing something far more potent than tomatoes.
They are likely to be part of an unprecedented boom in domestic cultivation of cannabis, which is now believed to account for more than half of all the drug consumed in Britain.
It is an expansion fuelled by a combination of factors, including the relaxation in the classification of cannabis to a class-C drug, the continuing demand from older, middle-class cannabis consumers who prefer not to become involved with dealers and the easy availability, via the internet, of the seeds and equipment that makes growing easy. But there are also many new commercial growing operations, based in small factory units or warehouses, some of which are now being tracked down by police with the help of power companies, concerned at the illegal use of electricity.
Since growing cannabis remains against the law, an accurate picture for the number of people cultivating it, whether for themselves or to sell to others, is hard to come by. But a series of new figures obtained by The Independent suggest that at least a dozen growers or "farms" are now being discovered every week by police. This is coupled with anecdotal evidence from the rapidly expanding and entirely legal industry of companies which supply both cannabis seeds and the hydroponic (water-based) growing equipment, strongly indicate a massive switch to domestic cultivation over the past two years.
Professor Mike Hough, professor of criminal policy at King's College London, who co-wrote a report published in April 2003 which predicted that home-grown cannabis would soon account for as much as half of all consumption, said yesterday: "I think these figures suggest that it is truer now that when we wrote that report.''
According to the Metropolitan Police, incidents involving domestic production of cannabis have risen from around 230 a year in 2002-03 to 420 last year and 242 so far this year, suggesting that the total for the year in London alone could reach 600. The force said it was concentrating on the "organised criminal networks" responsible for drug abuse.
The situation is thought likely to be similar around the country, but official figures are scarce, since many forces - including some large urban ones including the West Midlands and Greater Manchester Police - while publicising individual raids involving substantial seizures, do not collate figures centrally. However, Merseyside police were able to disclose that seizures of cannabis plants had almost doubled each year since 2001-02, rising from just 18 in that year to 91 in the year ending March 2005.
The number of cautions issued has risen from 458 in 2000 to 750 in 2003, the last year for which figures were available. Convictions rose from 1,500 to 1,890 in the same period. Although the vast majority of cannabis growers remain undetected, police will caution where smaller growers are uncovered and prosecute for supply only where there is direct evidence.
An alternative method of assessing the scale of cultivation has now emerged, through the high usage of electricity by larger growers. EDF Energy, which supplies electricity to a large area of the south-west and south-east of England, including London and East Anglia, told The Independent that its investigators, working with police, were discovering an average of more than 40 cannabis "farms" a month, a figure which has risen from just a handful over the past 18 months; this includes people with just a few plants, as well as commercial growers. Earlier this month, British Gas also said it had uncovered an additional 32 "farms" by this method in the first five months of this year. Other power companies admitted the problem exists, but were unable to provide figures.
Power investigators take action when there is evidence of tampering with meters or dramatic increases or decreases in consumption. They have now issued advice to employees to identify likely growers - such as curtains permanently drawn or bin bags taped to windows to block out light or humid and damp atmosphere. Police are also believed to be using helicopters equipped with heat-seeking equipment to pinpoint larger farms, which can use dozens of powerful lights.
Most agree that such figures only indicate a small proportion of the true extent of cannabis growing. An even better indication can be obtained from the network of companies which supply the home-grown market in an industry which now has a multimillion-pound turnover. Although there are some who still swear by their plot of cannabis plants in their garden or allotment, most domestic growers now chose a hydroponics system because it produces strong plants quickly and reliably.
"There has been a rapid rise in small-scale cultivation,'' said Brian Biggs, proprietor of Hempstead Hydroponics, based in Watford, which has just started selling a self-contained "growroom" for less than £1,500 - which contains lights, fans, a plant irrigation system and growth media and will produce four to six plants in about 12 weeks. A smaller growing kit would typically cost around £300-£350 and would contain a 400-watt light, irrigation and fans. Extra costs include the nutrients and electricity, estimated at around £10 a week. Some users grow three or four crops a year, using either fresh seeds or cuttings.
The concept of hydroponics is used extensively by commercial growers and intensive vegetable production in glasshouses. It is based on the principle of growing plants using a medium such as artificial pebbles or granules, with the roots fed by a nutrient-rich, water-based solution. Strong lights are used to encourage growth.
Mr Biggs said: "Although we cannot advertise our equipment for illegal purposes, we are aware than 90 per cent of our customers probably use it for growing cannabis, which of course we do not condone."
Mr Biggs and others said many of their new customers were older people with families, who preferred to grow their own, buying seeds and equipment over the internet rather than facing the risk of going to dealers. "We get a lot of people coming in with their kids and they tell us that it is the kids have the know-how to grow it for their parents.''
A number are also people with health problems, such as multiple sclerosis and migraines, which cannabis is said to alleviate.
Despite the increase in cases of electricity theft detected by power companies, Mr Biggs does not believe that relates to most of his customers. "For most of our customers, I suspect the small extra electricity cost doesn't really make it worthwhile. I think that tends to be the larger, commercial-growing operations.''
While all hydroponics sellers include disclaimers making it clear that they do not condone illegal use and it is not their responsibility what it is used for, less ambiguity exists over the sale of cannabis seeds, although most are sold under labels such as "For novelty purposes only.''
There are now many companies selling dozens of varieties of the seeds at between £9 and £40 a packet, depending on strength and number of seeds. Reading like conventional seed brochures, they extol the virtues of individual plants and the quality of the smoke they produce.
Our turnover has doubled in the past year,'' said Steve Kirkby, who runs another site, Cannabis Heaven. "The site is getting around 2,000 hits day and we are selling around 50 packets a week, half of them to UK customers.''
Mark Evans, of Everyonedoesit, who says his sales have quadrupled over the past two to three years, added: "A lot of customers settle their bills with their American Express cards. These are the kind of people who may have been smoking for years, but have moved on socially and prefer not to associate with dealers or ask around in pubs.''
Professor Hough and bodies such as DrugScope, the country's leading drugs "think-tank", believe there is a good and a bad side to the boom. "If the easy availability of growing equipment makes it possible to isolate people from criminal supply networks, that has to be a good thing,'' said Professor Hough.
But there is also concern that consumption of strong cannabis could have health implications, despite the fact that overall levels of consumption remain stable - and may even have dropped slightly among teenagers.
Martin Barnes, chief executive of DrugScope, said that despite the image of cannabis growing as a victimless crime, it was dangerous to ignore the health risks associated with regular smoking: "There is a clear consensus that regular use of strong cannabis can worsen mental health problems among those people who already suffer from them. There is less evidence it can cause such problems. But we must get the harm-reduction message across to people that, if they are going to smoke cannabis, it is better to be sensible and moderate in their use."