SUBURBS don't come much tidier than Chino Hills, 30 miles (50 km) east of downtown Los Angeles. Last year, the neighbourhood of coffee-coloured stucco houses and three-car garages boasted an average household income twice that of the nation as a whole. In Vista del Sol, one of its quiet enclaves, every house but one has a neatly-trimmed green lawn. And, until recently, the exception was verdant inside. When the police went in, they found more than 800 marijuana plants—a small part of what is turning out to be an enormous harvest.

Greg Garland, a local narcotics cop, used to discover about a dozen houses a year that had been turned into marijuana factories. So far this year he has raided more than 40. The production boom is not confined to the suburbs. Since April the state's annual “Campaign against Marijuana Planting” has pulled 2.9m plants worth some $10 billion from back gardens, timber forests and state lands (see chart). Marijuana is now by far California's most valuable agricultural crop. Assuming, very optimistically, that the cops are finding every other plant, it is worth even more than the state's famous wine industry.

The illicit crop is grown with a technical sophistication that Napa Valley's Robert Mondavi might envy. To supply outdoor plantations, rivers are dammed and water piped as far as two miles. Plants are nourished with fertilisers and tended by workers brought to America specifically for the purpose. Ageing hippies are responsible for only a few such operations. Kent Shaw, a state narcotics officer, reckons four-fifths of outdoor marijuana plantations are run by Mexican criminal gangs.

Indoor factories, by contrast, are largely the province of East Asian entrepreneurs. They prefer to buy houses rather than rent them, to avoid the attention of landlords. They tend to go for big ones in good neighbourhoods: the property in Vista del Sol cost more than $600,000. Like good horticulturalists, they propagate strains of the plant that produce a high proportion of tetrahydrocannabinol (THC, marijuana's active ingredient) and speed their growth by means of heat and artificial light.

Why the boom? The National Survey on Drug Use and Health shows that the rate of marijuana use in California has barely risen in the past few years, whereas production has hugely increased. Some 11% of the state's population indulge—just a puff over the national average, and less than every state in New England.

The likely explanation is a steady tightening of America's borders after the terrorist attacks of September 11th 2001 and the panic over illegal immigration. California used to import high-grade marijuana from Canada and low-grade weed from Mexico. Both routes are now more risky. As a result, Asian gangs have moved south from British Columbia, where they dominate the hydroponic trade. Mexican distributors, who may handle cocaine and methamphetamine as well as marijuana, have diversified into production.

In places like Chino Hills, the boom has also been helped by demographic change. Like many southern California suburbs, Chino Hills has been rapidly transformed from a mostly white area to a rainbow one. Residents of such a diverse place may be more inclined to ignore odd behaviour and a funny smell emanating from the house down the road—provided that the grass is kept short.