In the 1960s and early 1970s, most of the marijuana smoked in the country was smuggled in from northern Mexico. The botanical techniques were primitive and the potency was low.
Then the United States' crackdown on Mexican marijuana forced smugglers to go elsewhere for their pot, said Ronald Siegel, a psychopharmacologist at the UCLA School of Medicine. More potent marijuana from southern Mexico, Colombia and Thailand began hitting the streets.
At the same time, the small California pot growing industry began to boom, taking its seeds from the influx of the more potent marijuana.
The Reagan Administration's "War on Drugs" against Colombian marijuana further increased the demand for home-grown pot.
Domestic marijuana--California is the biggest growing state--has expanded from less than 10% of the market in 1980 to about 60% now, according to the National Organization for the Reform of Marijuana Laws. Local growers have produced varieties that are more than 10 times as potent as many of the foreign plants.
The northern Mexican marijuana that was once prevalent was weak because often both the male and female plants were harvested together. Growers soon learned that the male plants were much less potent.
Then, in a development that accounted for the dramatic increase in potency, sinsemilla (Spanish for without seeds) marijuana was discovered and California growers perfected the process. Growers discovered that if the male plants were uprooted and destroyed in their infancy, the female plants developed an extremely high content of THC, the active ingredient in marijuana.
High in THC
Female plants trap male pollination by manufacturing sticky resins extremely high in THC. But with the male plants gone, the females manufacture more resins--while waiting for pollination--and produce larger flower buds. Because they never are pollinated, they produce no seeds.
Domestic growers also began cross-pollinating variants, using better soil nutrients and fertilizers and studying climate and elevation patterns, said Ned Walsh, Mendocino County alcohol and drug program administrator. The most sophisticated horticulture technology is now being used on California marijuana crops.
"If growers used all their time and brainpower on legitimate farming," Walsh said, "they could grow 90-pound tomatoes."