Police officers seized $24 million worth of marijuana in Granada Hiils, California, in the San Fernando Valley. The grass was growing high at O'Melveny Park in Granada Hills this summer, but don't blame groundskeepers at the rugged city facility.

Authorities this week destroyed nearly 3,000 marijuana plants valued at $24 million that towered more than 10 feet high and thrived on a sophisticated irrigation system hidden off the main trail.

The plants were discovered in July but police spent months investigating the large operation, leading to the arrest of a West Hills man last week.

"It's the largest outdoor grow I've ever seen," said Detective Robert Holcomb, who has worked in LAPD's Narcotics Enforcement Detail in the San Fernando Valley for 20-plus years.

The plants were discovered just hundreds of yards off the trail and 1½ miles north of the parking lot. More arrests were expected, detectives said Wednesday.

"One person, I don't think, could have tended to all these plants," Holcomb said.

Natale Gabriele, 49, was the first person arrested in the case last Thursday after air surveillance photos were taken in April. He was charged with possession of marijuana for sale and released Friday after posting $100,000 bail.

"So what this guy did was he went about 1½ miles up, through the brush, up over a ridgeline, into another secluded part of the park," Holcomb said. "You'd never see it from the hiking trail or the biking trail. Never. You'd have to go through the brush."

The plants, which ranged from 2 feet to more than 10 feet high, were planted in a ½-acre area along the hillside and in a ravine and were interspersed among existing shrubbery, Holcomb said. Officers have since cleared out the site.

Gabriele, who investigators believe tended the plants, had created an elaborate watering system by damming up a stream that ran through the hillside in the city's second-largest park, Holcomb said. Gabriele allegedly rigged the system using irrigation tubing, a battery-powered pump and timer to water the plants.

The regular watering was what enabled investigators to find the site, which was no easy feat. Detectives enlisted the help of the California Air National Guard Condor Squadron, which uses a C-130 plane outfitted with high-tech imaging equipment, to find and photograph rural areas believed to be sites of illegal marijuana operations throughout state and federal parks.

"They look for symmetrically planted and different-colored plants," Holcomb said. "Because vibrant marijuana plants are a different color from the California chaparral that we normally get, it will maintain that color even in the dry season that we have now."

The pictures were verified to contain evidence of an illegal grow in April. Detectives hiked up to the site in July, discovered more than 100 plants, and set up surveillance of the area.

When investigators returned to destroy the plants after Gabriele's arrest, they discovered thousands more in various stages of growth, Holcomb said. Another illegal outdoor grow operation was found within park boundaries about a mile away in May 2007. Several have been discovered along La Tuna Canyon Road, in the foothills.

In 2009, the Drug Enforcement Agency destroyed nearly 10 million marijuana plants being cultivated outdoors across the country, up 31 percent from the 7.6 million eradicated in 2008. "It's not that common to discover them in the San Fernando Valley," Holcomb said. "It's not something you usually find in an urban area."

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