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On the morning after one of those record-breaking cold nights this past January, Mike Rasmusson checked on his charges. Rasmusson supervises the Kate O. Sessions Balboa Park Nursery -- the city facility where many of the park's plants get their start in life. Behind the nursery's hothouses and administrative offices, he stopped at an outdoor table and peered down at a group of four-inch plastic containers. "Ohhhhh, shoot!" he moaned. "This is Ficus macrophylla, the Moreton Bay fig. These are the babies." The fig-tree sprouts, grown from tiny, almost invisible seeds that had been buried in the pots for more than five months, had shriveled and turned black overnight. Had they withstood the frost and thrived for a hundred years, each might have rivaled the magnificent giant that reigns over the lawn next to the Natural History Museum. But now they were dead. Rasmusson shrugged. "You just have to take it in stride and say, 'You know, let's try it again.' "

It helps to be dauntless when it's your job to maintain the park's status as a horticultural showplace. Apart from the 150 acres of coastal sage scrub preserved in Florida Canyon, the landscape of Balboa Park's 1172 acres is a wholly unnatural creation, filled with species imported from every continent except Antarctica. The plants have to be watered and mulched and pruned and fertilized and protected from deleterious weeds and bugs. As they die or grow tiresome, they have to be replaced with others both eye-pleasing and apt to thrive. Who decides what goes where? "Right now, I'm pretty strongly involved with that," Paul Sirois said, "but we draw on a lot of folks."Sirois is one of two district managers in charge of the park. He holds a two-year degree in park management and design and a bachelor's in environmental horticulture, but when he joined the city parks staff in 1993, he started "from the ground up." As a maintenance worker, "I did everything from cleaning restrooms to edging turf," he says. He later advanced through the park department's supervisory ranks and in 2000 became the first city arborist -- looking after all the trees in all of the city's 340 parks. In December 2005 he was promoted to the position of horticulturist for Balboa Park, and when the district manager's position opened up last year, Sirois applied for it. "I love the park, so I thought I'd give it a go."

He now oversees about 60 people, including 45 grounds maintenance workers, 5 rangers, 4 gardeners, 5 supervisors, and a horticulturist. Augmenting his team's labors are citywide crews that handle certain tasks, such as mowing the expanses of lawn adjoining Sixth Avenue and at Morley Field. The central crews cut those weekly using seven-gang mowers and smaller trim machines, but Sirois's crews go behind to weed-whip and edge and blow the clippings. "We also do some hand-mowing in areas that are too small, such as the strips of lawn near the Organ Pavilion."

A separate citywide crew attends to routine pest management for San Diego's parks. "We're trying to get away from chemical use as much as possible," Sirois says. Mulch and mechanical weeding are used to discourage herbivorous invaders, but when herbicides and pesticides are unavoidable, the centralized teams apply them. Balboa Park's gardeners and grounds maintenance workers are on the front line in diagnosing problems. "It's really a combined effort of everybody being aware and watching what's going on." The volunteers who toil in the rose garden are constantly monitoring the roses to see when they've got too much rust and need to be sprayed, Sirois says. Because of his years of working as the city arborist, Sirois himself has been involved with diagnosing tree maladies.

No tree in the park has been more beleaguered in recent years than the eucalyptus. First brought to California during the gold rush, eucalyptuses were later thought to be a good source of wood for railroad ties, but people soon learned that the young trees grown in California lacked the strength of the several-hundred-year-old specimens that Australians had harvested from their virgin forests. Still, early Californians liked how fast eucalyptus matured, Sirois says. "And they were pretty drought resistant." He thinks that's why so many were planted for the 1915 Panama-California Exposition, the event that kicked off the transformation of the park from an almost treeless wasteland to a garden paradise. "They grew so fast, and you had a pretty good shade canopy pretty quickly."

During most of the 20th Century, the eucalyptuses seemed to be pest-free. Then in the 1980s, an Australian beetle known as the eucalyptus longhorn borer began chewing its way through eucalyptus species in Southern California, killing many of them. Since then an almost biblical series of plagues has descended upon the trees: "at least 16 different pests in the past 17 or 18 years," Sirois says. Dying eucalyptuses have become a common sight, not just in the park but throughout the city.

Most of the time, scientists have found ways to vanquish the pathogens. That was the case with the red gum lerp psyllid, a tiny insect that in 1998 began showing up on the leaves of the red gum eucalyptus and a few other species. The psyllids sucked the sap from leaves, an insult that caused defoliation. As the denuded trees weakened, Sirois and other local arborists feared that up to 1000 might be lost. But a Berkeley entomologist went to Australia and found a tiny stingless wasp that was a natural predator of the lerp psyllids. He imported the wasp to California. "For a while, they had to delay the release because a different type of psyllid was being used in Florida to control the overgrowth of melaleuca trees there." There was some concern that the wasp might attack the Florida psyllids, but it turned out not to be a problem, Sirois says. Once introduced here, the wasps spread, and the red gum lerp psyllid population came to be "very well under control." Sirois says another psyllid began attacking lemon-scented gum eucalyptus around the same time, but it turned out not to be a tree killer, as had been feared.

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