HORTICULTURE HERO: Wedged between the concrete walls of the Los Angeles River and the San Diego Freeway is a curious oasis of pepper trees, canna lilies, geraniums, raspberries and sweet peas. . . . Water, sun and the gentle imagination of 83-year-old Ernie LaMere have brought life to a once-barren patch of dirt along a quarter-mile stretch of Sherman Oaks. . . . "I wanted to prove to the world that one man could make a difference," said the retired maitre d'. He has (B1).