The poison garden, which opened 20 years ago as a ghoulish attraction, has taken on new life moonlighting as a destination for researchers of all kinds, from scientists curious about exotic toxins to crime writers looking for inspiration. One summer day, when the annual faint count was at about 70, Mikey Leach, the 38-year-old head gardener, cracked open a glossy laurel leaf protruding across a path. “This is one of the most common hedging plants in this country,” he said. “Cut it open? Cyanide.”