Plants may prove to be the most cost-effective and safe tool to clean up toxic sites, scientists say. In a process dubbed “phytoremediation,” plants like sunflower, clover, and mustard have been shown to absorb and break down such harmful chemicals as arsenic, zinc, uranium, and lead. “A stand of poplars is a self-assembling, solar-powered, pump-and-treat system,” says Steve A. Rock, an engineer with the U.S. EPA. The catch? Plants sometimes clean up sites more slowly than other detox methods, needing several crop cycles to remove pollution. Still, the market appears to like the plant method — one business analyst predicts the industry will see a dramatic increase in annual revenues to as much as $400 million in 2005.