More than a decade ago, Cheng and fellow NC State forestry professor Anne-Marie Stomp wondered whether fast-growing duckweed, commonly seen in shallow ponds, might remediate animal waste. Excrement from the billions of animals raised every year in America's factory farms has fouled watersheds, especially in the South, and fed oxygen-gobbling algae blooms responsible for rapidly-spreading coastal dead zones.

Duckweed, they discovered, has an appetite for animal waste, quickly converting it to leafy starch that can then be converted into ethanol. The current source for most U.S. ethanol is industrial-scale corn farming, which requires large amounts of toxic pesticides and dead zone-feeding, fuel-intensive fertilizers. When the costs are added up, corn-based ethanol may prove little cleaner than gasoline.