"Another advantage is that these trees are so efficient at photosynthesis," Meilan said. "They are capable of very rapid growth. We have some varieties that, when fertilized and irrigated, are capable of growing up to 15 feet a year in other locales. Some harvested trees are 90 feet tall at 6 years of age."

By early October some of the trees at Pinney-Purdue and Southwest-Purdue already were more than 15 feet tall.

Because poplars are a multiyear crop, they might not be as management-intensive as annual crops such as corn and soybeans, Meilan said. With a multiyear crop the ground isn't disturbed each year by planting and harvesting. And unlike row crops, poplars could be harvested at any time of the year and sent directly to ethanol plants, allowing growers to avoid drying and storage.

Meilan hopes Purdue research also can address three challenges to growing poplar as an ethanol feedstock: removing the sugars from cell walls and large-scale planting and harvesting.

Extracting sugar from cellulose is more difficult than it is with grain because of the presence of lignin, an organic polymer within the cell walls of biomass. Sugars must be separated from lignin before being converted into alcohol. Meilan is working with other Purdue researchers to develop genetically modified poplar varieties that have altered lignin composition and content.

Meilan and Purdue researcher Patrick T. Murphy of the Department of Agricultural and Biological Engineering are tackling the planting and harvesting issues. Meilan believes it might be possible to modify conventional farm machinery to conduct harvesting operations without requiring farmers to make sizable investments in new equipment. Harvest would involve cutting down trees a few inches above the ground. New trees then would grow from the stems.

"If specialized equipment is needed for harvest, maybe the farmer will only be responsible for growing the crop and will, in turn, hire a contractor to come in and do the harvesting," Meilan said. "A lot of that goes on right now with corn."

Cellulosic ethanol production is not far away, Meilan said. Whether poplar will be part of the mix is uncertain.

"There are two important criteria when evaluating a potential bioenergy crop," he said. "One is how much biomass it produces. The other, which is equally important, is how efficiently the sugars contained in the biomass can be converted into fuel. As the technology develops it will become even more profitable for growers."

Three corporations partnered with Purdue on the poplar research. ArborAmerica Inc. of West Point and GreenWood Resources Inc. of Portland, Ore., donated poplar sticks, while Hoosier Energy of Bloomington provided financial support.