Some could "jump the fence" and encroach on food crops or suffocate irrigation ditches and water systems in the US West, where water supplies are tight, he said.

Plants like miscanthus, switchgrass and giant reed grow rapidly in dense formations and have few pests and diseases -- traits companies say would make them ideal for biofuels.

As crude futures test US$100 a barrel and global concern grows about emissions of climate-changing gases, companies are racing to develop new biofuels using the plants like cellulosic ethanol for motors and biomass for power generation.

They say new crops could become better sources than corn, the current feedstock for US ethanol, because they can be grown on marginal areas rather than rich farmland, and producing them emits less greenhouse gas.

The Bush administration is granting hundreds of millions of dollars to companies to build cellulosic ethanol plants, which may use a range of feedstocks. And the farm bill being considered in the US Congress includes about US$1 billion in loans to encourage commercial-scale cellulosic ethanol plants.

"Several of the features deemed ideal in a biofuel crop species are the exact traits that raise red flags when we investigate risks of exotic plant introductions," Raghu Sathyamurthy, a plant expert at the Queensland University of Technology in Australia, said in an e-mail response to questions.

Many plants that had been introduced as beneficial species have had long-term costs due to their aggressiveness. A recent paper on biofuels in "Science" magazine said Johnsongrass, a type of sorghum, has become an invasive weed in 16 US states and costs cotton and soybean farmers in three states a conservative estimate of US$30 million annually.

Switchgrass is native in the eastern United States, but it is unknown how it would mingle with ecosystems in the rest of the country, said Jacob Barney, a scientist at University of California at Davis and co-author of a report this week on the risks of new biofuels by non-profit group, Council for Agricultural Science and Technology (CAST).

John Ferrell, a US Department of Energy biofuels expert, said any risk of new feedstocks to places where they are not native could be lessened by government cooperation in regulating them.

FOCUS ON GIANT REED

The risk of any crop becoming invasive is 10 percent or less, WSU's Mack said. But the amount of marginal land needed to be planted with new crops will be huge, could make the economic costs of any problems worse, he added.

The plant scientists say poses the most risk, the giant reed, will be used to generate electricity that will be bought by a utility in Florida. The highly flammable plant is listed as a noxious weed species in Texas, and California has spent millions trying to eradicate it.

Ferrell said giant reed worries him and that the DOE does not fund any projects using it as a biofuel.

Private company Biomass Investment Group, however, will sell power generated by burning giant reed, to Progress Energy Florida, a subsidiary of Progress Energy Inc.

"Ignorance is a dangerous thing," Allen Sharpe, president of BIG, said in an interview about the CAST report. He admitted that if giant reed, known scientifically as Arundo donax, and being marketed by BIG as E-grass, is grown near rivers it could migrate and become a problem.

He said when grown in an agricultural setting, it poses no risk. A spokesman for Progress said its confident BIG can grow the plant safely, but added that the utility is simply buying the power, not generating it.