If one of the goals of growing plants for biofuel is to be kinder to the environment than you are by extracting oil from the earth, you wouldn’t want to plant anything that could be harmful to the environment. But how could a plant harm the environment? Well, it could become invasive, outcompeting native species, altering the habitat and driving other species into extinction. The damage from and control of invasive plants already costs the United States more than $34 billion each year, according to one estimate. Bioenergy shouldn’t add to that number.

Recognizing this potential for danger, a group of biologists at the University of Florida recently set out to predict whether a dozen species being considered for biofuel cultivation could become invasive. Their study appears in Biomass and Bioenergy.

The researchers note that the characteristics that make a plant attractive as a biofuel source—high productivity, low input requirements, wide breadth of habitat—overlap with those of non-native invasive species. And when the biologists analyzed a dozen non-native species using an assessment system already used by Australia and New Zealand for more than a decade, only four species (miscantus, plume grass, sugarcane and sweet sorghum) had acceptable scores. Seven other species were rated as likely to become invasive, and the last needed further evaluation.

These results may be surprising to the people who proposed these species as biofuels because nearly all of the plants have been grown in Florida for decades for ornamental or agricultural purposes. And they may think, therefore, that this study can be ignored. But growing a tree in a garden is not the same thing as growing acres of them for regular harvesting. “Cultivation of large acreages of a species previously cultivated and introduced in low numbers over relatively low acreages, might so significantly alter propagule press that shifts in dispersal and colonization frequency occur,” the scientists write. In other words, growing something in large numbers can create the opportunities necessary for the species to take off and grow in even larger numbers in places you never intended.

And that has happened in the past. In Australia, for example, people grew a type of ornamental tree called Mimosa pigra for at least 60 years with no problems. But when the tree was moved to a new riparian habitat—the land near rivers or streams—the tree quickly became invasive; it’s now one of Australia’s worst invasive plants.

Not that long ago biofuels were touted as the easy solution to our energy future. We now know that’s not the case. And this study shows that it’s even more complex that we thought.