As biofuel sources go, weeds and grasses are looked on with more favor than land-ravaging, food-price-raising corn and palm. But there’s no such thing as a free lunch-in-your-tank, says a paper presented by green groups at a United Nations meeting Tuesday: “Some of the most commonly recommended species for biofuels production are also major invasive alien species.” The quick growth and need for little maintenance that make some grasses attractive as biofuels also make them liable to spread where they’re unwanted. The paper warns of the potential for both environmental and economic damage; the Global Invasive Species Program estimates that invasive species cost the world more than $1.4 trillion each year. The biofuels industry says it will be cautious, but that the risks are overstated.