Soil bugs keep plants in check.

European and the United States soils contain different microbes that can dramatically change the rate at which a plant grows. This may help explain why plants that are innocuous on one continent can turn into hugely damaging pests when they reach the other. Ragan Callaway of the University of Montana in Missoula and his team studied spotted knapweed (Centaurea maculosa), a plant brought to the United States 100 years ago, and now a rampant weed. The researchers compared the growth of knapweed plants in normal and sterilised soils collected from both continents, to see if native microbes helped or hindered plant growth.