Corn cultivation spread from Mesoamerica to what is now the American Southwest by about 4000 B.C., but how and when the crop made it to other parts of North America is still a subject of debate. In a new study, scientists report that corn was not grown in the ancient metropolis of Cahokia until sometime between A.D. 900 and 1000, a relatively late date that corresponds to the start of the city’s rapid expansion.