In the second week of November, central Indiana is a patchwork of tawny and black: here a field covered with a stubble of dried corn and soybean plants; a little far­ther on, bare earth where the farmer has plowed under the residue of last summer’s crop. This is soil that wants to grow things, and already if you look closely you can see some shoots of fall weeds: chickweed, cressleaf and purple nettle. In a greenhouse on the campus of Purdue University, Chad Brabham, a soft-spoken grad student in weed science, selects two pots, each holding one 18-inch-high plant, bearing serrated, three-lobed leaves on a coarse stem. If the plants look familiar, you might have seen them growing in a vacant lot or by a roadside almost anywhere in the lower 48 states. They are Ambrosia trifida, or giant ragweed—a plant as ugly as its name and as useless, well, as its cousin, common ragweed, A. artemisiifolia, a machine for sucking up water and spewing out highly allergenic pollen. If the farmers stopped farming, it would not take more than a few years before this part of Indiana would live up to the nickname that agronomists joke should appear on its license plates: Giant Ragweed National Forest.