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.
Over the past half a century or so, that fate has been kept at bay primarily by chemical herbicides. One of the most widely used is glyphosate, best known as the active ingredient in Monsanto’s Roundup weed killers, among others. Brabham positions the two pots in a spray chamber and fills a small tank with a solution of the potassium salt of glyphosate. A traveling spray head swiftly traverses the length of the chamber and soaks the drab-green leaves with what by all rights should be a lethal dose. Brabham removes the pots and returns them to the growing table. What happens to these weeds in the next 24 hours will show, in microcosm, what farmers will face across the Midwest this growing season.