John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Allowing the city to gradually reforest itself, rather than occasionally mowing weeds, could reduce pollen counts.
As whole sections of Detroit sit abandoned and untended, rampant vegetation is slowly overtaking parts of the city. But there might be at least one positive side to this encroaching urban jungle: Unlike vacant lots that are occasionally mowed, properties left to go to seed will produce less hay fever-triggering ragweed pollen.
As Hank Hill and, now, researchers from the University of Michigan attest, a regular mowing schedule is what's needed to keep a lawn healthy and clear of weeds. That's especially true for ragweed, which thrives in urban areas due to a preponderance of vacant lots. But regular weed management is obviously not possible across Detroit, where 85,000 blighted properties include more than 6,000 empty lots.
These vacated spaces are powerful pollen producers in Detroit; the density of noxious ragweed is six times higher on them than on occupied properties. It's a "common practice" in the city for property managers to run a mower over these cursed plots once every year or even two years, according to the UM's scientists. But they shouldn't do that, as such a haphazard schedule of mowing creates the perfect conditions for ragweed to grow.
Consider the pollen counts the team found at 62 vacant lots all over town. In the ones that were mowed every one to two years, between 63 and 70 percent had ragweed plants, each one capable of releasing a billion pollen grains in a single season. These grains can travel hundreds of miles, but the vast majority stay within the neighborhood, creating for allergy sufferers a highly localized plague of sneezing, itchy eyes and throats, and noses that run like busted faucets.
However, only 28 percent of the lots that were never mowed harbored ragweed. That's likely because in an undisturbed environment, ragweed struggles for resources with many different plants, including milkweed, chicory, and goldenrod. "When these lots are left alone completely, other plants rapidly outcompete ragweed," says Daniel Katz, who helped conduct the study.
It just so happens that in Detroit these forlorn lots are frequently situated in low-income areas with high minority populations. That makes how to care for them an issue of "environmental justice," according to the researchers. But it's clear what their preference is: Either mowing on a regular schedule, or leaving the native vegetation alone so it completely overwhelms the area:
"Although allowing vacant lots to reforest is controversial, it is already happening in many places across Detroit. Woody plants are establishing in vacant lots and reclaiming large chunks of Detroit," Katz said. "Regardless of whether people think that reforestation of vacant lots is a good or bad thing overall, it will have the benefit of reducing ragweed pollen exposure."
It's uncertain whether letting nature take its course on Detroit's vacant land will help the city's recovery prospects, but the idea is likely to appeal to anyone who dreads the thought of more pollen. Some 35 million people in the U.S. suffer from hay fever. And ragweed is a particularly nasty agent during allergy season, causing an annual financial burden of more than $3 billion due to lost production, medication, and trips to the doctor.