How to Stop Allergies by Letting Lawns Go Wild

Detroit and other cities plagued by abandoned properties can cut pollen by letting empty lots return to nature.

(Photo: Rebecca Cook/Reuters)

Katharine Gammon has written for Nature, Wired , Discover, and Popular Science. A new mom, she lives in Santa Monica.

Sneezing may seem unavoidable during allergy season, but a new study shows that letting vacant properties go wild can reduce the growth of eye-watering ragweed plants.

The study, to be published in the journal Urban Forestry and Urban Greening, focused on Detroit, where up to half of homes in some neighborhoods have been abandoned. Researchers from the University of Michigan categorized lots according to how frequently they were mowed and found that only 28 percent of the unmowed lots contained ragweed, whose pollen is a potent allergen. But 63 percent to 70 percent of lots mowed every year or two had the weed.

That’s because ragweed has to compete with other plants for space and light to grow. When a lot is mowed, the playing field is leveled and ragweed can dominate other flora. Over time, though, other plants grow taller, crowding out ragweed and cutting off its light.

Constant care, as home owners tend to give, can also kill the allergy-causing plants—when lots were mowed at least once a month during the growing season, ragweed disappeared. But regular mowing is not happening in bankrupt Detroit. The city can only afford to mow its 114,033 vacant lots every year or two, which creates the largest pollen load.

“Regardless of whether you view reforestation as a good thing or a bad thing, it is happening in Detroit right now, and that process is unlikely to stop anytime soon,” said Daniel Katz, a doctoral candidate at the University of Michigan School of Natural Resources and Environment.

“Although the recent Blight Task Force report calls for regular mowing of vacant lots, this is an extra expense of millions of dollars a year for a city that is struggling to provide basic services,” he added. “What our research shows is that allowing vacant lots to reforest has one unexpected side benefit: a reduction in ragweed pollen.”

But reforestation—letting cities go wild with plants—comes with its own complex set of issues. When Katz and colleagues went to Philadelphia for another study, people in neighborhoods with many vacant lots expressed concern that overgrown vacant lots attract illegal dumping, crime, and unwanted wildlife. It doesn’t have to be that way, though. Other research has shown vacant lots maintained with trees can improve neighborhood safety and health.

Baltimore, Philadelphia, and Cleveland also face challenges in managing abandoned lots. “Ragweed thrives in all of these cities as well as in many in Europe, where it is an invasive species, so our management recommendations apply beyond Detroit,” said Katz.

Detroit’s citizens have started looking for creative solutions for vacant lots, Katz pointed out. City leaders have suggested using the extra space for community gardens, urban farms, or forests.

Hantz Farms, the Greening of Detroit, and other groups have actively begun working to reforest Detroit’s vacant lots with desirable trees. “Doing so can have tremendous benefits for the people who live in neighborhoods with vacant lots, and all of these strategies will help reduce the amount of ragweed pollen in Detroit,” he said.

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