To mow, or not to mow — that is, and will continue to be a pressing question in Detroit.

According to a recent study conducted by researchers from the University of Michigan, the periodic clearing of vegetation, be it via traditional lawnmower or industrious ruminant, from the broke city’s wealth of overgrown vacant lots may encourage the spread of hay fever-triggering ragweed pollen instead of keeping it in check.

And as the study, published this month in the journal Urban Forestry and Urban Greening suggests, the most effective method of combating drippy, stuffed up noses and itchy eyes brought on by allergic rhinitis might be simply not mowing and permitting Mother Nature to reclaim the city's 114,033 forsaken parcels identified last month by the Detroit Blight Removal Task Force. It’s either that or mow these so-called “pollen factories” on a much more frequent (i.e. monthly) basis. Considering Detroit’s dire financial situation, this probably won’t happen any time soon as such an effort would require the city to employ a small army of John Deere-riding ragweed eradicators.

While the thought of letting ragweed grow uninterrupted instead of clearing it out may seem counterintuitive, taking the former approach might make the most sense in the long run.

Daniel Katz, a School of Nature Resources and Environment doctoral candidate and study co-author, explains: “When we surveyed vacant lots, we found that some mowing is worse than no mowing. This is because occasional mowing, say once a year or once every other year, creates the disturbed conditions in which ragweed plants thrive."

In a news release released by the University of Michigan, Katz goes onto to address the somewhat contentious “let 'em all grow wild” approach:

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. 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.
In conducting the study, Katz and his colleagues zeroed in on the growth of ragweed in city parks, occupied properties, and 62 different vacant lots spread across several different Detroit neighborhoods. Seventy percent of the lots mowed once every two years contained ragweed while 68 percent of lots that got the once-a-year treatment were filled with the notorious flowering plant.

On the flipside, only 28 percent of the completely neglected lots observed as part of the study contained ragweed. “When these lots are left alone completely, other plants rapidly outcompete ragweed,” notes Katz. These ragweed-defeating plants commonly include milk thistle, goldenrod, chicory, and Kentucky bluegrass along with a variety of trees that start to grow several years after a lot has been left untouched.

Vacant lots that have been treated to frequent upkeep and once-a-month mowing were observed to be completely ragweed-free.

All and all, vacant lots, primarily located in low-income neighborhoods, proved to be the primary habitat for Detroit’s ragweed population with densities being six times higher in those lots than on occupied properties.

Katz and his colleagues concluded that despite the fact that ragweed pollen is viewed as a regional problem by public health officials, it's adversely impacting residents on a much more local level in Mo(w)town: “Because pollen grains can travel long distances, sometimes people make the mistake of assuming that it usually does travel long distances. Our Detroit study shows that ragweed pollen is a local problem, and that's important because it means we can make local management decisions about how to reduce exposure," explains Katz.

Via [CityLab]

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