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Detroit's Vacant Lots Are Causing Hay Fever

As ragweed pollen rages, letting vacant lots grow is better than mowing them occasionally.
June 17, 2014, 8:30am
Image: Wikimedia Commons/Andrew Jameson

Open land in an urban environment is a peculiar thing. Cities are full of covert wild spaces, slivers of green insulating rail lines, the unspoken-for patch of thorns and faded soda cans next to an overpass, and, of course, the empty lots, unused space in its most naked form. In the case of Detroit, those empty lots have become an entirely new sort of feature—increasingly vast tracts of land that might receive some scraps of attention, but not often. The lots grow into tall weeds, only to be hacked down at infrequent, irregular intervals by either the city government or the Detroit Mower Gang. According to a new study from the University of Michigan, this particular limbo, between relative wildness and occasional taming, is making the city sick.

More specifically, the massive volumes of empty lots (114,033, to be specific) found in Detroit coupled with sporadic mowing are turning the city into a pollen factory, with ragweed in particular raging out of control. The basic idea is this: a long period of unchecked weed growth first allows the ragweed to flourish, and if that growth were allowed to continue naturally, the noxious weed would soon enough find competition from plants like goldenrod and Kentucky bluegrass. Given even more time, trees would move in, eating up yet more ragweed real estate.


Eventually, the open space would have taken care of its own weed problem, but if a mower moves in after nine months or a year, the ragweed, which thrives in "disturbed" environments, finds its competition conveniently eliminated. The cycle repeats; sneezing commences. The researchers, who collected data by counting plants in certain lots and via pollen collection stations distributed around the city, found that most of the pollen from lots settled within 100 meters of its source. The upshot is that most of the pollen harm unleashed by these lots is being felt by neighborhoods with the most empty space, typically low income and minority populations.

There are two opposing solutions to the pollen problem, according to researchers: mow regularly and aggressively, or don't mow at all. Let the trees grow. "Although allowing vacant lots to reforest is controversial, it is already happening in many places across Detroit," said study co-author Daniel Katz in a statement. "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."

Katz et al looked at 62 different lots spread across the city's neighborhoods, categorizing each by how frequently it sees a mower: monthly, annually, biennially, or never. The lots mowed monthly fared the best, with zero ragweed plants found. Second best were the lots that were never mowed, with 28 percent containing ragweed plants. Of lots mowed biennially and annually, 63 and 70 percent contained the plants, respectively.

The researchers found up to 42 plants per square meter on some of the properties, noting that a single plant is capable of kicking out a billion pollen grains per year. Moreover, ragweed infestations were on average six times more densely populated on vacant properties than around occupied homes.

In some sufferers, allergies are nothing to fuck around with. Allergic rhinitis (hay fever) can hit hard, keeping people effectively trapped indoors during growing seasons and leading to general life-inhibiting misery. The study notes that asthma rates in the Detroit area are a full 50 percent higher than in the rest of Michigan.

Detroit, currently still in financial freefall, isn't likely to hire an army of mowing crews to give vacant lots monthly attention. Forests, then, are really the only conclusion, as half-measures have thus been demonstrated to do more harm than good.

The idea of just letting it grow isn't too much of a stretch; as you read this, some 1,500 empty lots over 140 acres are being converted to woodlands by Hantz Farms, a well-funded outfit whose eventual goal includes development of the "world's largest urban farm." 1,500 lots is a drop in a bucket nearly 100 times that size, but a pressing public health concern may help kickstart the conversion of those hundred-thousand or so others to less destructive states of wildness.