Gardening safely in Detroit’s soil

Nearly 1,400 community gardens feed the city of Detroit. Per capita, that's more than any other city in the U.S., said Sarah Black, a Ph.D. candidate in biological sciences at the Wayne State University College of Liberal Arts and Sciences, and yet crops grown in urban environments face unique stressors.

"Two of the most pervasive are thermal pollution generated by urban heat islands (UHIs) and atmospheric deposition of heavy metals from industrial and vehicular emissions."

An urban heat island is a section of the city that's warmer by 5 to 6 degrees Fahrenheit than the rest of the city, Black said. For example, Midtown is much warmer than the Riverfront or Palmer Park, because there's less greenspace, less shade from trees, more buildings and reflective surfaces, like asphalt and windows that magnify sunlight.

Under the guidence of Associate Professor Glen Hood, Ph.D., and funded by the WSU One Health Initative, Black conducted a field experiment from 2022-2024 wherein she grew tomatoes, kale, chard, and carrots from seeds in pots in community gardens across the city to study heat island intensity and atmospheric metal disposition during growing seasons.

She will present her research at the Wayne State University Graduate Research Symposium on February 25, 2026, at the Student Center.

Black conferred with community gardens around the city to determine which plants are routinely eaten by Detroiters and which to study.

"I was interested in different organs of the plant. A carrot, we eat its root. Swiss chard, we eat the stem, sometimes the leaves. Kale, we eat its leaves. And tomatoes, we eat the fruit."

All organs retain metals differently, she explained, so it was important to study a spread.

Black discovered that up to an extent, UHIs actually resulted in an increase of micronutrients like copper and zinc that helped increase crop yield. But excessive heat forced a higher intake of metals, which lessened the protein content of crops. It's nuanced, Black said, but gardens near green space with fewer buildings and occasional shade, ultimately fared better.

The most measurable spikes in detriment levels of metals did not occur spread across particular UHIs but instead resulted on a block by block basis, Black said.

"I'd notice an influx and ask around the neighborhood about nearby events, and they'd tell me 'Oh, three houses were demoed on our block this week,' or 'It was the Fourth of July weekend, there were fireworks.'"

Such events result in heightened levels of heavy metals like chromium, lead, and aersinic in the air that then settle in the soil and are usurped by plants. Root vegetables in particular take the brunt, Black said. 10 to 15% of the carrots she grew tested above ambient concentrations. That's more than 200 parts per million (ppm) by EPA standards.

"A way to mediate that is to grow root vegetables in above ground beds or to not grow root vegetables at all," Black said, pointing out it's a suggestion when it comes to farming in urban areas supported by the Michigan Department of Health and Human Services. "Roots accumulate lead as a defense mechanism in plants to keep it from moving to other parts of the plant."

None of the other crops Black planted tested for meaningful levels of metals.

Black hopes her research can inform protections for produce grown in UHIs. The ultimate answer is to create more greenspace, she said. "Ferndale, for example, has made it a goal to be 40% forested by 2030. Meaning they're offering to plant trees in front of every home that doesn't have one in the median between the sidewalk and the street in an effort to reduce the amount of sunlight that reflects off the pavement directly into houses."

The same solution works to protect plants from excess sunlight and heat.

Black said other solutions that have been implemented in other cities include moving away from asphalt to other kinds of pavement that aren't so reflective, and green roofs on buildings that can absorb heat.

The greener the neighborhoods, the healthier the food that ends up on tables.