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City gardens are blooming. In backyards, rooftops, and community lots, the number of U.S. urban dwellers growing food jumped from 7 million to 9 million from 2008 to 2013. Nearly one third of the country’s public elementary schools had a gardening program as of 2013, up 11 percent since 2008. Urban schools are especially likely to have them, and for great reasons: Studies show health and learning benefits for young children exposed to growing plants.

But among the chard and cherry tomatoes are trace metals, enduring in the soil. Particularly worrisome is lead, a vestige of the industries, house paints, and gasoline mixtures of 19th- and 20th-century urban life.

Researchers at the City University of New York (CUNY) recently surveyed 1,652 garden soil samples from 904 New York City gardens, and found that 71 percent of home garden samples exceeded the state’s soil safety maximum for lead and arsenic. 21 percent of community gardens also surpassed those limits. (Because the study’s samples came from New York City gardeners who sent them in voluntarily, it is impossible to know how many came from store-bought versus city soil.) Brooklyn, with its factory-heavy past, fared the worst of all five boroughs.

Soil Science

Children are especially vulnerable to the health effects of ingested lead, according to the World Health Organization. Lead exposure can severely and irreversibly affect children’s cognitive development, and can cause anemia, hypertension, immune system deficiencies, and other conditions. Lead CUNY researcher Zhongqi “Joshua” Cheng says he hopes for a day when governments better regulate soil testing. For now, it’s critical that gardeners in any city set up strict standards about what kinds of soil they’re using, especially if they’ve got kids digging and playing in the dirt.

“We encourage people to get their soil tested, so that you know what you’re looking at,” says Cheng, who also helps lead the NYC Urban Soils Institute, which offers testing for a small fee. “If it’s extremely contaminated, then you help yourself avoid exposure.”

Try these tactics, too:

Learn about the land. Some businesses—such as dry cleaners, gas stations, and manufacturing facilities—leave more significant ecological footprints than others. Before you plant, consult historical materials, such as atlases of local businesses, to learn about what was on or adjacent to the property.

Build a better raised bed. If you do perform the soil testing and discover trace amounts of lead or other contaminants in the soil, you might choose to make a raised bed. That’s great, but it’s probably not enough, according to The Johns Hopkins Center for Livable Futures. That’s because crops with deep roots can extend to the dirt underneath. Consider adding a barrier in the form of a fabric cover, with holes big enough to let water through.

Choose plants wisely. Lead in urban soil doesn’t necessarily translate into contaminated fruits and veggies. Certain types of plants are more likely to take up lead than others. “Tomatoes aren’t a problem, while leafy greens and root vegetables you have have consider a bit more,” Cheng says. There’s also evidence to suggest that certain plants, such as sunflowers, are able to help extract harmful metals from soil through a process called phytoremediation.

Grow food in pots. If you don’t have space to build a raised bed, you can grow many varieties of vegetables in containers. Herbs and leafy greens are easy starter plants. Cheng suggests buying freshly bagged soil from the store; potted plants will do best in soil specifically designed for containers.