Q. Dear Umbra,

I was getting all excited to plant some herbs and maybe veggies in containers on the balcony of my new apartment this summer. But the balcony faces out to a busy road, and it occurred to me that maybe all that car exhaust might deposit some dangerous stuff on my cilantro. Can I do anything to make sure I’m not eating heavy metals or other weird pollutants? Is it safe to garden on my balcony at all?

Mateo P.
Seattle, Wash.

A. Dearest Mateo,

Benzene in your basil? PAHs in the parsley? Lead in the lemongrass? Heavens, what a buzzkill for the usually reliable high of backyard gardening. While loyal readers will know I’m all for the convenience, cost savings, and just plain locavore fun that comes with growing one’s own food, those of us living in urban areas do have a few more things to consider beyond whether we’re planting White Queen or Lemon Boy heirloom tomatoes this year.

So let’s turn our attention to the congested road outside your window. Might that idling traffic be depositing something unsavory on your leafy greens? We know exhaust contains a number of noxious ingredients: particulate matter, ozone-forming hydrocarbons, nitrogen oxides, and sulfur dioxides among them. The few studies I’ve found on airborne toxics getting into veggies (this one and this one) did find that gardens located near busy roads tested positive for heavy metals including lead and for polycyclic aromatic hydrocarbons (PAHs), and the EPA reports that garden contaminants like lead, zinc, and PAHs are indeed associated with traffic. But here’s the good news: All that stuff washes right off. So if you’re not already in the very good habit of giving your produce a hearty rinse before eating, it’s time to start. (Not to be a downer, but the good news might end with your salad. These substances are also linked to respiratory problems and cardiovascular disease, and the EPA says heavily traveled streets can affect air quality up to about 600 feet away.)

You can protect your little container garden from some of that airborne nastiness while your plants are growing. I spoke to Katie Vincent, garden hotline educator with your local nonprofit Seattle Tilth, and she recommended adding a physical shield to your balcony. “It’s worth having some sort of planting screen in front of the garden to keep [exhaust] drift from happening,” she said. “Any sort of physical barrier, whether that’s bamboo or a hedgerow, is preferable.” Walls and fences do the same job, as would a small, enclosed cold frame. Might you have room for a few larger potted plants to act as a screen out on the balcony, too?

Whatever set-up you choose, Mateo, container gardening is actually a good way to avoid another stew of problematic contaminants: those that settle in the ground. Thanks to an urban legacy of industrial activity, commercial pollution, and heavily trafficked freeways, heavy metals and other toxic traces in soil are a factor every urban gardener should keep in mind. Anyone wishing to dig directly in his backyard dirt should be aware of the potential for lead (left over from the bad old days of burning leaded gasoline, plus old leaded paint remnants from buildings), arsenic (from treated lumber and industrial pollution), and other pollutants like cadmium and PAHs. All of these substances pose potential health risks to us humanfolk, and unlike the airborne variety, they can’t be washed off. That said, it’s unclear exactly how many pollutants actually end up in the veggies. And there are steps one can take to deal with contaminated urban soil, starting with testing your dirt for toxics.

To be crystal clear, I don’t want all you urban veggie-growers out there getting so concerned about contamination that you give up gardening. The urban gardening movement is flourishing — it’s hard to get a handle on the exact number of city gardens across the country, but Detroit alone is home to more than 1,400 community, family, and school gardens — and every seed we plant sends a message to the titans of industry that the way we feed ourselves can and must change. “Be aware and do your best to keep that [exhaust] drift from happening,” advises Vincent of Seattle Tilth. “But it’s so much better for the food system for people to be growing at home. The benefits, in the larger scheme of things, far outweigh [the risks].” So plant away, break out the salad spinner, and bon appetit.

Purely,
Umbra