Dear Umbra,

Growing your own vegetables is supposed to be healthy and good for the environment, but I live on a heavily trafficked avenue in Manhattan and my plants stay on my roof. Should I be worried that dirt from car and truck exhaust is contaminating my buckets of soil? Could I be poisoning myself? Should I get my soil (a mix of bagged potting soil, bagged peat moss, compost I got from the city, and perlite) tested, and if so, where? I grow fruit (strawberries, blueberries, raspberries, blackberries, watermelon, figs, tomatoes, zucchini, squash), leafy greens (spinach, lettuce, Swiss chard, arugula), herbs (basil, thyme, chives), and root crops (beets and maybe others later).

Laura
New York, N.Y.

Dearest Laura,

You probably get some good watermelon and tomatoes on your hot rooftop garden, and you should go ahead and eat them. The basic answer is that you don’t need to get your soil tested, but you should exercise some caution by washing certain produce. Lettuce explain.

Are your plants safe up there?

Photo: Todd_Huffman via Flickr.

Soil contamination is mainly a problem for those who garden in actual soil: the stuff on the ground made of rocks, clay, sand, and humus that has been sitting there for a while absorbing everything we throw at it. You are container gardening, and your plants grow in an almost soil-free mixture. The “soil” ingredients — perlite (tiny white balls of popped rock), sand, compost, peat, and maybe a bit of topsoil — are unlikely to come from contaminated sources. Hopefully your compost source regularly tests for contaminants, for this is one area of potential problems. You could call and ask. In general, a from-scratch potting mix hasn’t been around long enough to be contaminated with heavy metals.

So you’ve stepped around the soil contamination issue by complete accident — lucky you. Anyone gardening in the ground in an urban area, or near a house that may have been painted with lead paint at one time, should get their soil tested for lead and other metals if possible. The University of Massachusetts has a respected and nationally available testing service. There is more to say about this topic; write in if you have questions.

For ye urban container gardeners, the concern is not long-term soil buildup but airborne toxics landing on your plants. Three solutions proposed by the Cornell Horticulture program (and others) are: locate your garden away from the heavily trafficked street, erect a fence or hedge as a shield, and wash vegetables. Laura, your only option seems to be Number 3, wash vegetables. In what, we all would like to know? In a 1 percent vinegar solution or a 0.5 percent dishwashing liquid solution (regular old vinegar, regular old dishwashing liquid). A 1 percent solution would be roughly a tablespoon of vinegar in three pints of water.

Make up a tub of the solution, dunk the vegetable, rinse off the solution, and eat. This tactic will solve part of your heavy metal problem. It will remove lead particles clinging to leaves — but not the lead absorbed into the leaves or on the outer parts of roots. The good news is, plants do not concentrate lead absorption in their fruits, so most of your crops are lead-free. I did read one assessment from Los Angeles that residues were minimal, even for crops grown in contaminated soil. I’m sure there are conflicting views — so peel root crops, and make your own decision about how comfortable you are eating your leafy greens and herbs.

I hope this helps put your mind at ease and frees you for the upcoming gardening season. Enjoy the fruits of your admirable rooftop labors!

Pepperly,
Umbra