The idea of depleting the ozone layer has bad connotations. We commonly associate it with a hole in the upper atmosphere that lets too much UV light pass through and causes skin cancer. That might be the only ozone that gardeners and farmers think about, so we put on hats and sunscreen. Ozone way up there, in the stratosphere, is a good thing. That ozone—stratospheric ozone—is doing its job, and we’ve reduced our use of chemicals that deplete the ozone layer in the past couple of decades.

Ozone at ground level is a different matter. Plants and people would be healthier if less of it were around. Ground-level, or tropospheric, ozone originates from a variety of air pollution sources, such as power plants and combustion engines. The mix of emissions in the air with sunlight and heat causes a chemical reaction that produces ozone. It’s a tasteless, odorless, invisible gas that is highly reactive and is regulated by the Clean Air Act because of its effect on human health.

Ozone is hard on lung tissue. It causes respiratory irritation and inflammation, especially in people with asthma or compromised immune systems. Children, whose lungs are still developing, are more susceptible, as are the elderly. Similarly, certain plants in the wild as well as cultivated crops will show signs of ozone damage at lower levels of concentration. Bio-indicators are plants that reflect the quality of health of the environment more apparently than others. Regardless of the parts per million that government agencies determine are safe, these plants display clear warning signs of poor air quality.

• Tobacco is the main bio-indicator of ozone stress in cultivated crops. Others that show sensitivity include low-growing plants such as potato plants, melons, squash and beans.
• Wild food and medicine plants are among ozone sensitive species: blackberries, elderberries, huckleberries, grapes, ground nut, sassafras and American hazelnut.
• Beloved trees, shrubs and wildflowers that provide wildlife habitat also make the list, such as certain types of milkweeds, elders, alders, sages, asters, coneflowers, redbuds, tulip poplars, sumacs, aspens and pines.

Plants breathe through stomata, the pores on the surface of leaves where gas exchange happens. After entering the leaves, ozone dissolves in the water within the plant and affects it on a cellular level. Membranes become leaky, photosynthesis is slowed down, and the ability to use water efficiently is impaired. Controlled studies have found that ozone exposure decreases tree growth. It also decreases production of flowers and fruit. With weakened defenses, plants become more susceptible to other types of pollutants, insects, disease and damage from severe weather. As with the effect of air pollution on humans, these symptoms are not easily observable at first. However, if plants get stressed enough, the leaves will show it.

• Stippling of purplish-black or brown spots on the upper leaf surface
• Chlorotic (whitish) spots
• Reddening of leaves
• Leaf tip burn
• Curling leaf edges
• Early leaf drop

The damage to growing plants doesn’t stop at the ground level. In soybeans, for example, plants with ozone damage metabolize less carbon dioxide. The roots need a flow of carbon to feed microbes in the soil as well as to fix nitrogen in the soil, a benefit that legumes such as soybeans are famous for. In cases of severe ozone damage, soybeans can’t mine enough nitrogen, and farmers add nitrogen fertilizer, an excess of which can run off and pollute waterways. All this could be prevented with less ground-level ozone in the fields in the first place.

Tropospheric ozone levels peak during the hot summer days and are at higher concentrations near large urban areas. However, ozone and the other constituents that it reacts with (oxides of nitrogen and volatile organic compounds) travel far and wide. Wind brings ozone with it, and higher elevations report higher concentrations of ozone. Even if you run a farm free of fossil fuel, your plants could be affected by air pollution from an industrial area several states away. A basic fact worth repeatedly reminding ourselves is that our industrial and household waste affects those downstream and downwind.

Make your own observations of the plants in your area, and check out the ozone levels as recorded at monitoring stations. The U.S. Forest Service has made its air quality monitoring data available to the public, and you can access an ozone calculator at its website. The National Park Service, meanwhile, provides access to its ozone risk assessments for different areas of the U.S.