ARMN’s Ozone Bioindicator Garden: Year Six

By Barbara Hoffheins

Introduction

What can a leaf tell us about the air we breathe? Quite a lot, as visitors to ARMN’s Ozone Bioindicator Garden at the Walter Reed Community Center discover during the garden’s sixth growing season. Established in 2020 as part of the National Ozone Garden Network, the garden uses specially selected ozone-sensitive and ozone-tolerant plants to provide a visual demonstration of the effects of ground-level ozone. The garden also connects local observations with a much larger story involving air quality, human health, plant health, citizen science, and atmospheric monitoring.

The 2026 season included the addition of ozone-sensitive and ozone-tolerant soybean varieties, participation in a National Ozone Garden Workshop, and the recurring Arlington County 55+ event that explains the connections between the observations made in the garden with NASA’s air-quality monitoring technology and Arlington County ozone measurements.(See 2025 ARMN Ozone Bioindicator Garden Blog piece for more on the history of the project.)

Background: What Is Ozone and Why Does It Matter?

Ozone is a gas with two very different roles in our atmosphere. High above Earth, ozone forms a protective layer that helps shield us from harmful ultraviolet radiation. At ground level, however, ozone is an air pollutant that can harm both people and plants. Ground-level ozone forms through chemical reactions involving pollutants such as volatile organic compounds (including fossil fuel fumes), nitrogen oxides (including automotive and industrial exhaust), and sunlight. Elevated ozone can irritate the lungs, and ozone levels tend to increase during the day. Ground-level ozone is usually highest in the afternoon during hot, sunny summer weather—conditions when many people are also spending more time outdoors.

Recognizing Ozone Damage in Plants

Plants provide another way to understand the effects of ozone. Ozone enters leaves through microscopic openings called stomata, the same pores that allow carbon dioxide to enter for photosynthesis. Once inside, ozone can damage plant tissue, reducing photosynthesis, slowing growth, and lowering yields in sensitive crops. On ozone sensitive plants, the injury can appear as tiny brown or tan spots, known as stippling, on the upper surface of leaves, often between the veins. Injury is frequently more apparent on older leaves following repeated exposure to elevated ozone levels in the presence of sun. Repeated and prolonged exposures result in leaf death. 

Learning to recognize these symptoms is important because leaves can be damaged by many other causes. Insect feeding, bacterial diseases, and fungal infections can produce spots or discoloration, but their patterns and symptoms differ from those associated with ozone.

Also, visible injuries to plants is what makes the Ozone Bioindicator Garden such a useful educational tool. Air pollution may be invisible, but its effects can sometimes be seen on a leaf.

Soybeans: A New Plant in the Garden

One of the most significant additions to the garden in 2026 was a pair of soybean varieties—one ozone-sensitive and one ozone-tolerant. The soybeans joined the garden’s established collection of ozone-sensitive snap beans and tobacco, as well as potatoes, cutleaf coneflower, and common milkweed.

In years past, we have observed distinct differences between ozone sensitive and ozone tolerant plants: the tolerant snap beans produced more legumes than the sensitive beans and the tolerant tobacco showed no sign of ozone damage, while the sensitive tobacco leaves suffered severely. The soybean plants provided an especially interesting comparison. They were vigorous, growing to approximately three feet tall and producing abundant foliage. Despite their healthy appearance, however, the plants showed evidence of ozone injury during periods of elevated ozone. This observation reinforces an important lesson: a plant does not have to look unhealthy overall for ozone damage to be present.

Also, the soybeans were so big and leafy that they were self-shading; ozone damage occurred early in the season when the plants had fewer leaves so there was increased sun exposure, and later, only at the edge of the bed where exposure to the sun was more direct.

Photos 1 and 2: Severe ozone damage (left) on younger small soybean leaves exposed to more sun (June 12, 2026), and moderate ozone damage (right) on older self-shaded leaves (June 20, 2026). Photo by B. Hoffheins.

National Ozone Garden Workshop

TheLarge Milkweed Bug (Oncopeltus fasciatus) and nymphs on milkweed pod on September 4, 2026. Photo by B. Hoffheins. ARMN garden is part of the national Ozone Garden Network, a partnership supported by the University Corporation for Atmospheric Research, NASA, and other scientific organizations. Gardens in the network use standardized ozone-sensitive and ozone-tolerant plant material, allowing researchers, educators, and citizen scientists to compare ozone injury observed in different locations.

In 2025, the ARMN ozone garden was awarded a grant, which included a $2,000 stipend with two years of one ARMN member participating in the National Ozone Bioindicator Garden Network Workshop in Boulder, Colorado. The workshop provides an opportunity for Team Ozone members to learn from other gardens, share observations, and strengthen the garden’s role in education and citizen science. In August, 2026, Team member Todd Minners attended the workshop and provided an insightful description of the event. (See “National Ozone Bioindicator Garden Network Workshop: a Great Learning Experience for ARMN’s Own Ozone Garden.”)

Bringing Ozone Science to the Community: The 55+ Event

On July 24, 2026, ARMN Team Ozone (Jon Bell, Anne Doll, Barbara Hoffheins, and Todd Minners (member Caitlin Buggeln could not attend) welcomed participants in an Arlington County’s 55+ event at the Walter Reed Community Center for a program that connected what visitors could see on a leaf with what scientists can measure from space. ARMN members Susan Berry, Jill Barker, and Carolyn Fleming also attended.

Team Ozone presented the history of the ARMN Ozone Bioindicator Garden. They then introduced the history of NASA pollution monitoring from space and highlighted the newest satellite, TEMPO—short for Tropospheric Emissions: Monitoring of Pollution—which represents a major advance in monitoring air pollution. They described why ozone matters, how ozone affects human health and plants, how to identify ozone injury on leaves, and the patterns of ozone concentrations measured locally and reported to the Virginia Department of Environmental Quality and the U. S. Environmental Protection Agency. NASA can observe pollution from space, monitoring stations can measure it from the ground, and sensitive plants can provide visible evidence of its effects right here in Arlington. The program demonstrated the unique educational value of the garden.

Looking Ahead

Team Ozone hopes at some point to acquire its own ozone sensor at the garden because it would be good to have a more accurate understanding of ozone patterns at the garden. Increasing shade from the nearby bald cypress, especially during the typically higher ozone levels in the afternoon, might also be preventing ozone injury in some parts of the garden. Given that part of the garden receives less sun, we may also rearrange plants in the beds to study differences in sun exposure.

In addition, next year, fewer plants and some pruning might be in order. However, there are some considerations that ultimately threaten the garden’s location. First, the above-noted increasing shade from the nearby bald cypress is likely preventing some ozone injury. Furthermore, the tree’s distinctive knees are beginning to crowd out plants in the garden’s beds.

On the bright side, an Arlington County Parks & Recreation staff member reported seeing numerous monarch butterflies at the garden this season, and recently, a milkweed seed pod was nourishing a Large Milkweed Bug and nymphs—an encouraging reminder that the garden provides benefits beyond its role as an ozone research and education site.