Seeing Air Pollution by Reading the Leaves: ARMN Ozone Bioindicator Garden 55+ Event

Text by Barbara Hoffheins, photos by Jon Bell.

On a warm Monday morning, July 21, 2025, ARMN Ozone Team members Jon Bell, Anne Doll, and Barbara Hoffheins discussed the purpose of the ARMN Ozone Bioindicator Garden for members of the “55+” community at the Walter Reed Community Center. The Ozone Bioindicator Garden was established in 2020 and has been tracked each year since the 2021 growing season. (See: https://armn.org/2024/08/15/2024-update-on-the-armn-ozone-bioindicator-
garden-a-wonderful-climate-tool-right-in-our-backyard/ for details and history.)

The event began with presentations by Anne, Jon, and Barbara who provided background about ARMN, the history of pollution monitoring by NASA, and the significance of the ozone garden. ARMN’s garden is part of National Ozone Garden Network, a collaboration with the University Center for Atmospheric Research (UCAR), the Harvard-Smithsonian Center for Astrophysics, and NASA Langley Research Center. The network uses genetically identical, ozone-sensitive and ozone-tolerant plants to track the impacts of ground-level ozone—the primary ingredient in smog.

Garden plants include snap beans, potatoes, tobacco, cutleaf coneflower, and common milkweed. Selected varieties of these plants are sensitive to ozone, showing telltale leaf stippling, yellowing, and tissue death, while tolerant versions of snap beans and tobacco plants in the garden are relatively unaffected. The 55+ attendees visited the garden outside the community center to see the plants for themselves.

The timing of this event was good for demonstrating ozone effects on plants. Just ten days earlier, on July 11, 2025, Arlington experienced its first ozone exceedance of the year, with levels surpassing the EPA’s health-based standard of 70 ppb (parts per billion), which lasted more than 5 hours. There were some earlier high levels, but at shorter durations. The location of the ARMN garden does not generally see much ozone damage throughout the growing season, which might indicate that ozone levels there are generally low. However, the hot, still weather of early July created ideal conditions for ozone formation, mostly from vehicle emissions in Arlington. (Volatile organic compound emissions from fossil-fueled power plants, factories, and ships are some of the other sources.)

In the ARMN ozone garden, sensitive bean plants showed visible stippling (small black dots) between leaf veins, while ozone tolerant plants stayed green and healthy. There was also stippling on some milkweed leaves. Air pollution damage was dramatic on sensitive tobacco leaves, but difficult to discern on the cutleaf coneflower, while the potato plants were already dead by mid-July. Ozone damage is usually observed on older leaves exposed to direct sunlight in the presence of ground-based ozone, providing a living illustration of what satellite data and air quality sensors detect. (See UCAR’s description of stippling on sensitive plants from excess ozone: https://research.cgd.ucar.edu/ozone-garden/files/ozone-observations.pdf.)

Beyond the science, the event was a chance for ARMN volunteers and visitors to connect. Most of the attendees live in the neighborhood around the park and visit often. One attendee, who was a Master Gardener trainee, said the training covered plant disease and insect damage, but not ozone damage, which can also affect many types of trees and plants in our area.

With careful monitoring by the ARMN Ozone Team and a little curiosity from visitors to the garden, the ARMN Ozone Bioindicator Garden can show how invisible pollution tells a story you can see with your own eyes—one leaf at a time.