Growing Awareness: How Ozone Gardens Help Us See the Air We Breathe

What’s Up, Forsyth? In our last post, we dug into how ozone affects plants and why some are more susceptible than others. Today, we're putting that knowledge to use by introducing you to a way to monitor air quality right in your own backyard! We are also sharing tips and preliminary findings from our very own ozone garden in Kernersville—a recent collaboration between the Paul J. Ciener Botanical Garden and Triad Air Awareness.

Figure 1. Air Awareness Coordinator, Sarah Coffey, standing next to the newly installed Ozone Garden sign at the Paul J. Ciener Botanical Garden in Kernersville, NC.

What’s an Ozone Garden?

An ozone garden is a specially designed garden filled with plants that are sensitive to ozone. In addition to beautifying any space and serving as a hub for native pollinators, these plants are bioindicators—living organisms that help scientists and citizen scientists monitor environmental health.

When ground-level ozone interacts with the cells inside leaves, it can cause visible damage if the plant is sensitive enough. By observing this damage over time, we can spot trends in local air quality, which is especially helpful in areas far from official air monitoring stations.

Meet the Ozone Dream Team

In North Carolina, especially the Piedmont region, the Ozone Garden Toolkit from CleanAIRE NC is the go-to resource. They've identified a list of top ozone-sensitive plants, including:

1. Common Milkweed (Asclepias syriaca)
2. Eastern Redbud (Cercis canadensis)
3. Yellow Poplar or Tuliptree (Liriodendron tulipfera)
4. American Sycamore (Platanus occidentalis)
5. Black Cherry (Prunus serotina)
6. Cut-leaf Coneflower (Rudbeckia laciniata)
7. Thornless Blackberry (Rubus canadensis)
8. American Elderberry (Sambucus canadensis)
9. Yellow Crownbeard (Verbesina occidentalis)
10. Black-eyed Susans (Rudbeckia hirta)

The two plants that were most highly recommended for the Piedmont Region of North Carolina were cut-leaf coneflower (Rudbeckia laciniata) and Southern/yellow crownbeard (Verbesina occidentalis), and we have planted both at the Paul J. Ciener Botanical Garden. The only catch was, we initially could not source the exact Verbesina species known to be ozone sensitive, V. occidentalis, and instead used its very close relative, V. alternifolia (other common names include wingstem and yellow ironweed). While not specifically listed as a bioindicator plant for ozone, we thought it would be a fun science experiment to compare this species to V. occidentalis, which was a very recent addition after we were finally able to acquire these plants in late August from the North Carolina Botanical Garden in Chapel Hill. All our other plants were sourced from a local native plant nursery, Frank’s Perennial Border in Winston-Salem and were planted in late May.

Figure 2. These are the three plants we have in our ozone garden in order from left to right: 1) cut-leaf coneflower (Rudbeckia laciniata), 2) Wingstem/Yellow Crownbeard (Verbesina alternifolia), and 3) Southern/Small Yellow Crownbeard (Verbesina occidentalis). While V. alternifolia is not a confirmed ozone sensitive species, it is very closely related to the ozone sensitive V. occidentalis. The first two plants shown above were planted in late May, and the last was planted in late August. 

You may be wondering, why didn’t we choose some of the other species, like milkweed and black eyed Susans? Common milkweed is already present in several areas of the Garden, and it tends to take over a space, which is why we did not plant it. While readily available in most garden stores and nurseries, we did not pick black-eyed Susans because they are an annual species that would need to be replanted each year. Our choices are beautiful and practical additions to the Garden that can be studied year after year.

Spotting the Signs: Ozone or Something Else?

As we described in “The Invisible Threat: The Impact of Ozone on Plants”, ozone damage often shows up as stippling—tiny, uniform looking dark spots on the upper surface of leaves. Stippling can progress into larger areas of brown/black tissue death (necrosis), which can be seen on the underside of the leaf, but for the most part, ozone damage is only seen on the top of the leaf. The damage should not cross leaf veins (which in contrast to the rest of the leaf, appear bright green), and the oldest leaves exhibit the greatest signs of injury. 

Figure 3. Stippling and chlorosis (yellowing) on a cut-leaf coneflower leaf at the Paul J. Ciener Botanical Garden. Photo taken on September 5, 2025. 

While this might sound straightforward, it can get tricky to identify when there are other sources of disease present, such as the following:

• Fungal infections (e.g., powdery mildew, rusts, botrytis)
• Bacterial leaf spots
• Mites and other pests
• Sun scorch from extreme heat

To help with the detective work, the National Center for Atmospheric Research (NCAR) offers a free ozone damage training game. You can test your observation skills across different species by scrolling to the bottom of the page.  

Building Your Own Ozone Garden

Thinking of starting an ozone garden? Here are some pro tips:

1. Sunlight is key. Plants need full sun to trigger their stomata open. (That’s where the gas exchange happens—and where ozone sneaks in).
2. Water regularly. Dry soil = closed stomata = less ozone damage = less useful data.
3. Choose your plants wisely. Go for ozone sensitive perennials that aren’t too aggressive, unless you want milkweed taking over your yard!
4. Prune and stake as needed. Some of these beauties get tall and flop over.
5. Don’t forget to weed and mulch—this keeps the plants happy so you can separate real ozone damage from other stress signals.

Monitoring the Garden: Science in Action

In North Carolina, “ozone season”—the period when our air quality agencies monitor and forecast ambient ground-level ozone concentrations using the Air Quality Index—is March 1^st through October 31^st. While the exact growing season varies from species to species, peak ozone damage usually occurs between July and September. During autumn, natural senescence (leaf die back) begins for deciduous perennial plants, making it nearly impossible to tell if they are losing their leaves because of ozone or this natural process.

Data collection should happen every 1-2 weeks as soon as leaves appear and continue until plants naturally senesce. This should provide the most comprehensive dataset to inform our understanding of how and when ozone affects plants in a certain location. However, our project with the Ciener Botanical Garden didn’t really get started until late May/early June 2025, and we first collected data on July 1, 2025. Here are the dates we’ve been out so far: 7/1, 7/9, 7/18, 7/30, 8/12, 8/20, and 9/5.

At this point, you’re probably wondering, what are we checking for? How do we record data? So far, we have used the NCAR Ozone Garden Network methodology. Every time we collected data, we did the following for each of our plants: 

1. We randomly selected 10 leaves from mid-to-lower parts of the plants. Again, we did not choose newer growth (leaves towards the tops of the plants) because they have not been exposed to the air as long and, therefore, would not show as much or any ozone damage.
2. We used a leaf index scoring method developed by NCAR. This progressive index goes from 1-6, with 1 being no damage and 6 being the most damage.
3. We also made qualitative notes about the plants, such as if there were other signs of damage (like mites or leaf spot disease), what the weather had been like recently, and if the plants were in bloom.
4. Almost every time, we also measured the heights of each plant, which came in handy with data interpretation.

Figure 4. Air Awareness Coordinator, Sarah Coffey, measures the height of a cut-leaf coneflower (Rudbeckia laciniata) at the Paul J. Ciener Botanical Garden’s ozone garden.

There are other, more detailed ways of recording these data, such as using the methods developed by the National Park Service. (See the Ozone Garden Toolkit pages 23-27 to read these methods). These data collection techniques involve tracking the same exact leaves each time rather than randomly selecting any 10 leaves from that plant. By studying the same leaves, it is possible to track the progression from healthy tissue to stippling, chlorosis, necrosis, and senescence, which can be extremely useful to understand how quickly ozone damage occurs in different species.

There are several reasons why we chose the NCAR method over the NPS method:

1. Since it is our first season with the ozone garden, we have been more interested in developing a big picture understanding of how these plants are responding to ambient ozone concentrations before spending the time needed for a more meticulous approach. 
2. It is easier to train others for citizen science purposes, and our ultimate goal is to get the community involved in monitoring these plants. 
3. Lastly, and perhaps most importantly, we might want to join the National Ozone Garden Network and compare what we observe with other ozone gardens across the US, and all participants in this program use these methods.

What Did We Find at the Paul J. Ciener Botanical Garden?

Our first season brought lots of learning:

Only our cut-leaf coneflowers (Rudbeckia laciniata) showed clear ozone damage. Even so, it was minimal. The Southern crownbeard (Verbesina occidentalis) likely would have shown some signs as well, but we planted them very late in the season.

Figure 5. Cut-leaf coneflower leaf index scores over two months of data collection at the Paul J. Ciener Botanical Garden. The crownbeard/yellow ironweed are not shown since they always had a leaf index score of 1.

 

In Figure 5, notice that the average leaf index score increased (from around day 200) and then decreased (by day 220) for all plants. It is possible that this was due to the random selection of leaves during each data collection session, but it might also be explained by the fact that some of the injured leaves that were observed in the earlier weeks of data collection could have fallen off. Then, due to the frequency and abundance of precipitation (conditions which inhibit ozone formation) that we experienced much of the summer, the older growth that remained on the plants did not experience further damage—hence the lower average leaf index scores. In fact, we have not had a Triad Air Quality Alert for ozone all summer!

We are not surprised that V. alternifolia did not show signs of foliar injury from ozone since it was not specifically listed as ozone sensitive. With its ozone-sensitive sister plant, V. occidentalis, only just having been planted at our ozone garden in late August, we may not have enough time during the rest of this growing season to observe the signs of ozone damage. We will surely find out next year, and we are glad to have baseline data for the other plants from 2025.

Another thing to consider is that all five plants experienced some degree of transplant shock this season. Most of the other damage? Fungal and bacterial diseases—again, thanks to a very wet summer. We suspect these mites were also part of the problem:

Figure 6. A mite found on one of the R. laciniata plants at the Ciener Botanical Garden.

We also observed that plants lower on the slope (which received more water) showed more signs of ozone damage—possibly because their stomata stayed open more (meaning they took in more ozone). The plant higher on the hill (with drier soil) may have closed its stomata more often to prevent water loss—less ozone got in. This helped us realize that slope, affecting water distribution, is another factor to consider in garden design.

Figure 7. Ozone garden plant heights over roughly two months at the Paul J. Ciener Botanical Garden. While we did not officially collect data on the degree of ozone injury in June, we did get the height shortly after all plants were in the ground on June 11.

Figure 8. The ozone garden has grown a lot since it was first installed. The photo on the left was taken on June 2nd, and the photo on the right was taken on September 5th. In the coming months, natural senescence (leaf die back) will occur, but being perennials, they'll start growing again in spring.

Why Do Ozone Gardens Matter?

From asthma to crop damage, high ozone levels affect people, wildlife, and native ecosystems alike. Ozone gardens help these unseen and under-discussed problems become more visible in our communities. They're not just pretty—they're powerful tools for education, research, and public awareness.

So, the next time you see stippled leaves on a tulip poplar or a milkweed, you might just be witnessing a live air quality report—no smartphone required. (We still recommend using AirNow to make informed decisions to protect your health).

Want to Start Your Own Ozone Garden?

Check out the Ozone Garden Toolkit from CleanAIRE NC, as well as the NPS and NCAR’s ozone garden resources. Whether you’re a teacher, gardener, student, or scientist, you can be part of the solution—one leaf at a time.

Acknowledgements

Huge thanks to the staff at the Paul J. Ciener Botanical Garden for making this project possible and for regularly tending to the ozone garden! I especially want to thank the Garden Curator, Devin Popenfuss, who readily agreed to implement the garden and bought/installed the plants and signage. We are so excited to continue this partnership in the coming years!

We also want to thank Susan Sachs with the National Park Service and Keith Bamberger with the North Carolina Department of Environmental Quality for introducing us to the topic of monitoring ozone sensitive plants at the 2023 Advanced Ground-Level Ozone Workshop in the Great Smoky Mountains National Park. This project was inspired by their great teaching!