Late spring is the perfect time to start a garden filled with ozone-sensitive plants and natives that are beneficial to pollinators.
Your garden can tell you about the air you breathe because, like people, plants need clean air, too. Some plants can even tell you when they have been exposed to air pollution. With a few carefully selected plants, some water and sunshine, you can create your own ozone garden that will help you learn more about the air quality in your neighborhood.
An ozone garden is simply a collection of plants that are known to be sensitive
to ozone. When exposed to ozone over time, these plants develop characteristic
signs of injury. Plants that serve as a signal of environmental conditions like air
pollution are called bioindicators.
What is ozone? Ozone is a molecule composed of three oxygen atoms. In the upper atmosphere, naturally-occurring ozone acts as a shield that protects us from the sun’s harmful ultraviolet radiation. But in the lower atmosphere, ozone forms through chemical reactions between nitrogen oxides, volatile organic compounds and sunlight. It is most likely to form during the warm months from May through October, otherwise known as ozone season in Kentucky.
Both animals and plants are vulnerable to the highly reactive ozone molecule. For this reason, ground-level ozone is one of the criteria pollutants with health-based standards that is regulated by the Clean Air Act.
In humans, ozone causes breathing problems and aggravates asthma by causing
acute respiratory irritation and inflammation. Children are most at risk from ozone exposure because they breathe at a faster rate than adults and they are more likely to spend time outdoors engaged in vigorous physical activity. Developing lungs are also more susceptible to damage.
In plants, gases are exchanged through openings called stomata, which open and close in response to different stimuli such as light, heat and humidity. When the stomata are open, ozone can enter into the plant and cause damage to the cells responsible for photosynthesis. Ozone exposure also weakens the plant, making it more susceptible
to disease and insect damage.
Even very low levels of ozone in the air can cause health problems. The National Ambient Air Quality Standard for ozone is set at 75 parts per billion (ppb). The U.S. Environmental Protection Agency is currently considering revising that standard to a level between 65 and 70 ppb. Ozone-sensitive plant species can show signs of injury at even lower levels of exposure—as low as 40 ppb.
How much space do you have? An ozone garden can be as small as a few plants in pots or as big as a large garden containing dozens of plants. Regardless of the size, choose a sunny spot for your ozone garden, ideally one that receives six hours or more of sunlight a day. Generally, it’s safe to plant outside by early to mid-May in Kentucky, when the danger of frost is past.
“What plants you grow depends on your space,” said Roberta Burnes, environmental education specialist with the Division for Air Quality. “For smaller gardens, try bush-variety snap beans, milkweed and evening primrose. If you have a little more space, add cut-leaf coneflower, which grows from 2 feet to 8 feet high. Potatoes, soybeans, squash and melons are also sensitive to ozone,” she said. (See sidebar for a more complete list of plants, including some tree species.)
“Many ozone-sensitive plants grow tall and for this reason an ozone garden would be best located in your backyard, a meadow or field,” Burnes continued.
If you’re growing ozone-sensitive vegetables, any variety will do. “For other plant species, especially milkweed, native species are recommended because they are far more beneficial to native pollinators like monarch butterflies,” said Burnes. Several species of milkweed and its cousin dogbane are native to Kentucky. (See informational sidebar on for suppliers of native seed and plants in Kentucky)
Ozone may be invisible, but the injury it causes to plants is very visible. The first signs of damage resemble tiny specks of pepper on the upper surface of the leaves. These purplish-black spots are known as “stippling,” and they only occur on the upper leaf surface.
Stippling never crosses the leaf veins; if you see spots that cross the veins, you’re looking at damage caused by something else.
“As ozone injury worsens, a leaf may begin to turn yellow—a process called chlorosis. In cases of severe injury, the leaf may die and
fall off the plant,” explains Burnes. “Ozone damage is cumulative, which means you’re more likely to see it on older leaves that are closer to the ground.” Leaf exposure to sunlight seems to play a role as well. A leaf that is partially shaded by another leaf above it may show no damage to the shaded portion, while the fully exposed part of the leaf may exhibit injury.
“As your plants leaf out and mature, check your garden at least once a week. Taking
notes, sketches and pictures of the plants in your ozone garden is a great way to get a
better picture of what’s happening throughout the growing season. In fact, you’ll be joining
thousands of other ‘citizen scientists’ who observe and record their observations of
the natural world in order to better understand the health of their environment,” Burnes
Will you see ozone injury in your garden? “That depends,” said Burnes, “on the ozone concentration over the growing season, plant sensitivity, and how long the plant has been exposed. Since ozone forms in the presence of sunlight, extended periods of hot, sunny weather can lead to higher ozone levels.”
Human activity also plays a big role in ozone formation since the chemical ingredients
for ozone come largely from human activities. Combustion activities such as open
burning, driving your vehicle, and mowing your lawn emit nitrogen oxides. Volatile
organic compounds are emitted when you pump gasoline and use paints or solvents.
Ozone injury is a symptom of revious exposure, so seeing ozone damage doesn’t necessarily mean the air is polluted right now. For nearly real-time data, use the Air Quality Index (AQI) that tells you how clean or polluted your air is, and what associated health effects you may experience within a few hours or days after breathing polluted air. You can access the AQI at http://air.ky.gov/Pages/
“Growing an ozone garden is a way of making the invisible visible,” said Burnes. “By observing your plants through the growing season, as well as keeping an eye on the AQI, you’ll gain a deeper understanding of the connections between chemistry, weather, air quality and living things.”