Ozone Gardens

A group of interns inspecting evidence of ozone damage in an ozone garden in Great Smoky Mountains National Park
Students have been observing ozone sensitive plants at an ozone garden in Great Smoky Mountains NP for more than 15 years.

Can you look at plants in a garden to learn if the air you breathe is clean? Students and teachers have been doing just that by planting and monitoring ozone gardens in national parks around the country.

By carefully observing the leaves on plants in ozone gardens, these budding scientists have been tracking changing ozone levels in national parks. When ozone levels are high, garden plants develop leaf damage over time, such as yellowing or black spots. Just as a doctor checks your pulse, we can look at plant leaves for signs of illness from air pollution. Ozone gardens have been established at Great Smoky Mountains and Rocky Mountain National Parks.

How ozone affects plants

Ozone in the upper atmosphere forms a layer that absorbs the sun's harmful ultraviolet rays and protects life on Earth. At ground level, ozone is harmful to human health and the environment. Ground-level ozone does not come directly from smokestacks or vehicles, but instead is formed when other pollutants, mainly nitrogen oxides and volatile organic compounds, react in the presence of sunlight. Ozone levels are generally highest during the warmer months of the year and peak during the middle to end of the day.

As plants “breathe” (respire), pollutants enter their leaves. Ozone is a highly reactive molecule, and once inside a leaf, it can damage plant tissues, making it harder for plants to produce and store food. Not all plants are sensitive to ozone pollution, but those that are will grow slower and can be less resistant to disease and insect infestation. Plants that are highly sensitive to ozone are all around us — ponderosa pines, quaking aspens, red maple, cut-leaf coneflower, skunkbush, and milkweed. In fact, over 130 ozone sensitive plant species have been identified in parks. Check out the full list or search by park to learn more.

What does ozone damage look like?

Milkweed leaves

Three leaves from milkweed plants showing increasing levels of ozone damage from left to right. Credit: Robin Rohrback, Adapted by American Geosciences Institute

Plant leaves can be damaged by frost, insects, or fungus but ozone damage has some characteristic clues that you can look for. Ozone injury starts out as just a few angular spots or “stipples.” These stipples are only visible on top of the leaf. The undersides of leaves remain free of visible injury. In addition, ozone injury does not appear on leaf veins or veinlets. Older leaves at the base of plants show more injury because they have been exposed to ozone the longest.

Try finding ozone damage on plants in your area. The best time to look for ozone damage is the later part of the summer season. The stipple areas are quite prominent and may appear dark purple, red, or tan to dark brown, depending upon the plant species.

Why gardens?

Rocky Mountain National Park ozone garden

Ozone garden at Rocky Mountain National Park, Colorado

Planting ozone gardens increases opportunities for citizen scientists and park staff to observe and document plant responses to ozone over the course of a growing season. Planting multiple sensitive species that may not usually grow near one another in one place allows park managers to understand parkwide responses to ozone pollution. It also gives visitors the opportunity to see bioindicators of current air quality. When visitors can interact with data collection, they are more likely to connect human action and environmental response.

Well designed ozone gardens include:

  • Ozone sensitive plants that are native to the area
  • A location that receives the amount of sunlight needed for ozone sensitive plants
  • Access to water and a plan to keep the garden watered once or twice a week in drier climates

Note, gardens located near heavy traffic, parking lots, or other pollution sources may have more ozone than locations that are behind belts of trees or open green spaces.

How to monitor

Inspecting ozone damage

National park employee examining signs of ozone damage.

Monitoring for ozone damage to sensitive plants in a garden can be irregular and informal or quite structured. Informal monitoring may consist of periodic observations, photographs, and notes. More formal monitoring will record the percentage of ozone damage on the leaf surface from a set number of plants and leaves regularly during the growing season. Ozone injury training with instruction on identifying percent damage is available through the Hands on the Land webpage.

Last updated: January 31, 2019

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