Plants Need Clean Air, Too


A Bay Weekly reader asked me why his Heritage birch was dropping its leaves despite the fact that it was under irrigation. The answer was simple: air pollution.

The Heritage birch is a clone of river birch, and river birch trees are extremely sensitive to ozone and sulfur dioxide. Since the middle of June, we have experienced several days of Code Orange, and in early July we have also experienced Code Red. This means that air pollutants are sufficiently high to affect humans, and the foliage of river birch trees is even more sensitive to air pollutants than humans.

If the air pollution had occurred at night, the plants would not have been exposed. At night, the stomata — openings in the leaves that allow for the exchange of carbon dioxide and oxygen — are closed, preventing polluted air from entering and damaging the tissues within. Once inside the leaves, the polluted air kills the conducting tissues that allow water to enter the leaves. Thus the leaves turn yellow and die, then fall to the ground. 

If you examine an affected tree closely, you will notice that it is the older leaves that are primarily affected. The stomata in younger leaves close quickly, thus preventing the pollutants from entering. The stomata of older leaves are sluggish, thus allowing the pollutants to enter and damage tissues.

Vegetable gardeners may notice that the leaves of summer squash have a silvery sheen. This is also a symptom of air pollution injury, especially if the air pollutant is sulfur dioxide.

Had the air pollution alerts occurred in mid-May to early June, you would have noticed white pine trees developing short yellow-green needles. But since the needles are now fully extended, air pollution will cause early needle cast in mid-August to early September. White pine is a good indicator of how clean the air is. Matter of fact, you will not find white pine trees growing in downtown Baltimore or Washington, D.C., because of high levels of air pollution.

Another good indicator of the absence of air pollution are lichens — organisms composed of algae and fungus. Lichens grow on the north side of trees and on rocks, but only in regions of clean air. They tend to be gray-green and appear like scales or flattened discs. If they are present in abundance, it means that the air is clean. If they are absent, it means that the air is polluted.

Name the Mystery Plant

Q: This is a cutting from a large shrub or a big plant. The owner of the plant does not know what it is. It came with the new home. He says it blooms like crazy each year, and when I saw it, it was loaded with these flowers. Can you tell me what it is and where I can get it?

Someone told me it was a Martha Washington, but when I looked it up the info said Martha was an annual. 

–Vicki Marsh, Deale


A: It’s a hibiscus, and from the looks of the flower color, I believe that it is Lord Calvert, which is hardy and now blooming mightily in this region.

Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.