The next time you are outside for a walk, take time to closely look at the leaves of the common milkweed plant. They could be telling you something about the quality of the air you are breathing. Common Milkweed Photo from Wikipedia - http://en.wikipedia.org/wiki/Asclepias
How To Identify Ozone Injury
on Eastern Forest Bioindicator Plants (Milkweed)
Ozone [O3] is an air pollutant (in the lower atmosphere) formed from the reactions of hydrocarbons and nitrous oxides in the presence of sunlight. Automobile engines and industrial processes produce most of the compounds that result in ozone. Natural sources of ozone exist, but they are minor compared to those resulting from man-made sources. The ozone in the upper atmosphere is the same chemical compound, but serves as a protective layer that shields the Earth from harmful ultraviolet rays.
There is increasing concern about how lower atmospheric ozone is influencing the health of our forests. One way to assess the impact is to document visible injury on sensitive plant species, known as bioindicators. Bioindicators are plants that show a well defined and consistent response to elevated ground level ozone concentrations in the air. This brochure will help you identify ozone injury on the leaves of sensitive plants such as Milkweed.
List of selected eastern forest bioindicator plants and most common visible symptoms of ozone injury:
Blackberry, secondary canes (Rabus spp.): Red to purple stipple.
Black cherry (Prunus serotina): Red to purple stipple, may drop the injured leaves early.
Common milkweed (Asclepias syriaca): Purple to black stipple, leaves may be chlorotic (yellow).
Yellow-poplar (Liriodendron tulipifera): Brown stipple, may drop the injured leaves early.
White ash (Fraxinus americana): Red to brown stipple. Similar injury is also found on green ash.
Milkweed Slide Show to identify Milkweed Injury
Photos of milkweed species: http://www.monarchwatch.org/milkweed/guide/index.htm
OSUE Weed Guide info: http://www.oardc.ohio-state.edu/weedguide/singlerecord.asp?id=580