How Are Plants Allergy-Ranked?

OPALS™ was developed based on the following considerations: “What do plants that are well known to cause allergies have in common?” and “What do plants that are well known NOT to cause allergies have in common?” With these two questions it was possible to build two opposing sets of medical-botanical-allergy criteria. One set is positive and one set is negative. Examples of negative criteria: tiny flowers, excerted stamens, small (less than 25 microns in diameter) sized pollen grains, extended bloom period. Examples of positive criteria: complete flowered, sticky, heavy pollen grains, presence of nectaries, brief bloom period.

There are now over a hundred criteria used to develop OPALS™ rankings. Individual landscapers, city arborists, the USDA and the American Lung Association have already use the scale to make better landscaping decisions.
Based on the plant-allergy scale it is now possible to state, for example, that Acer rubrum – ‘Red Sunset’ maple, is ranked number one and causes no allergies. By comparison, ‘Autumn Spire,’ a male cultivar of red maple does cause allergies and is ranked number nine. Most Pine trees will rank at numbers 4 to 5 and will cause some allergy. Platanus species (sycamore) rank number eight, and cause quite a bit of allergy. A male Canary Island Palm, Phoenix canariensis, which is considered one of the worst at a ranking of 10, will produce an abundance of pollen that will cause severe allergic reactions to many living nearby.

Pollen dispersal rates have been measured since 1972 (Gilbert Raynor, NY meteorologist) and although many pollen grains travel far in the air, research shows that most often 99% of a tree's pollen falls out and sticks within fifty feet of the tree. This means that the closer one is to the pollinating tree the greater the exposure. Thus, the job for arborists is to plant trees that will not expose everyone near them to allergenic pollen.

So, How Do You Tell the Boys from the Girls?

It isn’t always that obvious by looking at a tree (especially a young tree) as to whether or not it is a pollen-free female or a pollen-producing male. But since so many city trees are now asexually produced cultivars, the sex is predetermined. In the book Allergy-Free Gardening, which is the result of 15 years of research on this subject, several thousand trees were individually sexed and allergy-ranked. In some cities, pollen control ordinances are already on the books, although most of these could be improved an updated. Albuquerque, New Mexico is particular interesting, since it has attempted to curb allergies by prohibiting the sale and planting of any male cultivars.
As the public grows more knowledgeable about allergy-free landscapes, municipal arborists and landscape specialists will want to be ahead of the curve. They will want to show their clients that they are well-informed on the subject. In the future, instead of planting high allergy-trees, they will need to plan and plant ‘healthy’ urban landscapes.

References:

1.Lewis, Walter H., Airborne and Allergenic Pollen of North America, John Hopkins University Press, Baltimore, Maryland, 1976.
2.Jacobson, Arthur Lee, North American Landscape Trees, Ten Speed Press, Berkeley, California, 1996.
3.Koch, Christopher Von, Allergy, Die Woche, pg. 27, July 7, 2000, Hamburg, Germany.
4.Dworschak, Manfred, Der Spiegel, Feind am StraBenrad, Pp. 174, 175, Nr. 29, 2000.
5.Ogren, Thomas Leo, Turn Back the Pollen Clock, New Scientist, London, Pp. 46, 47, June 3rd, 2000.

 About the Author

Thomas Ogren is the author of Allergy-Free Gardening, Ten Speed Press. Tom does consulting work on plants and allergies for the USDA, county asthma coalitions, and the Canadian and American Lung Associations. He has appeared on HGTV and The Discovery Channel. His book, Safe Sex in the Garden, was published in 2003. In 2004 Time Warner Books published his latest book: What the Experts May NOT Tell You About: Growing the Perfect Lawn. His website: www.allergyfree-gardening.com