How to Improve Health and Air Quality through Tree Selection Landscaping to reduce allergies, especially pollen allergies, is an idea that is coming into its own. In past few years several books have been written on subject and interest is growing rapidly. With addition of OPALS™, (Ogren Plant-Allergy Scale) arborists now have a means to design allergy-free plantings. This scale ranks all landscape plant materials on a simple 1 to 10 allergy basis. Trees that produce zero pollen, e.g., female cultivars, usually rank best – number one. Trees that have abundant, highly allergenic pollen, especially those with very long bloom periods, are usually ranked worst – in 9-10 range. There are many trees and shrubs, however, that fall somewhere in between. Using a list of over 100 factors, OPALS™ numerically ranks each species and then further ranks individual cultivars. There are often dramatic allergy differences even between two species in same genus.

How Are Plants Allergy-Ranked? OPALS™ was developed based on 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, USDA and American Lung Association have already use scale to make better landscaping decisions. Based on 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 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 air, research shows that most often 99% of a tree's pollen falls out and sticks within fifty feet of tree. This means that closer one is to pollinating tree greater exposure. Thus, job for arborists is to plant trees that will not expose everyone near them to allergenic pollen.

So, How Do You Tell Boys from 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, sex is predetermined. In book Allergy-Free Gardening, which is 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 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 sale and planting of any male cultivars. As public grows more knowledgeable about allergy-free landscapes, municipal arborists and landscape specialists will want to be ahead of curve. They will want to show their clients that they are well-informed on subject. In 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 Pollen Clock, New Scientist, London, Pp. 46, 47, June 3rd, 2000.