Natives and Urban Landscapes There are many native trees and shrubs used in our landscapes. Maples, oaks, locust, poplars, willows, catalpa, birch, junipers, and many more native species are extensively used. Unfortunately plant breeders and propagators discovered how to “sex-out” trees and shrubs. They learned to use only male plants, ironically, as “mother plants,” as source for their scion wood for asexual propagation. First they just used male plants from dioecious (separate-sexed) species, but later they learned how to produce all-male clones from species that in Nature were never unisexual (the monoecious species). For example, Honey Locust trees, (Gleditsia triacanthos) are native to our Southeastern US. Look at these trees in wild and you will see that all of them are almost always covered with long seedpods. But go to a nursery now and look at Honey Locust trees for sale. The ones on sale now are called “seedless” and they are in effect, all-male clones. What exactly is effect of using all male cloned trees and shrubs in our landscapes? Very simply, this translates to an excess of allergenic pollen. Only male flowers produce this airborne pollen. Unisexual female flowers produce no pollen.

Why Emphasis on Male Plants? Horticulturists knew that female plants produced seeds, seedpods, and fruit. This “litter” fell on sidewalks and created a “mess.” By using only asexually (no sex involved) propagated cultivars (cultivated varieties), they were able to create “litter-free” landscapes. These required less maintenance and were (and still are) very popular with city arborists and public. In US today, four of five of top-selling street tree cultivars are male clones. Female flowers (pistillate) on female trees or shrubs produce an electrical (-) current. Their stigmas are broad and sticky. Airborne pollen from male plants has a negative electrical impulse before release and a positive charge after release, and this pollen is light and dry. Because of + and – electrical charges pollen and stigmas are drawn to each other. They are mutually attractive. Mother Nature saw to it that pollen would land, and stick, exactly where it was needed. Female plants are nature’s pollen traps, our natural air-cleaners. Today though, most of female plants are long gone from our landscapes. The pollen from males floats about, seeking a moist, sticky, positive-charged target. We humans emit a positive electrical charge, and our mucus membranes, our eyes, skin and especially linings of our nose and throat, now trap this wayward pollen. We have become targets Allergy develops from repeated over-exposure to same allergens. If your own yard is full of pollen-pumping trees and shrubs, you and your family are ones who will be exposed most.

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.