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Allergies, Asthma And City Trees
By Thomas Ogren, Fri Dec 9th
Allergies, and City Trees Thomas Leo Ogren Some urban tree species cause an inordinate amount of andallergy problems, while other tree species cause little or nohealth problems. A large part of the problem is that thearborists and landscape professionals, who plant these trees,often don’t know the difference. The type of trees (and shrubs)used in modern city landscapes has changed dramatically in thepast three decades. In the past, the majority of street treesused were perfect-flowered, insect-pollinated trees, such as theonce so common American elm tree. Today though, many of the mostwidely used city trees are wind-pollinated species. Most ofthese species are unisexually flowered (dioecious and/ormonoecious) and further compounding the problem, thousands ofpopular cultivars sold today are touted to be “seedless,”“low-maintenance,” “pod-free” or “litter-free.” These fruitless,seedless trees are male plants, all male, and male trees produceprodigious amounts of allergenic pollen. Female trees produce NOpollen what so ever. In dioecious-flowered trees such as mostash, willow and poplars, it is easy to propagate male only treesbecause they are separate-sexed. Monoecious trees, which inNature always have both sexes (male and female flowers) on thesame tree, also usually produce abundant pollen. It is possibleto have all-male trees from the monoecious species. On manyspecies the sexes will be born on separate branches, such as ona Honey Locust tree. If you take cuttings, or budwood, only fromthe branches with male flowers, then, you'll get an all-maletree. Lots of monoecious Acer spp. cultivars are male-onlyplants. In a somewhat different way, there are also numerousmonoecious species where only the top or only the bottom willhave either male or female flowers. For example, the bottom halfof a mature Italian Cypress for example is all-male. Female woodis found only at the top of the plant. Thus, scion wood takenfrom the bottom usually produces "seedless" plants. The terms“dioecious,” and “monoecious,” are botanical terms, nothorticultural terms. We could perhaps say that a manipulated,asexually propagated all-male cultivar, taken from a monoeciousspecies, is now “dioecious,” but this is not quite correct. Aproper dioecious tree is one that in Nature is separate-sexed.These modern engineered trees are never found in Nature.Interestingly, the first reference in print I ever found to thissingle sexing-out with monoecious street trees, was in a USDAbooklet, from 1982, called “Genetic Improvement of Urban StreetTrees.” How Bad Is Allergy Now? In 1959 the official rate ofallergy in the U.S. was between 2 to 5% of the public. By 1999the official rate of allergy had increased to an incredible 38%of Americans. Asthma, which was once considered rare, is now thenumber one chronic childhood disease in the US.
Where are Allergies and the Worst? The most commonallergen of all is pollen and since there are so many moreplants growing in the country than in the city, it would makesense then that there is more allergy and in thecountryside. Right? No, wrong! Allergies and are farworse in the city than they are in the country. Several things contribute to this: 1.Pavement makes a poorpollen trap. Pollen in the city often lands on pavement wherewind can cause it to become airborne again. In naturallyvegetated areas where there is much more vegetation, pollenoften lands on and becomes stuck in grasses, shrubs and vines orin trees. 2.Cities have more air pollution, which weakens theimmune system and lung function. 3.Stress, which is generallyhigher in cities, can contribute to both and allergydevelopment. 4.Increased carbon dioxide levels within citiescauses pollen-forming plants to produce more pollen with eachbloom cycle, and also often causes urban plants to bloom moreoften. 5.Pollen loads are actually far greater in cities becausethere is a sexual imbalance within the plant community. In thecity there is a preponderance of male trees and shrubs, while inthe rural areas there is almost always a complete balance ofplant sexuality. The excess of male plants in the city resultsin an excess of pollen. 6.The very lack of female plantmaterials in the urban environment also is a prime factor in theepidemic of allergy and asthma. Female flowers carry anelectrical negative (-) charge (the trees are grounded withtheir roots) and airborne pollen holds a positive (+) charge.The tree and the pollen are mutually attractive; thus a femaleplant becomes a very effective pollen trap for pollen of its ownspecies. But with almost no female trees and shrubs in modernlandscapes, most of the pollen remains airborne. How to Improve Health and Air Quality through Tree SelectionLandscaping
to reduce allergies, especially pollen allergies, isan idea that is coming into its own. In the past few yearsseveral books have been written on the subject and interest isgrowing rapidly. With the addition of OPALS™, (OgrenPlant-Allergy Scale) arborists now have a means to designallergy-free plantings. This scale ranks all landscape plantmaterials on a simple 1 to 10 allergy basis. Trees that producezero pollen, e.g., female cultivars, usually rank the best –number one. Trees that have abundant, highly allergenic pollen,especially those with very long bloom periods, are usuallyranked the worst – in the 9-10 range. There are many trees andshrubs, however, that fall somewhere in between. Using a list ofover 100 factors, OPALS™ numerically ranks each species and thenfurther ranks the individual cultivars. There are often dramaticallergy differences even between two species in the same genus. How Are Plants Allergy-Ranked? OPALS™ was developed based on thefollowing considerations: “What do plants that are well known tocause allergies have in common?” and “What do plants that arewell known NOT to cause allergies have in common?” With thesetwo questions it was possible to build two opposing sets ofmedical-botanical-allergy criteria. One set is positive and oneset is negative. Examples of negative criteria: tiny flowers,excerted stamens, small (less than 25 microns in diameter) sizedpollen grains, extended bloom period. Examples of positivecriteria: complete flowered, sticky, heavy pollen grains,presence of nectaries, brief bloom period. There are now over ahundred criteria used to develop OPALS™ rankings. Individuallandscapers, city arborists, the USDA and the American LungAssociation have already use the scale to make betterlandscaping decisions. Based on the plant-allergy scale it isnow possible to state, for example, that Acer rubrum – ‘RedSunset’ maple, is ranked number one and causes no allergies. Bycomparison, ‘Autumn Spire,’ a male cultivar of red maple doescause allergies and is ranked number nine. Most Pine trees willrank at numbers 4 to 5 and will cause some allergy. Platanusspecies (sycamore) rank number eight, and cause quite a bit ofallergy. A male Canary Island Palm, Phoenix canariensis, whichis considered one of the worst at a ranking of 10, will producean abundance of pollen that will cause severe allergic reactionsto many living nearby. Pollen dispersal rates have been measuredsince 1972 (Gilbert Raynor, NY meteorologist) and although manypollen grains travel far in the air, research shows that mostoften 99% of a tree's pollen falls out and sticks within fiftyfeet of the tree. This means that the closer one is to thepollinating tree the greater the exposure. Thus, the job forarborists is to plant trees that will not expose everyone nearthem to allergenic pollen. So, How Do You Tell the Boys from the Girls? It isn’t alwaysthat obvious by looking at a tree (especially a young tree) asto whether or not it is a pollen-free female or apollen-producing male. But since so many city trees are nowasexually produced cultivars, the sex is predetermined. In thebook Allergy-Free Gardening, which is the result of 15 years ofresearch on this subject, several thousand trees wereindividually sexed and allergy-ranked. In some cities, pollencontrol ordinances are already on the books, although most ofthese could be improved an updated. Albuquerque, New Mexico isparticular interesting, since it has attempted to curb allergiesby prohibiting the sale and planting of any male cultivars. Asthe public grows more knowledgeable about allergy-freelandscapes, municipal arborists and landscape specialists willwant to be ahead of the curve. They will want to show theirclients that they are well-informed on the subject. In thefuture, instead of planting high allergy-trees, they will needto plan and plant ‘healthy’ urban landscapes. References: 1.Lewis, Walter H., Airborne and Allergenic Pollenof North America, John Hopkins University Press, Baltimore,Maryland, 1976. 2.Jacobson, Arthur Lee, North American LandscapeTrees, 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 amStraBenrad, 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 SpeedPress. Tom does consulting work on plants and allergies for theUSDA, county coalitions, and the Canadian and AmericanLung Associations. He has appeared on HGTV and The DiscoveryChannel. His book, Safe Sex in the Garden, was published in2003. In 2004 Time Warner Books published his latest book: Whatthe Experts May NOT Tell You About: Growing the Perfect Lawn.His website: www.allergyfree-gardening.com
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