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Allergies and Asthma Can Be Triggered
By Pollen From Urban Landscaping

By Thomas Leo Ogren
Years ago horticulturists and nursery people discovered that with separate-sexed plants, such as junipers, yews, willows, ash, red maples, mulberries, silver maples, poplars, yew pines, box elders, aspens, pepper trees, and all sorts of other species, that if they only propagated with scion wood from male plants; ah, that the resulting trees and shrubs would all be “litter-free.”  They wouldn’t drop any messy fruit or seeds or old seed pods; they would be true “low-maintenance” plants for the modern landscape. These were produced by the hundreds of millions. They sold like crazy and were planted everywhere. The only problem with these wonder plants, these litter-free selections was this: they were all male, every one of them.

When plants are dioecious (separate-sexed) only the female plants make any fruit or seeds—the males, they only produce one thing, and they produce huge amounts of it: pollen. Female plants, which never produce any pollen ever, are a rare find today; but the male plants, fully grown now, they’re all over the place and each year they explode with clouds of highly allergenic pollen. And, of course, along with all of this came great increases in allergies and asthma. Many in horticulture excused this new phenomenon, saying over and over, “pollen just blows in from hundreds of miles away. Nothing you can do about it.” This self-serving proclamation was never true; most allergies are in fact, homegrown.

Researchers have been measuring the distance pollen grains can travel in the air for years. Some pollen grains that are (relatively) very small, dry, rounded in shape, and exceptionally low in specific gravity are able to travel long distances. Nonetheless, most species have pollen grains that are larger, heavier, odd-shaped, and often sticky, and some are be able to travel less than a dozen feet unless picked up by a very stiff wind.

In 1972 meteorologist Gilbert Raynor measured pollen dispersal from a square mile stand of pure Timothy grass (1). Timothy pollen is well known to be exceptionally light, dry, rounded and very buoyant. Raynor found that while he was able to still trap some pollen a mile out from the field, the closer he was to the field, the greater the amount of pollen in his traps. More than 99.9% of pollen grains trapped were on the traps closest to the field itself.

Locality must always be considered with all persistent cases of “hay fever.” Allergy is not triggered by minor exposure to a few pollen grains, but rather is initiated by exposure to a large amount of pollen. This principle is the same for all types of allergens and related allergic responses; allergy is almost always caused by repeated over-exposure(s) to an allergen over an extended period of time. With this in mind, allergens that come from sources that are very close, these are the allergens that are most likely to trigger symptoms. Below, some examples: (8)

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Las Vegas, Clark County Nevada: March, 17, 2009: Clark County Department of Air Quality and Environmental Management stated that: “The department considers mulberry (pollen) levels higher than 1,500 grains per cubic meter of air to be "very high." On this same day they also made this note: “The highest level, 10,887 grains (per cubic meter of air space), was reported Tuesday at Griffith Elementary School, near Meadows Mall.” (2)

In the same article local allergists were interviewed. "Dr. Mulberry is a very good referring physician,'' Dr. Joram Seggev, a Las Vegas board certified allergist and immunologist, said about the number of patients streaming in and out of his office with runny noses and irritated eyes. Dr. Seggev, along with Dr. Jim Christensen, a Las Vegas internal medicine and allergy specialist, said, “the allergy season actually began in mid-February with Arizona Ash tree (pollen) pollutants.” (2)

It is important to note in the above discussion of Las Vegas, that all of the trees causing the allergy referred to here—the male mulberries and the male clonal Arizona Ash selections—100% of them are planted male trees, growing in the area’s planted landscapes. It is useful too, to note, that the highest pollen counts were found in area elementary schools because schools are almost always found to have been landscaped with an abundance of both trees and shrubs that are (“litter-free”) clonal male selections.

Asthma: The pollen from these planted urban trees do not simply trigger episodes of allergy. “Pollen allergies also worsen asthma conditions,” Dr. Seggev said. "Some of my patients only have asthma during the allergy season. I had three such patients today (Tuesday) and their stories are typical. One is asthmatic March through August, and the others February till May. It all depends on what's blossoming." (2)

One last note on mulberry pollen and Las Vegas: “Clark County began monitoring pollen in 1988, and the record mulberry pollen count is 69,224 grains per cubic meter, set on March 2005 at the Griffith station.” (3) Please note here, again, that the “Griffith station” is once more in the news, and the pollen trap at this “station” is located on roof of the Griffith Elementary School! It should come as little surprise that the school has numerous fruitless mulberry trees (and asthmatic students).

One of the cities in the west trying to do something positive about pollen allergies from their planted landscape is Albuquerque, New Mexico. It may be worth mentioning, too, that Albuquerque was successfully sued over the death of a local 6 year old boy, who died after heavy exposure to the City-planted male junipers at the City Zoo. After this lawsuit, Albuquerque passed a Pollen Control Ordinance that forbids the sale or planting of certain plants, including cypress, male juniper and mulberry. The following was gleaned from the City’s Ordinance and reflects an accurate understanding of pollen dispersal and proximity pollinosis: “The majority of tree pollen will likely be deposited within a few hundred feet of the tree. To limit pollen in your home, you should not plant (possibly allergenic) trees near air conditioning, windows, vents, or other openings to your home. It is appropriate to consider your neighbors prior to planting new trees and consider how potential pollen may be an impact.  It is a violation to purchase a prohibited tree outside the city limits, transport the tree to your home and have the prohibited tree planted within the city limits. (5)

An interesting study was done in a large city in Spain in 2006. Pollen traps were set at 48 locations, with 47 of them inside the city, and one outside the city limits. Some of their findings: “Cypress pollen concentrations were much higher at the urban locations than at the fixed trap site. Plane tree pollen levels could be locally very high, reflecting the proximity of the source.” (7)

There are many challenges to landscapes and urban forests—adequate funding, public opinion, insects, drought, disease, unusual cold fronts, storms, industrial pollution, etc.—but it sheds considerable light on this discussion of pollen dispersal and proximity pollinosis to read these next two paragraphs, reprinted here, with permission, from the authors of a recently published study in the Journal of Landscape and City Planning:

 “Perhaps the most serious challenge posed by urban green spaces, though, is related to human allergic reactions to the airborne vascular plant pollen released during pollination. Recent data suggest that people living in urban areas are 20% more likely to suffer airborne pollen allergies than people living in rural areas. This situation has emerged due to several factors, chief among which are the uniformity of green spaces, where a small number of species that have proved highly suited to urban environmental conditions are overwhelmingly used, and the interaction of pollen with air pollutants (Carinanos, Prieto, Galán, & Dominguez, 2004; Carinanos, Galán, Alcázar, & Dominguez, 2007), which can even prompt an increase in pollen production by certain herbaceous species (Ziska et al., 2003).” This same article, referring to the over-abundance of male landscape plants, describes the situation as “botanical sexism.”

Prior to 1950 in most cities dioecious (separate-sexed) street trees were represented by roughly an equal number of pistillate (female) and staminate (male) trees. This however soon began to change. Starting in 1949 with the USDA Yearbook, TREES, an emphasis on planting male street trees was promoted, pushed because the males “do not produce litter." (18)

This "litter-free" or "seedless" trend became so common after 1949 that today there are a number of landscape species where it is now impossible to find any grafted varieties sold that are NOT male. 

The situation became even worse in the 1980’s.  In a 1982 USDA booklet titled "Genetic Improvement of Urban Trees," a method was described whereby male-only trees could also be propagated from the monoecious species; thus there now is not only an overabundance of clonal male trees from naturally separate-sexed (dioecious) species, but additionally there are now many male trees sold and planted from monoecious species that in nature never were unisexual. Because of this booklet and the propagation that followed it, we can now, for example, buy single-sexed (male) honeylocust and male Italian Cypress trees. Outside of the planted landscape, in nature, these trees do not exist.

We now know that higher temperatures from climate change speeds up plants flowering (Bradley et al. 1999; Menzel et al. 2006) and causes an earlier start to the pollen season (Van Vliet et al. 2002). Recently more intensive pollen seasons have been determined (Frei and Gassner 2008), higher amounts of pollen have been measured (Wayne et al. 2002) and stronger allergy to pollen has been ascertained (Beggs and Bambrick 2005; D'Amato et al. 2007). (11)

Air pollution is worst in large cities with heavy traffic. Particulates from chemical air pollution have been found to adhere to intact pollen grains, and also to smaller particles of fractured pollen grains. The resulting pollen is considerably more allergenic. Those city dwellers prone to allergies/asthma are also at increased risk from pollen, because exposure to air pollution increases airway responsiveness to aeroallergens (pollen & particulates). (19) In this regard, pollen today is not the same as it was in our parents’ or grandparents’ time; it now represents a health problem not seen in the past.

Lastly: Most pollen lands close to its source; Dr. Walter H. Lewis, author of Medical Botany, once wrote, “It makes no sense to plant highly allergenic trees or shrubs close to where we live.”  Researchers are becoming ever more aware of the increasing health problems caused by using poor plant selection in the landscape. Unfortunately there’s a serious lag in the awareness of those who do the actual planning and plantings of these landscapes. Most still ignore the newer data, and cling to outdated ideas, ignoring the importance of selecting allergy-free trees and shrubs for urban landscape projects.

The result has been a steadily increasing number of humans, year after year, suffering needlessly from allergies/asthma. Many who now have serious allergies might never have had them at all, had the landscapes near where they lived and worked been designed with a greater emphasis on human health.

It should be noted, that for over a decade now there has been in use a published and well-publicized numerical Plant-Allergy scale, in which most commonly used landscape plants have been rated according to their propensity for triggering allergies/ asthma. The scale, OPALS™, uses a simple 1-10 system, where 1 is completely non-allergenic (usually pollen-free), and 10 is the most allergenic. The scale has been successfully used for over a decade now by many individuals, a few concerned cities, universities, state public health departments, a small but growing number of landscape designers and landscape architects, the American Lung Association, and the USDA Urban Foresters. (20) (21) (22) (10) (23)

Those in landscape design and horticulture who continue to fail to address the link between the typically planted landscape and urban allergies and asthma have much in common with deniers of global warming. They chose to ignore the considerable accumulated data and science involved, and then plead that there is nothing they can do, or that there is nothing they should have to do. But in the end, ignorance is no defense, and it certainly does nothing to solve any of these problems. The health ramifications are perfectly real for millions of adults and children, and will only be solved with the full cooperation of those who do the planning and planting of our city trees.

1. Airborne & Allergenic Pollen of North America, Dr. Walter H. Lewis, John Hopkins University Press 1979

2. Achoo! Mulberry woes in Valley nothing to sneeze at,  Maggie Lillis and Annette Wells, LAS VEGAS REVIEW-JOURNAL, March 18, 2009

3. Pollen hangs thick in the air, Keith Rogers, LAS VEGAS REVIEW-JOURNAL, Las Vegas, Nevada, Mar. 27, 2010

4. Trees and Allergies, Marianne C. Ophardt, Washington State University Cooperative Extension paper, 2002

5. Amendment to the Pollen Ordinance, Air Quality Tree Flyer handout, Albuquerque City Government, Albuquerque, NM 2009.

6.  Genetic Improvement of Urban Trees, USDA Urban Forestry Booklet, Urban Tree Biology, Washington DC, 1982.

7. Differences in the spatial distribution of airborne pollen concentrations at different urban locations within a city. Gonzalo-Garjo MA, Tormo-Molina R, Muñoz-Rodríguez AF, Silva-Palacios I., Journal of Investigative Clinical Allergy, Allergy Section of Children’s University Hospital of Christ, Badajoz, Spain, 2006.

8. Travels of airborne pollen, Eugene Cecil Ogden, Gilbert S. Raynor, Janet V. Hayes, published Washington DC,  by U.S. Environmental Protection Agency, 1975

9. Patterns of pollen dispersal in a small population of Pinus sylvestris L. revealed by total-exclusion paternity analysis, Robledo-Arnuncio JJ, Gil L., Heredity, Ciudad University, Madrid, Spain 2005

10. Urban green zones and related pollen allergy: A review. Some guidelines for designing spaces with low allergy impact, Paloma Carinanos, Manuel Casares-Porcel, Department of Botany, Faculty of Pharmacy, Campus de Cartuja, University of Granada, 18071 Granada, Spain, Journal of Landscape and Urban Planning, December 2010

11. Analysis of changes in flowering phases and airborne pollen, Journal of Environmental Engineering and Landscape Management, The Lithuanian Academy of Sciences and The International Academy of Ecological and Life Protection Sciences, Lithuania, 2010

12. Although not highly allergenic, mimosa has been known to cause proximity pollinosis; it's best to avoid smelling these flowers. See: Pollinosis op proximity, a concept to be taken into account. Pollen as a domestic allergen/

13. Japanese study shows that the prevalence of cedar pollinosis varied not only with the proximity to cedar trees but also with the volume of road traffic. David W. Kennedy, William E. Bolger, S. James Zinreich – 2001, Diseases of the Sinuses, BC Decker, Inc. Ontario, Canada L8N 3K7

14. Insect-pollinated, holly isn't a strong source of pollinosis, but could cause problems for those sensitive when grown in close proximity to living spaces/ The Pollen Library,

15. Aerobiology, Plane tree pollen levels could be locally very high, reflecting the proximity of the source. MA Gonzalo-Garijo, Differences in spatial distributions of airborne pollen concentrations at different urban locations within a city, Children’s’ Hospital, Badajoz, Spain 2006

16. The increasing trend of seasonal respiratory allergy in urban areas, G.D’Amato MD, G. Liccardi, Division of Pneumology and Allergology, Naples, Italy, Allergy 2002

17. Subjects living in urban areas are more likely to experience allergic respiratory symptoms, particularly those induced by pollen allergens, than subjects living in rural areas. North American pollinosis due to insect-pollinated plants. Lewis WH, Vinay P., John Hopkins University Press, 1979

18. Trees; The Yearbook of Agriculture 1949, United States Department of Agriculture, U.S. Government Printing Office, Washington D.C., 1949

19. Outdoor air pollution, climatic changes and allergic bronchial asthma, Exposure to air pollution increases airway responsiveness to aeroallergens, G. D'Amato, Division of Pneumology and Allergology, Hospital A.Cardarelli, Via Rione Sirignano 10, 80121, Napoli, Italy, 2002

20. UFORE, Urban Forest Effects Model, USDA, 2001, Dr. David J. Nowak, project leader, Northeastern Research Station, USDA Forest Service, USDA, Newtown Square, PA.

21. American Lung Association recommends plants which have pollen rating of 6.0 or less, using the OPALS TM scale, University of California at Davis, Florasource, UC Verde, 2009

22. You can control pollen allergies, Asthma and Allergy Foundation of America, Article for public distribution, Fort Worth, Texas, 2007

23. Strategic Plan for Asthma in California, California Department of Public Health, Mark B. Horton, MD, MSPH, Director, Identify the asthma triggers associated with landscaping (for example, pollen as major organic/biologic trigger) and promote low allergen landscaping around public and private properties. Asthmagenic Landscaping ( ), section 5.1.9, 2008-2012, Link:

Bio: Thomas Leo Ogren is the author of six published books and hundreds of newspaper and magazine articles about health, gardening, landscaping, and allergies. His work has appeared in per reviewed scientific journals and in popular print publications such as the New York Times, the London Times, Der Spiegel, The Farmer’s Almanac, and New Scientist. His research on allergies caused by common landscape plants has been aired on ABC, NBC, and other major TV stations, and he’s been interviewed on radio programs across the world, including NPR, CBC, and the BBC. His book, Allergy-Free Gardening (Random House Publishers) is considered the “bible” for many landscape designers who wish to limit pollen exposure. Mr. Ogren’s own website, live for more than a dozen years now, is