A Plan for Cities Wishing to Reduce Pollen-Allergies and Related Allergic-Asthma

With the hundredth anniversary of Chelsea Flower Show upon us, Tom Ogren of
www.allergyfree-gardening.com sets out his plan for cities wishing to reduce pollen allergies and related allergic asthma.

birch treeThe problem:

Rates of allergies and asthma both continue to grow worse in cities around the world. Modern landscaping (with its extensive use of newer clonal horticultural male selections of “litter-free” trees or shrubs) is often highly allergenic. Pollen levels in the city are high and getting worse. Compounding this problem, due to global warming and pollen’s interaction with urban air pollution, pollen production today is greater, and individual (urban) pollen grains are far more allergenic than ever before. Many of the very most allergenic landscapes are at area elementary schools. The current situation is no longer sustainable.


Having a pollen-allergy is frequently a precursor to the development of asthma. More than 80% of those with asthma also have pollen-allergies. Allergies are also the most common trigger that initiates episodes of asthma.

The solution:

These recommendations below will not “cure” pollen-allergies or asthma, but they will greatly reduce many of the triggers, they will reduce the symptoms people have, and unlike many drug therapies, there are no negative physical reactions to these proactive programs.

Any one of these recommendations will help to reduce exposure to urban pollen; the more recommendations a city decides to apply, the better, and the quicker, the results will be.

Action # 1.

Follow the innovative lead of cities such as Albuquerque, New Mexico, and enact a tough local Pollen Control Ordinance, one with some teeth in it. Ban the sale and planting in city limits of the very most allergenic plants. Fruitless (male) mulberry trees, male box elder clones, male junipers and male yews should be on every list... however, in each area, given its own geography and climate, there will be other highly allergenic trees and shrubs that should also be included. (Example, in Los Angeles, olive trees, cypress and male Podocarpus should be included in the ban.)

Action # 2.

A plan should be approved and set in motion so that each year a certain percentage of the very worst, the most highly allergenic trees and shrubs will be removed, and then immediately replaced with allergy-friendly choices. The more ambitious this plan is, the more effective it will be.

Action # 3.

Because of the horticultural practices of the last 40 years almost every modern city now has far more male plants than females. Male plants shed large amounts of allergenic pollen; female plants trap pollen and shed no pollen. The current sexual imbalance is not healthy or sustainable. Because of systemic reluctance toward removing already growing trees, it is not expected that enough males will be removed, therefore: a serious programme should be initiated, where every year a large number of known female trees (and shrubs) are planted with the aim of eventually achieving some form of natural balance of the sexes. As with action # 2, the more aggressively this is done, the sooner positive results will be seen.

Action # 4.

Species biodiversity is a worthy goal, but with certain allergenic species, diversity of that particular species is, from a human health perspective, a flawed goal. For example, birch trees (Betula species) are well known to cause pollen allergies and birch pollen cross-reacts with many other species, causing an extended negative effect. Each species of birch tree has a short bloom period when it sheds pollen.

However, if numerous different birch species are planted in a city, as each species will bloom in its own time, this effectively greatly extends the time of exposure to this pollen. With birch and similar allergenic tree species, oaks for example, it is recommended that if any more are planted at all, that only one or at the very most two species be used.  

Action # 5.

Because insects make up so much of the diet of virtually all song birds, allergy-friendly trees that provide food for song birds should be planted in much larger numbers. The importance of wild birds in allergy/asthma reduction should not be underestimated. Insect pests often infest landscape trees and shrubs; if unchecked their numbers soon explode and the plants become covered in insect dander and sticky secretions (“honeydew”). Airborne mold spores land on these secretions and quickly proliferate.

The result is a tree or shrub that is now highly allergenic. Song birds are our best defense against these pests (usually some species of aphids, whitefly, scale, and or mealybug) and keep their numbers in check. Each year cities should plant a good number of plants specifically to attract and feed these wild native song birds. Many fruiting native species in particular are useful for attracting wild birds. (It should be noted that male trees & shrubs produce no food for wild birds.)

Action # 6.

In many cities there is at least one common weed that triggers more than its share of pollen allergy. Each city should consider a long range plan for eradication of these. Many decades ago, the city of Montreal, Canada, undertook just such a programme. They enlisted the help of school children, boy and girl scouts, church and other civic groups. The city posted and passed out fliers with tips on identification at all stages, and within seven years 100% of the ragweed in Montreal had been eradicated. For many years after this Montreal was widely advertised as the place to go in the fall to escape ragweed allergy. (Alas, in subsequent generations Montreal became lax about this, and now has a great deal of ragweed.)

In each area there may be a different noxious weed species to focus on. In San Diego and in Santa Barbara, California, for example, there is little ragweed, but castor bean (Ricinus communis) has become a noxious invasive weed, and its pollen is a prime asthma trigger.

Action # 7.

Lawns are popular in almost all cities, but some species of lawn grasses cause more allergy than others. With the exception of the mild-winter area grass, Common Bermudagrass, almost all other types of grasses will not flower and release pollen if they are kept mowed on a regular basis (hybrid Bermuda grasses will not flower if regularly mowed). Many cities already have existing regulations requiring property owners to keep their lawns mowed, however the focus of this has been on tidiness, on beauty, and those are important considerations.  However, regulations on keeping lawns mowed, in order to keep lawn grasses from getting too tall and releasing allergenic pollen should also be enforced as a public health issue. Stricter regulation of this would cut down considerably on grass pollen in urban areas.

In addition: whenever possible, use of female clones of grasses such as buffalograss should be encouraged. These cultivars use very little water, require less fertilization, and they produce no pollen. There are now selections of female buffalograss (Buchloe dactyloide) suitable for almost all areas.

It should also be understood that a thick, healthy, regularly mowed lawn makes an excellent trap for pollen that comes from nearby trees or shrubs. When this pollen lands on the lawn, it eventually gravitates down below the surface of the grass, and effectively is taken out of circulation.

*Note: to identify the most allergenic trees or shrubs (and the least allergenic also) OPALS™ (Ogren Plant Allergy Scale) can be a useful tool. OPALS™ has been in available to the public since 1998 and is used by many allergists and numerous organizations including the USDA Urban Foresters and numerous state and local offices of the American Lung Association. Its use is endorsed by the Public Health Department in the most current Strategic Plan for Asthma in California. 

"All plants, just like all people, are not created equal. The very best treatment for allergy is to avoid the offending substance." Allergist David A Stadtner, MD, Stockton, California

N.B. You can find all of Tom's books on allergen-free gardening here on Amazon in the UK and here on Amazon in the US.


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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

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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 (www.allergyfree-gardening.com ), section 5.1.9, 2008-2012, Link: https://www.cdph.ca.gov/programs/caphi/Documents/AsthmaStrategicPlan.5-5-08.pdf

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

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 

First published in 2009

Articles on allergen-free gardening



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