Pollen Dispersal And Allergies

Reviewed by Phillip Waite, Ph.D.

I am frequently asked some variation of this question:  "What is the point of making my own yard into an allergy-free oasis, if pollen from my neighbors will just blow in on me anyhow?" I'm glad you asked that!

I have been studying pollen dispersal for the past fourteen years. There is data back to meteorologist Gilbert Raynor, 1972, who measured pollen dispersal from a square mile stand of pure Timothy grass( Airborne & Allergenic Pollen of North America, Lewis W.H., John Hopkins University Press 1979 ). Timothy pollen is well known to be exceptionally light and 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. At half a mile from the field more than 99.9 percent of all the pollen had already fallen out and stuck. Pollen dispersal tests on typical landscape trees such as oak, maple, birch and poplar have consistently shown that more than 99 percent of the source plant's pollen falls out, lands and sticks, usually within 30 feet of the source plant's drip line. It has been casually estimated by a number of pollen scientists that an allergenic pollen- producing tree in your own yard, will expose you to ten times the amount of pollen as would the same tree planted just down the block. The closer the source, the greater will be the total exposure.

Perhaps we ought to clear one thing up. In the spring and summer, and often in the fall too, there is always a certain amount of pollen in the air. We may typically be breathing in several hundred grains of pollen with every breath of air we take. However, if your own yard has some highly allergenic, heavy pollinating trees and shrubs in it, at certain times of bloom you may easily be breathing in several thousands of pollen grains with each breath of air. Directly underneath male mulberry trees (Las Vegas, May 1999) airborne pollen levels have been measured that were over 65,000 grains per cubic yard of air space. Were one to accidentally shake a branch of such a tree, and be close and directly downwind of it, exposure could momentarily easily exceed 1,000,000 grains of pollen per cubic yard of air space.

It is precisely this over-exposure that leads to development of allergy. Allergy always develops from repeated, over-exposure, resulting in eventual hyper-sensitivity. Think of someone with allergies as having a large empty glass. Each day into this glass goes different allergens, cat or dog dander, dust mites, dust, particulates, mold spores, VOCs, diesel fumes, smoke, all manner of allergens. The glass starts to get full. Along comes a huge inhalation of pollen and suddenly the "glass" is not only full, it is now over-flowing. Once this happens the individual gets sick. One can only take so much.

The point of all this, is that we need to limit the total amount of allergens we come in contact with. Avoidance is the real key with allergy. Dr. David Stadtner, the experienced and well-respected allergist, who wrote the Foreword to Allergy-Free Gardening, once wrote me that allergists had many ways to alleviate the suffering of allergies, HOWEVER, everything they did had side affects. "The beauty of allergy-free gardening," he wrote, "is that there are no side affects. It's all positive."

There are of course other benefits to this besides simple avoidance. Female trees in one's own yard will attract and then trap incoming airborne pollen from males of their own species. We could easily think of female trees as our first line of defense. It is, to me at least, quite incredible that the female stigmas, of wind-pollinated dioecious and monoecious species, produce a positive electrical impulse, and that airborne pollen is itself negative. Thus, of course, the pollen is actually attracted like a magnet, right to the female tree, and rendered harmless as it is turned into fruit or seed.

When I first learned of this mutual attraction of the two, I was literally blown away with the wonder of it all. But nature has many such fantastic gifts for us, even if we don't at first discover them. When I drive around my own town of San Luis Obispo, I see so many landscapes that look as though they were designed entirely to cause allergies for whoever lived there. Many times people have killed off their lawns and replaced them with a mass of male juniper bushes, that literally "smoke" with highly allergenic pollen whenever they are brushed against in early spring. I know of one landscape here where half the yard is all in male junipers and the other half is all in male Coyote Brush ground cover, a close relative of ragweed. This landscape will be creating havoc all spring and then again all fall.

I once stopped at the house I just mentioned, to take some photos of the Coyote Brush in full bloom. A man came out of the house and asked me, "What in the world could be worth photographing in my front yard?"  Indeed it was a plain and drab landscape at that. "I'm taking pictures of highly-allergenic landscapes," I told him. "That figures," he said. I talked to him about what I was doing and then asked him if anyone there had any allergies. "Yep," he said, "I have them a little and my poor wife has lots of allergies."

When we get rid of the highly allergenic plants from our own yards we immediately improve our own personal airspace. Yes, pollen will blow in on us but we will never be as exposed as people who have these air-polluters right in their own yards. It is my hope that someday this subject will get the respect I feel that it deserves. Someday I hope the rights of those with allergies will be respected. If a whole block of people got rid of the worst polluting plants, everyone there would be so much healthier. If the whole town did it, the effect would be incredible.

Often people make the mistake of thinking that pollen allergy just causes asthma or hay fever. Over-exposure to pollen has also been shown to cause headaches, tiredness, skin rash, dry, itchy skin, itchy throat, itchy eyes, cough, sore throat, change of voice, loss of taste or smell, plugged or itchy ears, drippy noses, difficulty in breathing, insomnia, and irritability. Many more people actually have allergies than realize it. So, why should we clean up our own yards and replace high allergy plants with allergy-free plants? Why? We should do it, if for no other reason, simply out of self respect.

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Thomas Leo Ogren is the author of "Allergy-Free Gardening" (Ten Speed Press). He has an M.S. in Horticulture and has been published in, among others, New Scientist, American Rose, Pacific Coast Nurseryman, California Landscaping, Environmental Builders News, Awareness and Alternative Medicine. Visit his web site at http://www.allergyfree-gardening.com

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