Well, I posted this some time ago in the "news" forum...& I just thought that mebbe y'all might enjoy it as well....
This is from
http://www.conciouschoice.com
June 2000
Safe Sex Landscaping
by Thomas Ogren
Female trees and shrubs in the landscape produce powerful symbiotic effects on local air quality.
Modern landscapes are heavily loaded with male-only trees and shrubs, favored by landscapers because they are litter-free. Dioecious (separate-sexed) plants are individually either male or female. Since male plants do not produce any seed, pods, or fruit (that’s the female’s job), males have long been considered to be the most maintenance-free landscape plants available. What has been completely overlooked however, is the fact that these male plants do indeed produce litter. Male plant "litter" is called pollen! Dioecious male trees shed huge amounts of allergenic airborne pollen, which causes allergies. Dioecious male shrubs also produce abundant allergenic pollen. Dioecious female trees and shrubs, on the other hand, produce flowers, seeds, and fruit, but they shed no pollen. Female-only plants do not have stamens — the male pollen-bearing sexual parts — and so produce no pollen at all.
In nature there is a balance between dioecious males and females, with roughly equal percentages of each sex. In urban landscapes, however, the ratio is now usually 90-95 percent male, and 10 percent or less female. With some urban landscape species, male clones (male plants produced through asexual propagation, not fertilization) now represent fully 100 percent of the landscape plants utilized. The result has been a constant rise in total urban pollen loads, and a corresponding rapid rise in the numbers of people affected with pollen allergies.
Airborne pollen floats, lands on dry surfaces, and then becomes airborne again with the slightest breeze. Individual pollen grains are so tiny that they can not be seen with the naked eye. To view pollen grains individually requires magnification of 1,000 power or more. Pollen grains are so small that they easily can pass through the tightest window screens. The grains of windborne pollen are light and dry and are negatively charged. Like heat-seeking guided missiles, these tiny, dry pollen grains — often shaped like a sharp-spined, minute ball of cactus — seek out moist, receptive surfaces.
Mother Nature designed female plants to receive these grains of pollen. In dioecious species (some examples are ash, poplar, willow, cedar, juniper, cottonwood, mulberry, osage orange, xylosma, yew, box elder, podocarpus, fringe tree, holly, pepper tree, smoke tree, coffee tree, sassafras, and maple; there are thousands more) the females act as specialized pollen traps. Each female tree has thousands of pistillate flowers, each one designed by nature as a highly efficient collector for the airborne male pollen of that particular species.
Female flowers stand up in the wind. With their large positively charged surface areas of moist stigmas, they attract, hold, absorb, and ultimately use the male pollen. Female plants are nature’s air-scrubbers, trapping ambient pollen grains and leaving the surrounding air free of this allergenic form of biopollution.
In the spring of the year in almost any modern city, we are all breathing in several hundred grains of pollen with each breath of air we take. In some areas people are breathing in thousands of pollen grains with every breath. The female trees and shrubs that used to clear the air of pollen are no longer used, and instead, the dry pollen grains land and stick on other moist, receptive surfaces: our eyes, skin, mouths, throats, noses, and mucous membranes. In the sterile male-predominant modern urban landscape, humans have replaced the female plants, and we are now the most effective natural pollen traps.
Interest in the connection between allergy and urban landscapes has increased recently. This has been driven by several factors. First, allergy rates are rising at an epidemic pace. This is true in the United States and in all cities worldwide. Twenty-five years ago only 10 percent of the U.S. population suffered from allergies. Today the official figure is 38 percent and rising. Second, deaths from asthma, long ago almost unknown, are now becoming common. Seven thousand people are expected to die in the United States in the year 2000 "from complications due to asthma." A very high percentage of the asthma victims will be children.
Years ago I found that simply describing all plants as either high-allergy or low-allergy is a vast oversimplification. There are many fine plants that are capable of causing some allergy, but rarely serious allergy, like asthma. There are other plants that can cause allergy but only from contact with the sap, or from their fragrance.
Certain species of plants were found to cause allergy but only to very few people. After years of research it became more and more evident that what was needed was a numerical scale, one that would give an accurate prediction of each plant’s potential to cause allergy.
The scale that emerged was built on a foundation of two fundamental questions. First, "What do plants that are known not to cause allergies have in common with each other?" And second, "What do plants that are well known to cause allergies have in common?"
Using these two questions, two large sets of allergy criteria factors were established, one set with positive values, and the other with negative values. Eventually the criteria lists included more than seventy individual factors, and these were weighted according to importance. Not all factors apply to each plant evaluated.
The criteria lists allowed me to develop a scale that measures the allergy potential of all garden and landscape plants. The new, trademarked scale is called OPALS, or Ogren Plant Allergy Scale. Using a simple one-to-ten ranking system, it lists the safest, least allergenic plants at one, and the worst, most allergenic at ten.
For instance, a completely pollen-free tree like red maple "autumn glory" ranks at one (the best). Double hollyhocks are ranked two, and single hollyhocks are ranked three. The common shrub abelia ranks at five, while Japanese boxwood is ranked seven. Few realize boxwood even has flowers, but it does cause allergy unless it is kept closely clipped.
At the high allergy end are plants like bottlebrush, ranked nine, and staminate pepper trees (the ones without the berries) ranking ten, the most allergenic.
An astute landscaper can now look over a customer’s yard and with a high degree of reliability, give the entire landscape a numerical ranking. Any landscape that would rank five or less would be considered to be a fairly low allergy landscape, but in the near future, many customers may demand gardens that rate even better; fours or threes or even lower. It is actually possible to create an entire landscape now, from the lawns to the trees, that ranks at one. Such a landscape would in effect be completely pollen-free, and would also not pose any risks from contact allergy, or from what allergists often refer to as negative-odor challenges.
In 1999 the United States Department of Agriculture (USDA) Urban Foresters from Northeastern Research Station, Syracuse, New York, led by David Nowak, Ph.D., adopted the OPALS scale for use in formatting pollen-allergy projections. The USDA is combining its own very extensive species bio-mass data with the OPALS allergy numbers. The results will be released as individual allergy projections for all major U.S. urban areas.
A number of quite significant changes may be expected soon because of OPALS. As well-educated customers become aware of the scale, everyone in the horticulture trade will also be expected to understand it.
Many currently popular plants that are high-allergy rated, will quickly fall out of fashion, and demand for them will drop dramatically. Plant material that ranks favorably, plants in the one-to-three range in particular, will rapidly gain in popularity, and demand can be expected to initially outstrip supply.
Local and state government organizations can be expected in many cases to stipulate that all of their new landscapes conform to the best OPALS rankings. Communities that have promoted this kind of landscaping will advertise it, and will attract people who wish to live in areas with low pollen-allergy potential. In some cases this will affect the actual sales value of area real estate. It will be found, as more urban areas are evaluated with these data, that one part of town may have much lower allergy levels than another part of the same town.
In the future it can well be expected that certain nurseries will only sell plants that are individually tagged with the OPALS numerical ranking. At least one large Web-based nursery is already in the process of doing just this. Other nurseries can be expected to follow suit, and in the near future, savvy customers may refuse to buy any nursery material that isn’t tagged with a numerical allergy rating.
Thomas Ogren is a former landscape gardening instructor, nursery owner, gardening radio show host, and author, with an M.S. in agricultural science. His first book, Allergy- Free Gardening (Ten Speed Press), provides a thorough introduction to this theory of landscaping. The complete OPALS scale is in Allergy-Free Gardening, where more than 5,000 plants are individually allergy-ranked. It can be ordered in all area bookstores or directly from the publisher, Ten Speed Press, at 800-841-2665. Portions of this article are excerpts from his next book, Safe Sex in the Garden, scheduled for publication by Ten Speed Press in March of 2001. For more information e-mail tloallergyfree@earthlink.net.