How Female Trees & Shrubs in Modern Landscape
 Are Affecting Our Air Quality
By Tom Ogren



 Female trees and shrubs in the landscape produce powerful symbiotic effects on local air quality. Dioecious (or separate-sexed) plants are individually either male or female. Dioecious male trees shed huge amounts of allergenic airborne pollen causing a great deal of allergy. Dioecious male shrubs, likewise, also produce abundant allergenic pollen. 

Modern landscapes are heavily loaded with these male-only trees and shrubs, favored by landscapers because they are "litter-free." Because male plants do not produce any seed, seedpods, or fruit, males have long been considered to be the most choice landscape plants available.

 What was completely overlooked however, was the fact that these male plants do indeed produce litter. Male plant "litter" is called 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 fifty-fifty of each sex present. In urban landscapes, however, the ratio is now usually 90-95 percent male, and 10% or less, female. With some urban landscape species, male clones now represent fully 100% 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. 

In dioecious species (think: ash, poplar, willow, cedar, juniper, cottonwood, mulberry, osage orange, xylosma, yew, box elder, podocarpus, fringe tree, holly, pepper tree, smoke tree, coffee tree, sassafras, maple, and thousands more), the females of each species act as specialized pollen traps. Each female tree will have thousands of pistillate flowers, each one designed by nature as a highly efficient collector for the airborne male pollen of that particular species. 

Airborne pollen floats around, lands on dry surfaces, and then becomes airborne again with the slightest breeze. Individual pollen grains are so tiny they can not be seen with the naked eye. To view pollen grains individually, magnification of a 1000 power or more is required. Pollen grains are so small they easily can pass through the tightest window screens. The grains of windborne pollen are light and dry and are negatively electrically charged. Like heat-seeking guided missiles, these tiny pollen grains, each often shaped like a sharp-spined minute ball of cactus these irritating dry pollen grains seek out moist, receptive surfaces. 

Mother Nature designed female plants to receive these grains of pollen. Female flowers stand up in the wind, and with their positively-charged, perfectly-designed large surface areas of moist stigmas, they attract, hold, absorb, and ultimately use this male pollen rain. 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 of today, 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, our skin, in our mouths, throats, and noses, in our mucus membranes. In the sterile male-predominant modern urban landscape, humans have replaced the female plants, and we are now the most natural effective pollen traps. 

Before 1950 most of the billions of trees that made up the U.S. urban forest were seedling-grown trees. A great number of these trees ash, box elders, many maples, ginkgo, aspens, poplars, cottonwoods, mulberries, pepper trees, junipers, willows, and other species were dioecious, or separate-sexed, trees. Since these were mostly seedling-grown trees, about half of them would have been male and half female. 

Material from a remarkable new book, "Allergy-Free Gardening", by Thomas L. Ogren, from Ten Speed Press, has exposed the root causes of the alarming huge increases in hay fever and asthma.

Starting in 1949 with the USDA Yearbook, TREES, an emphasis on planting male street trees was promoted, pushed because the males did not produce "litter." This "litter-free" or "seedless" trend became more and more common and today there are a good number of tree species where it is now almost impossible to find any grafted varieties for sale that are NOT male. 

Around 1950 these separate-sexed trees were represented in urban areas with a ratio of approximately 50% female trees. These large female trees produced no pollen of their own and did not contribute at all to pollen allergy. What is often over-looked though, is that these same large urban female trees were also wonderful natural "pollen traps." 

In separate-sexed species the female flowers often have large clusters of pistils with broad, sticky stigmas that are positioned in the branches in such a way as to trap windborne pollen. For catching and stopping airborne pollen of, for example, Red Cedar pollen, there is no organism in nature as perfectly designed for this job as a large female Red Cedar tree. For trapping and stopping airborne pollen of ANY species, the most efficient creation is the female of that species. Not only did these billions of female trees produce no pollen themselves, they were also highly effective natural "air-scrubbers," or pollen removers. 

Today's urban forests have very few of these large female trees left. As the old female trees died off naturally, from harsh urban conditions, or as they were cut down because they produced "litter," they were usually replaced with male clones or with monoecious species that also produce large amounts of airborne pollens.

 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 we now not only have an overabundance of males from naturally separate-sexed species, but we additionally have many male trees from species that in nature never were unisexual. 

In the year 1950 American Elm trees were the predominant street trees in thousands of neighborhoods across the United States and also in many other countries around the world. DED, or Dutch Elm Disease, swept across the land killing off literally billions of elms. Ulmus Americana, the American Elm, is a tall stately vase-shaped deciduous tree that is perfect flowered. Elm flowers have both the male and female parts in the same flowers and are largely pollinated by insects, especially honeybees and butterflies. 

As DED moved westward, killing off almost all the elms in its path, the dead elms were cut down and were replaced usually with unisexual-flowered trees, the majority of which are wind-pollinated. Elm trees themselves do shed some limited amounts of airborne pollen, and allergy from elm pollen was not in the least bit rare. However, in most instances the replacement trees for the elms produced far greater amounts of airborne pollen. These wind-pollinated urban trees usually lacked nectar sources in their flowers, and thus with the loss of the elms, not only did we get huge increases in ambient pollen, but at the same time countless numbers of urban bees and butterflies lost one of their major early spring food sources. Our urban forests are now heavily dominated by asexually-propagated, wind-pollinated trees. 

Fifty years ago less than 5 percent of our population suffered from allergies. Today it is estimated that some 38% of the U.S. population now has allergies. The honeybees and the butterflies, once so common, are themselves disappearing in many areas. So what are we to do? How can we clean up this biopollution and bring back the bees and the butterflies? The answer is actually quite simple. First, we need more diversity in our urban plantings. Never again should we over-rely on just a few species. 

Second, we need to start planting as many non-polluting female-only, pollen-trapping trees and shrubs as possible. Thirdly, we should also increase the planting of those perfect-flowered trees that are known to have especially low allergy potentials. The bombardment of urban pollen, the resulting epidemics of allergy, and the terrible loss of biodiversity, all of these were avoidable man-made problems. Now is the time to start getting back to the benevolent urban forests of yesteryear. 

In many areas tree pollen makes up more than 70% of the total urban pollen load. As the number of people with pollen-allergies grows, attitudes toward trees themselves are already changing. Our great life-producing urban forests, once so beloved by one and all, are now more and more being seen, as one North Carolina man suffering through yet another attack of allergy, so succinctly put it, "It's all these damn trees." 

Thomas Ogren is the author of "Allergy-Free Gardening", just released by Ten Speed Press. He holds a Master of Science in Agriculture from Cal Poly University, with an emphasis on plant flowering systems and the connections between landscape plant materials and allergy.

Return to the March/April Index page