Botany As A Weapon Against Allergy

A new book, Allergy-Free Gardening by Thomas L. Ogren, from Ten Speed Press, has exposed what the author says are the root causes of alarming increases in hay fever and asthma in the United States.

Before 1950 most of the billions of trees that made up the US 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.

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 large number of tree species in which 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 percent female trees. These large female trees produce no pollen therefore do not contribute at all to pollen allergy. What is often over-looked, however, is that these same large urban female trees were also wonderful natural “pollen traps,” Mr Orgen said.

 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-scubbers,” or pollen removers.

Today’s urban forests have very few of these large female trees left. As the old female trees died off naturally, or 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 through which male-only trees also could be propagated from the monoecious species; thus there are now not only an over abundance of males from naturally separate-sexed species, but there also are many male trees from species that in nature never were unisexual.

When Dutch elm disease swept the nation decades ago, killing billions of elms, the dead trees were cut down and generally replaced with unisexual flowered trees, the majority of which are wind-pollinated. Elm trees have both the male and female parts in the same flowers and are largely pollinated by insects, especially honeybees and butterflies.

Wind-pollinated urban trees usually lack 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 their major early spring food source. and are disappearing in many areas.

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 percent of the US population now have allergies.

In many areas tree pollen makes up more than 70 percent of the total urban pollen load. As the number of people with pollen-allergies grows, attitudes toward trees themselves are already changing. The 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.”

So what should be done? Thomas Ogren believes the answer is simple: First, more diversity is needed in urban plantings. Never again should we 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, Mr Ogren says.

“The bombardment of urban pollen, the resulting epidemics of allergy, and the loss of biodiversity, all of these were avoidable, man-made problems,” he said. “Now is the time to start getting back to the benevolent urban forests of yesteryear.”