Allergies, Asthma and City Trees Written by Thomas Ogren
Allergies, Asthma and City Trees
Thomas Leo Ogren
Some urban tree species cause an inordinate amount of asthma and allergy problems, while other tree species cause little or no health problems. A large part of problem is that arborists and landscape professionals, who plant these trees, often don’t know difference. The type of trees (and shrubs) used in modern city landscapes has changed dramatically in past three decades. In past, majority of street trees used were perfect-flowered, insect-pollinated trees, such as once so common American elm tree. Today though, many of most widely used city trees are wind-pollinated species. Most of these species are unisexually flowered (dioecious and/or monoecious) and further compounding problem, thousands of popular cultivars sold today are touted to be “seedless,” “low-maintenance,” “pod-free” or “litter-free.” These fruitless, seedless trees are male plants, all male, and male trees produce prodigious amounts of allergenic pollen. Female trees produce NO pollen what so ever. In dioecious-flowered trees such as most ash, willow and poplars, it is easy to propagate male only trees because they are separate-sexed. Monoecious trees, which in Nature always have both sexes (male and female flowers) on same tree, also usually produce abundant pollen. It is possible to have all-male trees from monoecious species. On many species sexes will be born on separate branches, such as on a Honey Locust tree. If you take cuttings, or budwood, only from branches with male flowers, then, you'll get an all-male tree. Lots of monoecious Acer spp. cultivars are male-only plants. In a somewhat different way, there are also numerous monoecious species where only top or only bottom will have either male or female flowers. For example, bottom half of a mature Italian Cypress for example is all-male. Female wood is found only at top of plant. Thus, scion wood taken from bottom usually produces "seedless" plants. The terms “dioecious,” and “monoecious,” are botanical terms, not horticultural terms. We could perhaps say that a manipulated, asexually propagated all-male cultivar, taken from a monoecious species, is now “dioecious,” but this is not quite correct. A proper dioecious tree is one that in Nature is separate-sexed. These modern engineered trees are never found in Nature. Interestingly, first reference in print I ever found to this single sexing-out with monoecious street trees, was in a USDA booklet, from 1982, called “Genetic Improvement of Urban Street Trees.” How Bad Is Allergy Now? In 1959 official rate of allergy in U.S. was between 2 to 5% of public. By 1999 official rate of allergy had increased to an incredible 38% of Americans. Asthma, which was once considered rare, is now number one chronic childhood disease in US.
Where are Allergies and Asthma Worst? The most common allergen of all is pollen and since there are so many more plants growing in country than in city, it would make sense then that there is more allergy and asthma in countryside. Right? No, wrong! Allergies and asthma are far worse in city than they are in country.
Several things contribute to this: 1.Pavement makes a poor pollen trap. Pollen in city often lands on pavement where wind can cause it to become airborne again. In naturally vegetated areas where there is much more vegetation, pollen often lands on and becomes stuck in grasses, shrubs and vines or in trees. 2.Cities have more air pollution, which weakens immune system and lung function. 3.Stress, which is generally higher in cities, can contribute to both asthma and allergy development. 4.Increased carbon dioxide levels within cities causes pollen-forming plants to produce more pollen with each bloom cycle, and also often causes urban plants to bloom more often. 5.Pollen loads are actually far greater in cities because there is a sexual imbalance within plant community. In city there is a preponderance of male trees and shrubs, while in rural areas there is almost always a complete balance of plant sexuality. The excess of male plants in city results in an excess of pollen. 6.The very lack of female plant materials in urban environment also is a prime factor in epidemic of allergy and asthma. Female flowers carry an electrical negative (-) charge (the trees are grounded with their roots) and airborne pollen holds a positive (+) charge. The tree and pollen are mutually attractive; thus a female plant becomes a very effective pollen trap for pollen of its own species. But with almost no female trees and shrubs in modern landscapes, most of pollen remains airborne.
Cottonwood “cotton” is flying/ Bad CompanyWritten by Thomas Ogren
Cottonwood “cotton” is flying/ Bad Company
Ó Thomas Leo Ogren
I originally saw question below posted in an Internet gardening forum and decided to answer it. It was a question I’d been asked many times before and I knew my answer would be useful in an article on springtime allergies caused by city trees. So I saved it to share with you here.
“Does anyone else here really suffer from allergies when seeds of Cottonwood are flying? I KNOW it is not cottonwood but I am really curious as to what is pollinating at same time.” Diana Pederson, Ingham County, Michigan, Zone 5, United States, author of Landscaping With Bible Plants:
That’s a very good question. Around here, as “cotton” (the seeds) of female poplars (cottonwoods and aspens) and willows is flying about, so is a good deal of pollen from different, unrelated species of trees. It is very common at this precise time that many people are suffering from extreme bouts of hay fever and often it is this “cotton” that gets blame. Some city arborists refuse to plant female willows or poplars because of their firm (if mistaken) belief that this “cotton” is really some kind of pollen. But it isn’t pollen; it is seed. It is NOT what is causing allergies at that time. By time seeds of female willow and cottonwoods are flying, pollen from males of these two species is already spent. However this flying of seed coincides with pollen release of many allergenic plants. Out West this is same time that millions of urban “fruitless” male mulberry trees are shedding their highly allergenic pollen. It is also time that olive trees are starting to release pollen. The cypress trees and shrubs are releasing very large amounts of pollen at this time too, as are many male Ailanthus trees. At or about same time walnut trees are releasing a large amount of pollen, as are many species of hickory, butternut, and pecan. Perhaps most pervasive at this point are oaks, many species of which are still at this time covered with staminate flowers and just loaded with pollen. At same time that female willows and cottonwoods are releasing all that harmless fluff into air, birch trees have just finished shedding large amounts of pollen, much of which is still lying around on ground. In southern areas alders often bloom twice (as will many birch and junipers) and second bloom of alders sometimes will coincide precisely with flying of “cotton.”