Many of our most allergenic plants commonly used in landscaping in United States and Canada are indeed natives. However, it is manipulation of these plants by commercial horticulture that has, and is, causing most of huge increases we are now experiencing with allergy problems. Thirty years ago fewer than 10 percent of Americans had allergies. The official figure today is that a whopping 38 percent of us now suffer from allergies.(December 99, American College of Asthma, Allergy, and Immunology) Not too many years ago death from asthma was fairly rare. Today it is all too common and is considered epidemic. Asthma has now become number one chronic childhood disease in America. Furthermore, there is new data coming in recently that shows a strong connection between over-exposure to pollen and or mold spores and increases in other diseases such as heart disease, autism, pneumonia, and reflux disease.

American Elms The landscape tree in most of America for many years was tall, stately American Elm. The American Elm used to grace streets of thousands of towns and cities and when DED, Dutch Elm Disease, started to spread and kill off these native elms, insect-pollinated, perfect-flowered elms were most often replaced with wind-pollinated, unisexual-flowered, street trees. Many things happened because of big switch from elms to these other tree species. First, elm flowers had a rich nectar source and since these trees bloomed very early in season, at a time when insect food sources were severely limited urban honeybees and butterflies depended on this food source. Since majority of street trees used to replace elms were wind-pollinated, they often lacked these nectaries and supplied no early-season food source. Soon we started to see a rapid decline in total numbers of urban honeybees and butterflies. There were other factors as well behind this decline, pollution, insecticides, and disease, but loss of crucial early-season food sources should not be underestimated. DED spread mostly from East to West across US and so has rise in allergy rates. You can actually track spread of allergy from decline of elms. The American Elms, Ulmus americana, did cause a certain amount of low-level, early spring allergy, simply because they were so very common. The over-planting of elms resulted in a lack of biodiversity and set stage for massive kill from DED. We now know that it is always a mistake to use a monoculture, to plant too much of just one species. Diversity is always a good idea in horticulture.

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.