Many of our most allergenic plants commonly used in landscaping in the United States and Canada are indeed natives. However, it is the manipulation of these plants by commercial horticulture that has, and is, causing most of the 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 the 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 the tall, stately American Elm. The American Elm used to grace the streets of thousands of towns and cities and when DED, Dutch Elm Disease, started to spread and kill off these native elms, the insect-pollinated, perfect-flowered elms were most often replaced with wind-pollinated, unisexual-flowered, street trees.

Many things happened because of the big switch from the elms to these other tree species. First, the elm flowers had a rich nectar source and since these trees bloomed very early in the season, at a time when insect food sources were severely limited urban honeybees and butterflies depended on this food source.

Since the majority of the street trees used to replace the 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 the total numbers of urban honeybees and butterflies. There were other factors as well behind this decline, pollution, insecticides, and disease, but the loss of the crucial early-season food sources should not be underestimated.

DED spread mostly from East to West across the US and so has the rise in allergy rates. You can actually track the spread of allergy from the decline of the 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 the stage for the massive kill from the 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.

Diversity

Biodiversity is the way to go when we are creating landscapes that will limit allergenic exposure. Almost any species of plants can eventually cause allergies if it is over-planted enough. All to often in our urban landscapes of today we see that landscapers have used the same old plants over and over again. This overly simplistic approach to landscaping results in landscapes that lack originality and produce a numbing “sameness" to far too much of our urbanscape. When residential houses are professionally landscaped with the exact same plant materials used to landscape banks, real estate offices, and dentist’s shops, we all lose.

Allergy rates today are far worse in urban areas than they are out in the country. Pollen allergies are worse in cities than in the country, despite the fact that there is much more total green matter in the countryside than in the city. Plant selection has been the main problem.

Natives and Urban Landscapes

There are many native trees and shrubs used in our landscapes. Maples, oaks, locust, poplars, willows, catalpa, birch, junipers, and many more native species are extensively used. Unfortunately the plant breeders and propagators discovered how to “sex-out" the trees and shrubs. They learned to use only male plants, ironically, as “mother plants," as the source for their scion wood for asexual propagation. First they just used male plants from the dioecious (separate-sexed) species, but later they learned how to produce all-male clones from species that in Nature were never unisexual (the monoecious species).

For example, Honey Locust trees, (Gleditsia triacanthos) are native to our Southeastern US. Look at these trees in the wild and you will see that all of them are almost always covered with long seedpods. But go to a nursery now and look at the Honey Locust trees for sale. The ones on sale now are called “seedless" and they are in effect, all-male clones.

What exactly is the effect of using all male cloned trees and shrubs in our landscapes? Very simply, this translates to an excess of allergenic pollen. Only male flowers produce this airborne pollen. Unisexual female flowers produce no pollen.

Why the Emphasis on Male Plants?

Horticulturists knew that female plants produced seeds, seedpods, and fruit. This “litter" fell on the sidewalks and created a “mess." By using only asexually (no sex involved) propagated cultivars (cultivated varieties), they were able to create “litter-free" landscapes. These required less maintenance and were (and still are) very popular with city arborists and the public. In the US today, four of five of the top-selling street tree cultivars are male clones.

Female flowers (pistillate) on female trees or shrubs produce an electrical (-) current. Their stigmas are broad and sticky. Airborne pollen from male plants has a negative electrical impulse before release and a positive charge after release, and this pollen is light and dry. Because of the + and – electrical charges the pollen and the stigmas are drawn to each other. They are mutually attractive. Mother Nature saw to it that pollen would land, and stick, exactly where it was needed. Female plants are nature’s pollen traps, our natural air-cleaners.

Today though, most of the female plants are long gone from our landscapes. The pollen from the males floats about, seeking a moist, sticky, positive-charged target. We humans emit a positive electrical charge, and our mucus membranes, our eyes, skin and especially the linings of our nose and throat, now trap this wayward pollen. We have become the targets Allergy develops from repeated over-exposure to the same allergens. If your own yard is full of pollen-pumping trees and shrubs, you and your family are the ones who will be exposed the most.