Avoiding Allergies by Use of the Right Native Plants in the
Landscape
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 “litterâ€
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
â€" 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.
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