Trees and Allergies

Trees and Allergies

Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist

Are gardeners, horticulturists and plant breeders the ones to blame for the rise of urban allergies?  Thomas Ogren seems to think that they are largely responsible.  Hay fever or seasonal allergic rhinitis is a big problem in this country with more than 35 million people being affected by airborne plant pollens and molds.  Chronic rhinitis is one of the leading causes of missed work days in this country. Treatment for this problem costs over 6 billion dollars a year and one out of every six doctor’s office visits are allergy related..  Ogren is pointing the finger of blame at people who love and work with plants.  How could such an innocent passion be the cause of all this?

Ogren is a former nursery owner with a Master of Science in Agriculture from Cal Poly University.  About 15 years ago he changed directions and has become an allergy researcher. Ogren seems to think a big part of increasing allergy problems is due to the increased exposure to air-borne plant pollens.  Allergies develop from repeated exposure to specific allergens, with plant pollens being the most common outdoor allergens. So why might plant enthusiasts be responsible?  It’s because they unwittingly changed the tree and shrub community in cities and towns to a predominantly male dominant population.  Yes, plant enthusiasts have been sexist.

Ogren points out that this sexist discrimination began back in 1949 when the USDA Yearbook “Trees”  recommended selecting male trees that didn’t produce litter or seeds.  Since then plant people have encouraged planting male trees and shrubs because of the less mess, lower maintenance characteristics they offer. 

At the time UDSA “Trees” yearbook was published, about fifty per cent of the trees in our cities and towns were female.  Since that time there has been a shift to mostly male, pollen producing trees. Also in the 50's, American elms were the predominant street tree across much of the nation. The elms which are monecious have both female and male flowers on the same tree and are generally insect pollinated.  Because they’re not wind pollinated, the elms caused limited allergy problems for city dwellers. 

Along came a disease, called Dutch Elm Disease, that destroyed the population of elms in most American cities.  That left many cities without trees along their streets.  Knowing the value of trees, urban and rural communities replaced the stately elms with other types of trees.  Regrettably these replacements have turned out to be predominantly male, pollen-producing, wind pollinated species.  Are you starting to see the problem? 

To make things a little clearer let’s talk about sex and pollination.  Some trees are “monecious”, having separate male and female flowers on the same plant. Examples include honey locust,  oak, sweetgum, pine, spruce, and birch. Other trees and shrubs are “dioecious” having female and male flowers on separate plants.   Examples include ash, willow, cedar, juniper, cottonwood, mulberry, box elder, holly, yew, and smoke tree. Yet another type are “perfectly flowered” with flowers being both female and male. Examples include dogwood, crabapple, cherry, redbud, magnolia, flowering pear, plum, and hawthorn. 

From an allergy perspective, perfectly flowered plants don’t cause as many problems.  Their pollen tends to be heavy and sticky.  Pollen is usually transferred from the male to the female parts of the plant by insects.  The dioecious and monecious plants are more likely to cause allergy problems because most are wind pollinated.  For wind pollination to be successful they must produce lots more pollen.  From the pollen standpoint, Ogren feels that dioecious males are worst plants because they only bear pollen and dioecious females are the best because the don’t produce any pollen.

Let’s consider that two-thirds of the pollen from wind pollinated trees and shrubs is distributed within 60 feet of the source and 90 per cent of the pollen within 90 feet. Grasses and herbaceous plant pollen tends to travel only a few yards from the plant.  That means that for most of us the pollen that aggravates our allergies is in the landscape at home, school, or work. That means we can have partial control over the situation by selecting and planting trees, shrubs, and flowers that won’t cause as many allergy problems caused by plant pollens. 

Some communities have already come to the realization that certain pollen producing plants are causing a problem.  Tempe, Arizona and Las Vegas, Nevada have outlawed the planting of olive and mulberry trees because of they’re extremely allergenic.  Las Vegas already has 200,000 male mulberry trees.  These male trees were no doubt planted to avoid the female trees and their very messy fruit. Other communities are banning the planting of Bermuda grass lawns because of the large amounts of pollen they produce. Albuquerque, New Mexico has banned the planting of many male plants trees and shrubs and has ordered the labeling of allergenic plants in nurseries and garden centers.  We will no doubt see more regulations and planting ordinances enacted in the future. 

I’m not sure we need laws and ordinances, as much as we need awareness and encouragement to plant trees, shrubs and flowers that won’t cause as many allergy problems.  However, we need to know which are the “good” plants and which are the “bad” plants.  Ogren developed a trademarked scale called OPALS(TM) or the Ogren Plant Allergy Scale.  With this scale plants are ranked from 1 to 10, with 10 having the greatest potential to cause allergy problems.  Various criteria involving 70 factors are used to rank plants.  The scale has been reviewed by allergists and botanists and judged to be a useful tool. USDA researchers at the Northeastern Research Station in Syracuse, New York have started using OPALS(TM) for its allergy projections in major U.S. urban areas.

How can we possibly use the scale to protect ourselves from allergy attacks?  The answer is to start planting landscape with plants that are on the low end of the OPALS (TM), five or less.  However, this is a major shift in thinking for gardeners, nurserymen, plant breeders, and municipalities. It will take time to create awareness and acceptance. We’ll need to have more tolerance for litter and some “messiness”, but a little extra work may well be worth it to save us from runny noses, watering eyes, and other allergy miseries.

To learn more about OPALS (TM) you might want to pick up Thomas Ogren’s book, Allergy Free Gardening published by Ten Speed Press.  In the book Ogren discusses his theories about the increase of allergies and asthma in this country and he lists thousands of plants with their OPALS (TM) rating.  If you’re an allergy sufferer, you may want to use it as a guide when shopping for garden and landscape plants.

I bet you’re curious about how some plants are ranked on the scale.  Here’s a sample:

Male Ash Trees: 9-10

Female Ash Trees: 1

Male Red Maple: 8-9

Female Red Maple: 1

Male Honeylocust: 7   

Female Honeylocust: 1

Male Junipers: 10       

Female Junipers: 1

Dogwoo d: 5


Flowering Pear: 4


Flowering Plum: 3


Flowering Cherry: 7


Double-flowered Flowering Cherry: 1


Birch: 7


Red Oak: 8


Pine: 4


Spruce: 3