Allergies? The Culprit Could Be Your Own Backyard


About one out of every eight adults and one out of every ten children suffers from hay fever in America. Asthma occurs in more than one out of every fifteen adults and in more than one out of every eleven children.

Plant pollen is a major cause of hay fever and asthma and authorities caution susceptible individuals to limit their exposure when possible. Until recently, that meant limiting all kinds of pleasurable outdoor activities. Thanks to the work of a botanist named Thomas L. Ogren, there is another approach. Through his work, we can learn how to make our yards, gardens, and cities more pollen free, partly through understanding the sex characteristics of our plants. 

Mr. Ogren created the first plant-allergy ranking system in existence. The Ogren Plant Allergy Scale (OPALS) is now being used by the United States Department of Agriculture (USDA) to develop allergy rankings for all major U.S. urban areas. Some plant nurseries are already selling plants that are tagged with their OPALS rankings.

Thomas Ogren is very approachable and he seems happy to share his valuable work with those who are genuinely interested. His goal is to teach us how to correct our allergy-aggravating planting trends. I was thrilled that he granted me an interview. I was able to ask specific questions about the plants common to our area. The following information is what I learned.

Understanding How the Sex of Plants Affects Your Allergies
Pollen from plants is a leading cause of allergies, but plants differ in the way they produce pollen based on their sex characteristics. Only the male parts of plants produce pollen. 

The females of “separate-sexed” plants are our sinuses’ and lungs’ best friends. These plants are referred to as separate-sexed because each plant has exclusively either male flowers or female flowers. Examples are ash, hollies, some maples, mulberry, ginkgo, and palm. The male plants are the worst allergy offenders because their pollen becomes airborne to be carried by the wind to the female plant. 

When the pollen from the male plants becomes airborne, it is tumbled through the air until it develops a positive electrical charge. Its female counterpart has a negative electrical charge and this causes a magnetic attraction, drawing the pollen out of the air. This makes the female plant an effective natural air cleaner. Females also provide food for our birds and bees. 

The second most allergy-friendly are plants that have both male and female parts in the same flower. These plants are considered by botanists to be “perfectly flowered.” The pollen from the male part of the flower is already in close proximity to the female part so nature has no purpose in causing the pollen to become airborne. It is heavy and sticky and it stays on the plant and is moved from the male part to the female part by the activities of birds and insects. Roses are an example. 

The third best plants for our allergies, but still contributing to the levels of airborne pollen, are plants that have separate male and female flowers on the same plant. The pollen must be airborne but is carried only a short distance between the flowers. Examples are oak and cypress. 

How Can OPALS Help Us Decrease Allergens in Our Area?
Controlling the sex characteristics of the plants you have in your environment will go a long way toward decreasing the amount of pollen in the air you breathe. However, OPALS can help us fine-tune a low-allergy garden. The scale takes into consideration two fundamental questions: (1) What do plants that are well known not to cause allergies have in common? (2) What do plants that are well known to cause allergies have in common? OPALS considers and weights more than 70 individual plant factors. Ratings are from 1 (the most allergy-friendly) to 10 (the worst allergy offenders). Thomas Ogren’s book, Allergy-Free Gardening, gives the allergy rating for over 5,000 different plants.

Landscapers can use OPALS to evaluate all of the plants in a given area and come up with a numerical ranking for that area. The goal for a low-allergy landscape is one that ranks under 5. It is possible to create an entire yard, trees and lawns included, that ranks 1. This low-ranking landscape would be virtually pollen-free and would not offer any contact allergy risk either!

Historical Planting Preferences
The USDA began recommending the planting of male trees and shrubs in 1949 in an effort to make cities more “litter-free.” Therefore, female plantings in urban areas have been avoided because they produce the fruit, flowers, and seeds. Pursuant to the USDA’s advice, horticulturists have historically chosen “cleaner” male plants. This habit has continued for aesthetic reasons, and with low-maintenance as a goal. As a result, we now have cities full of plants that cause the worst allergies.

Of special interest are our schoolyards. Our children, including those with asthma, are having their lunches sitting under the most allergenic trees. They are also playing in grasses that cause the most problems. Mr. Ogren says, “School after school is landscaped with the most allergenic plants possible. Even at hospitals I see landscaping so explosively allergenic that it makes me shudder.”

Based on Mr. Ogren’s work, that trend is being thrown into reverse. Many localities, including Tucson, Phoenix, Beverly Hills, Las Vegas, and Albuquerque, are beginning to pass local ordinances against, and to publish lists identifying, the worst allergy offenders.

Our communities are where the most improvements are needed because rural areas, though sometimes supporting abundant plant life, reflect nature’s own balance of sex characteristics. Our allergies worsen with repeated exposure to offending pollen, especially if the amount that is airborne is great. The goal is not to eliminate pollen altogether, but to reduce it to a much healthier level. 

Implementing what we learn from this work can be a real boon to our community. Improving our health is the number one payoff but there may be other benefits too. If our landscapes contain mostly plants that are very low in pollen and other allergy-causing characteristics, we can promote and advertise that. We can attract people who yearn to live in a more allergy-friendly area. Reversing the allergenicity of entire communities is even predicted to increase real estate values.

Though some argue that local plantings are not the entire answer and that pollen can travel great distances, improving the situation locally can only have beneficial results. Having fewer allergenic plants in one’s yard and community drastically decreases exposure, as most windborne pollen stays in the immediate area. In 1972, a scientist named Raynor was able to show that a male pepper tree in one’s own yard exposes him to ten times the pollen that he would have to breathe if that same tree was located in a neighbor’s yard down the block. Besides, as more and more communities take responsibility in this regard, it follows that there will be less airborne pollen traveling to other locales.

General Advice For Our Area
The worst tree we have in our area is a male separate-sexed tree referred to as fruitless mulberry. It has an OPALS rating of 10. It is commonly used as a shade tree, especially in schoolyards. This tree is responsible for a great many allergy and asthma problems and it has even been the cause of deaths from asthma. You may know someone who is forced to leave town while these trees are in bloom? 

The good news is that if you have a fruitless mulberry, it is “as easy as riding a bicycle” to convert it to a female. During the winter months, when the tree is dormant, it can be cut back to a stub and then it can be grafted with a cutting from a female tree, giving it an overnight sex-change operation. Though some people want to avoid the messy female mulberry, there is a variety called a weeping mulberry that hangs like a willow but doesn’t produce much fruit and it can be used in the graft. The end result is a beautiful tree that produces no pollen and is not very messy.

How can John Q. Public accomplish a sex-change operation on his mulberry? The best way is to learn how to graft. “Once you have gotten the hang of it,” Mr. Ogren says, “it is very easy.” Alternatively, mulberry owners could hire a gardening expert to do the grafting.

Our worst grass is common Bermuda. It is bad for allergies and blooms all the time. Hybrid Bermudas are more expensive, but they look better and produce no pollen. A hybrid called “Princess 77” is available as seed or sod and it is pollen-free. Buffalograss is also specifically recommended for our area. It comes in male or female plants. The two best all-female buffalograss varieties are “609” and “Legacy.” Both of these can be purchased as sod or as plugs. They are low growing, blue-green in color, very soft under bare feet, and they are pollen-free. They need less fertilizer and water than most lawn grasses but they do go dormant for a few months during the winter and then they green up again in early spring. If you already have an allergy-offending variety of grass, Thomas Ogren’s book, Growing a Perfect Lawn, can help you remedy the situation.

Most grapevines are pollen-free. Pomegranates are low-pollen shrubs that sometimes grow to be small trees. Any poplar or willow or cottonwood or aspen tree that makes “cotton” is a female tree, and is pollen-free. All of these are very easy and fast to grow from dormant cuttings that are stuck into the ground during winter months. Thomas Ogren recommends collecting a cutting from the female tree, in a thickness somewhere between that of a pencil and an index finger, about one foot long, and sticking three-fourths of it into the ground. He uses that same approach for grapevines.

Anyone in the area who chops his own firewood should only chop down male trees, leaving the females alone. In west Texas, the native junipers (red cedar) are separate-sexed trees and only the ones that do not have any berries should be used for firewood. 

The Best Trees for Our Area
Thomas Ogren has been kind enough to provide us with the following list of the ten best allergy-friendly plants for our area. These are not listed in any particular order:

  • Female red cedar trees (Juniperus virginiana). Identify the females by looking for juniper berries or ask for “Manhattan Blue,” which is always female.
  • Female mulberry trees, especially “Illinois Everbearing Mulberry.”
  • All-female persimmon trees such as “Fuyu,” “Eureka,” or “Suruga.”
  • Crape myrtle trees are low-pollen trees and they are heat-tolerant.
  • Weeping willow (Salix babylonica). Don’t buy a golden willow.
  • “Theves Poplar” (Populus thevestina). This all-female poplar grows up tall and narrow and is similar in shape to the “Lombardy Poplar,” but is better-shaped and lives longer.
  • “Variegated Box Elder” (Acer negundo) is a female tree. 
  • Fruit trees such as plum, peach, pear, and apricot are all very low-pollen trees.
  • “Red Sunset” (Acer rubrum) is the southern red maple cultivar. It is a female tree with good fall color. It is heat-tolerant and grows quickly.
  • “Bradford Pear” trees produce some pollen, but not too much, and allergy to them is rare. They grow fast and have great fall color. 
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