Are Unisex Trees Making Us Sick?
by Wendy Priesnitz
 The trees are leafing out and we’re back working in our gardens and walking in parks. Itchy, watery eyes, and runny noses are also on their way back...and multiplying in number and severity year by year.
This situation is no coincidence, according to landscape gardener, teacher and writer Thomas Ogren. He believes the steady increase in the number of seasonal allergy sufferers has its roots in a trend toward using male trees and shrubs in urban landscaping. Among the trees believed to be problematic for allergy sufferers are such popular varieties as cottonwood, willow, elm, aspen, maple, and poplar.
Ogren is a former nursery owner with a Master of Science in Agriculture from Cal Poly University, who lives in San Luis Obispo, California. He is also the author of Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping, published in 2000 by Ten Speed Press, in which he presents his case for claiming that our gardening and landscaping practices may be the culprit for the continual increase in the number of allergy or asthma sufferers.
He says homeowners and commercial landscapers like male clone trees because they are “litter-free”, meaning they do not drop seeds, seed-pods, or fruit on lawns and sidewalks. However, these male plants all produce large amounts of allergenic pollen.
Most tree species are “monecious,” having separate male and female flowers on the same plant. Examples include honey locust, oak, sweetgum, pine, spruce, juniper, alder, and birch.
Other trees and shrubs are “dioecious,” having female and male flowers on separate plants. Examples include ash, poplar, willow, cedar, juniper, cottonwood, mulberry, box elder, holly, yew, pussy willow, Chinese ginkgo, and smoke tree.
Yet another type is “perfectly flowered” with flowers being both female and male. Examples include dogwood, crabapple, cherry, redbud, magnolia, flowering pear, plum, horse chestnut, and hawthorn.
From an allergy perspective, perfectly flowered plants don’t cause many problems. Their pollen tends to be heavy and sticky, and is usually transferred from the male to the female parts of the plant by insects. Most dioecious and monecious plants are wind pollinated.
| Thomas Ogren believes the steady increase in the number of seasonal allergy sufferers has its roots in a trend toward using male trees and shrubs in urban landscaping. |
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.
A half century ago, an estimated 50 percent 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, the elm was the predominant street tree across much of North America. 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 the devastation wrought by Dutch Elm Disease, and the subsequent replanting with predominantly male, pollen-producing, wind pollinated species.
Largely ignored for years, Ogren’s work on the subject is now attracting considerable international attention. He has recently been hired to do consulting work for the US Department of Agriculture (USDA), the American Lung Association, various landscape and nursery organizations, and for some pharmaceutical companies.
Ogren says that children are especially at risk from the “no-litter” landscaping mentality. “Because no one bothered to consider the effect of the pollen from these male trees, we now have many elementary schools, ringed with male shade trees, and full of asthmatic children. Pollen counts exceeding 60,000 grains of tree pollen per cubic yard of airspace have been found in elementary school yards. What does this mean? Simply, it means that on these playgrounds, every child there is inhaling several thousand grains of allergenic pollen with each breath of air they take!”
The solution, according to Ogren, is allergy-free landscaping. Planting a wide variety of pollen-free or low pollen producers is not only healthier for humans, but it also creates a more diverse, resilient landscape. Some of the best low pollen trees are the females of dioecious species such as ash, poplar, willow, cedar, juniper, cottonwood, mulberry, osage orange, xylosma, yew, box elder, podocarpus, fringe tree, holly, pepper tree, smoke tree, coffee tree, sassafras and red maple.
The type of grass you grow can also have an allergenic effect. Bermuda grass can pollinate even when the lawn is very short, sometimes as quickly as just a few days after mowing. Other grasses, such as perennial rye, Kentucky Blue, and fescue will not flower unless permitted to grow to a height of one foot or higher. Keeping the height of your lawn about two inches will help reduce pollination. Alternatively, replace your lawn with female versions of groundcover plants like Irish moss or phlox.
Even if you can’t directly influence what your neighbors and municipality are planting, you have control over your own garden. It has been estimated that an allergenic pollen-producing tree in your own yard, will expose you to ten times the amount of pollen as would the same tree planted just down the block. The closer the source, the greater the total exposure. if your own yard has some highly allergenic, heavy pollinating trees and shrubs in it, you may easily breathe in several thousands of pollen grains with each breath of air.
Ogren would like to see local ordinances that forbid the further sale and planting of wind-pollinated male clones of trees and shrubs. And, he says, we need to train people in tree grafting in order to change the multitude of male trees into female trees. He would also like to see allergy labeling of all landscape plants for sale in nurseries. To this end, he has created a trademarked scale that measures the allergy potential of all garden and landscape plants. Called OPALS, or Ogren Plant Allergy Scale, it uses a simple 1 to 10 ranking system.
To lessen chances of allergic reactions, garden in the morning or late afternoon when pollen is higher in the sky, or on cloudy or cool days when it is not as prevalent. Use a mask when mowing, and don’t forget to shower and change clothes after working in the yard!
In addition to pollen, mold can also present problems for allergic gardeners. Take care to rake up and compost dead leaves lying under shrubs and use less bark mulch or chips which may harbor fungus. During wet springs, clouds of spores can actually be seen rising like smoke from moldy twigs or fruit. If you’re sensitive to molds, wear a mask or at least a scarf over your face when you garden in damp weather.
Wendy Priesnitz is the Editor of Natural Life Magazine and a journalist with 40 years of experience. She has also authored 13 books.
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