Gardening Articles: Landscaping :: Trees, Shrubs, & Vines

Are Trees Making You Sneeze?

by Susan Littlefield


Does it seem that your allergy symptoms are getting worse lately? Changes in the types of trees planted in many landscapes and along many city streets may be part of the problem.

Many common landscape trees, such as maple, pine, spruce, birch, and oak, naturally have both male and female flowers on the same tree; these are known as monecious. Other trees, called dioecious, bear male and female flowers on separate plants; examples are ash, ginkgo, and juniper. It is the female flowers that go on to produce fruits and nuts, but it's the male flowers that release the pollen that fertilizes the female blossoms and that troubles allergy sufferers. (Some trees, such as crabapples and dogwoods, have "perfect flowers" with male and female parts in the same flower. These generally don't release their pollen as widely and are less likely to trouble those with allergies.)

Both home owners and city maintenance crews often prefer not to have to deal with the messy fallout of fruits and nuts, choosing instead to plant clones or varieties that have been selected only to bear male flowers. Sometimes this makes a lot of sense, as in the case of ginkgo trees, whose female flowers produce fruit that is not only messy, but very smelly to boot. But in some cities, the decision to plant mainly male clones of trees can result in higher pollen levels and increased allergy problems, according to pollen researcher Thomas Ogren, who has been hired by an allergy medication manufacturer to inventory Canadian urban forests and tree planting practices and create allergen maps.

In a recent article on the Vancouver Sun website, city arborist Bill Stevens noted that about 30 percent of the trees planted by the city of Vancouver are all-male clones, commonly maples and some oaks. Ogren pointed out that planting large blocks of male trees can result in "pollen corridors" that can be highly irritating to sensitive individuals; planting female clones or a mix of trees instead helps reduce pollen levels. He also noted that plants such as yews and junipers, which are commonly used as foundation plants near windows, are often male clones that release lots of pollen. "If you have one of these outside your bedroom window you are going to get dusted at night," he says.

So if spring and summer bring on itchy eyes and a runny nose, it's a good idea to do some research before choosing and placing plants in your landscape to avoid male clones as much as possible. You can also encourage the municipal department responsible for street tree planting in your community to plant a mix of trees rather than relying heavily on male clones.

To read the entire article, go to: Vancouver Sun.

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