In the Garden:
New England
April, 2005
Regional Report

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If you suffer from plant allergies, avoid plants that produce copious amounts of pollen, such as male junipers, and plant female versions instead.

Gardening to Minimize Allergies

It's that time of year again -- the sneezing, sniffling, itching eyes time that signals spring is here. The month of May, when maple trees are flowering, is prime allergy season for me. But since it's also prime planting time, I've had to find ways to adjust. I take an antihistamine and keep my "outdoor" clothes in the garage so I don't inadvertently bring pollen inside the house. I can't remove the source of my allergies because large maples surround my yard. But I can consider the allergenic potential of new plants I bring home.

Most allergies arise from wind-blown pollen. Unlike insect- or animal-transferred pollen, the grains of which are relatively large and sticky, the pollen of wind-pollinated plants is light and easily carried on the slightest breeze. Most grasses and many trees are wind pollinated.

Unfortunately, allergies are on the rise in the US. In 1959, 2 to 5 percent of the population suffered from them. In 1984, the number was 12 to 15 percent. In 1999, it spiked to 38 percent. Tom Ogren, author of Allergy-Free Gardening, attributes these increases to major changes in our landscaping practices. An allergy researcher with a horticulture background, Ogren theorizes that it all started back in 1949 when the USDA published its Trees Yearbook. The book recommended choosing male cultivars because male trees don't produce "messy" fruits or seeds. Thus, our current philosophy of tree sex discrimination began! While it's true that male trees don't produce fruits, what they do produce is copious amounts of pollen.

Tree Categories
Trees can be categorized by the types of flowers they bear. Some tree species are "perfectly flowered" -- a perfect flower is one that has both male and female parts. Perfectly flowered trees are usually insect-pollinated, and include crab apple, cherry, dogwood, magnolia, and redbud.

Some tree species bear separate male and female flowers on the same plant; these are termed "monecious." Examples include honey locust, oak, sweetgum, pine, spruce, and birch.

"Dioecious" tree species bear male and female flowers on separate plants. A familiar example of this is holly; you need a male holly plant nearby if you want your female holly to produce berries. Other dioecious trees include ash, box elder, cedar, cottonwood, juniper, mulberry, and yew.

Ogren points out that perfectly flowered trees, which are generally insect-pollinated, pose few problems, allergy-wise. However, many monoecious and most dioecious species are wind-pollinated, and it's their light, wind-blown pollen that triggers allergic responses. Male cultivars of dioecious trees are the worst offenders.

OPALS Scale
How can we determine which trees are "good" and which are "bad," allergy-wise? Ogren developed a trademarked scale called OPALS (the Ogren Plant Allergy Scale). He ranks plants from 1 to 10; the higher the ranking, the more potential the plant has to cause allergy problems. If your city has an urban forestry program, you might ask what criteria they use when deciding what to plant along city streets. Do they take allergy problems into consideration?

In Your Yard
You may not be able to influence the trees planted on your street, but you can make wise choices about what you plant in your own yard. Keep these steps in mind:

1. If you are allergy-prone, determine a species' OPALS rating before planting it in your yard.

2. If the plant variety you desire has female flowers on one plant and male flowers on another, choose the female plants because they will produce no pollen. Ask your nursery or garden center for female ash, willow, mulberry, juniper, and maple trees.

3. Plant a diverse garden so you won't end up with large numbers of any one allergen-producing plant.

4. Install any troublesome plant at the edge of your property, away from windows and doors.

5. Choose plants that are well-adapted for your area and keep them healthy. A stressed male plant can put out two to three times the normal amount of pollen in an attempt to reproduce itself before it dies.


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