CBC radio interviewed Horticulturist Thomas Ogren on July 18, 2011. This is what he said about pollen and allergies.
Plants have sex. No, it’s not what you’re thinking! What he means is most plants are either male or female. The male plants produce pollen; female plants produce fruit, seeds or seed-pods.
Nurseries produce and sell mostly male trees. The market demands male trees because they are lower maintenance. Female trees produce messy flowers, seeds, fruits and seed-pods that need to be cleaned up. However, female trees do NOT produce pollen.
Most cities are planted with male trees for lower maintenance. This is why we have so much pollen in cities. This is why so many people develop allergies to pollen.
In a natural state there would be roughly an equal number of male and female trees. By planting mostly male trees in cities, we create a double whammy. You see, female trees attract and trap pollen. With an abundance of male trees, not only do we create more pollen but we have fewer female trees available to trap this overabundance of pollen.
If you’re going to plant trees or bushes; select females. They’re the ones with flowers and berries.
If you’re going to cut down some trees for firewood, select the male trees. Over time you’ll surround your home with mostly female trees and bushes and thus have less pollen around your home.
Anti-allergy web-sites list plants that produce the most allergic pollen and advise you against planting them. You can still plant them, just plant the females.
He also suggested buying locally produced honey if you have pollen allergies. Locally produced honey has trace amounts of pollen that increase your immunity to pollen and reduce allergic reaction.
Thomas has a web-site that you might consider whether you have pollen allergies or not. By increasing female trees and bushes and reducing male plants, we help balance our environment and improve our health. Here’s his web address for you to copy and paste on your browser:
http://www.allergyfree-gardening.com/
Gerold
July 18, 2011
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Gerold's Blog 
Yes, it is true that many trees and shrubs are “monoecious,” having both sexes on the same plant…BUT, horticulture long ago figured out how to manipulate many of these species so that they will be single-sexed (dioecious). This sort of tree never occurs in nature, but only in the man-made environment.
For example, with many kinds of cypress trees, male flowers appear only on lower branches…so, the propagators only take scion wood from these lower branches. The end result? All the new cypress grown this way will be male. Another example, honey locust trees. On this species, one branch will have all male flowers & another all female flowers. The propagators only take scion wood from the male branches…the result? So-called “pod-less” honeylocust trees…all of them of course are male clones.
There are many more examples. The allergy-asthma-horticultural-sexism phenomena is a serious health problem across the world, and will only be put in check by greater public awareness.
My thanks to Gerold for this blog.
Thomas Ogren
author of Allergy-Free Gardening
Many thanks, Thomas, for your very informative comment.
About 6% of flowering plants are both male and female. They have both types of single sexed flowers on the same plant. These are monoecious plants like alder, hickory, fig, birch, beech, chestnut, walnut.
Most conifers bear both small male and large female cones that are wind pollinated.
That’s interesting. Thanks for the information.
The tricky part is figuring out which are your male trees and which are female for each type of tree on your property.
That’s a good point. Readers Can check Ogren’s website for more information.