To alleviate his wife’s allergies years ago, Thomas Leo Ogren, a California horticulturist, began searching for pollen-free trees and shrubs to landscape his yard. One of his most interesting discoveries was that dioecious species (those that are either all male or all female) were the worst allergy offenders. More specifically, he found it was the males that release all the pollen. Meaning one way to avoid red, itchy eyes and relentless sneezing might be to plant more female trees and plants, which don’t produce the tiny, allergy-inducing grains.

Intrigued, Ogren set out to photograph male and female examples of each dioecious species, which include ash, gingko, poplar, mulberry, junipers, silver maples, honey locust, yew and willow. But he kept coming up short on females.

"I thought maybe my city was unusual," he told Global News.

But in place after place where trees and bushes had been purposely planted instead of growing naturally, he saw the same preponderance of males.

Ogren, who has written several books on allergy-free gardening, calls it botanical sexism, and says this lopsided preference for male trees and shrubs is one factor behind the rise in pollen allergies.

Battle of the sexes

Why would people eschew female trees and plants? Turns out they’re the messier sex. They produce all the seeds, pods and fruits — and that makes males "cleaner" by comparison. Less debris to sweep off decks and sidewalks or scoop out of clogged roof gutters. Fewer pests stopping by for free goodies on the ground.

seed pod from honey locust tree One female honey locust tree can drop 100 pounds or more of long, twisted seed pods each year – a major reason why most honey locusts in planted landscapes are male clones. They may not produce pods, but they do produce lots of pollen. (Photo: Andrew Dunn/Wikimedia Commons)

The practice of planting male trees dates back to the 1940s when the U.S. Department of Agriculture recommended avoiding "nuisance" seed-bearing trees along streets. Cities took heed, particularly after Dutch elm disease decimated millions of American elms in the 20th century. Faced with an explosion of barren thoroughfares, cities jumped at the chance to repopulate their environs with non-litterers.

Commercial growers were happy to oblige, providing "low-maintenance" male trees to meet rising demand. They even figured out how to cut off male branches from species that have both male and female branches on the same tree (monoecious species) and clone them into new all-male versions. And they did it with landscaping shrubs too, creating an abundance of clonal male plants that dominate the market to this day.

It sounds harmless enough. But male trees and plants aren’t entirely mess-free. Not only do they release pollen, but without enough female flowers to catch the powdery stuff and trap it – which is how they’re fertilized to create seeds – we’re left with a massive overload of tiny air-borne pollen grains that find their way into eyes and noses everywhere.

Allergy epidemic

People who are allergic to pollen (better known as hay fever) have immune systems that mistake it for an outside invader. In response, their bodies release helpful chemicals called histamines that prompt eyes to water and noses to run in order to fight off (wash away) the offending pollen.

Learn more about how seasonal allergies develop in this video.

An estimated 30 percent of American adults and 40 percent of kids now have allergies of some sort, with pollen allergies being one of the most common.

Certainly the imbalance between male and female trees and plants isn’t the only reason for the growth of pollen allergies. Climate change is boosting temperatures and CO2, spurring plants to grow faster and release more pollen. Other contributors include air pollutants that cover pollen grains and increase their allergic potency, as well as a lack of tree and plant biodiversity and too many of the same kinds of pollen-producers spaced too close together.

Still, any full picture of the pollen allergy crisis has to also include the overload of male trees and plants in outdoor spaces.

As Ogren writes in Scientific American, "In order to put the brakes on America’s allergy epidemic, we need to reverse the trend toward male-dominated landscapes and stop selling and planting any more of the most allergenic trees, shrubs and grasses in our cities … It is time for a less sexually discriminatory planting strategy."

Allergy busting

Some cities like Las Vegas and Tucson now have ordinances banning the sale or planting of clonal male trees. Las Vegas resorted to regulations after recording the highest U.S. pollen count ever in 2005 at an elementary school – almost 70,000 grains per cubic meter. The culprit: 200,000 male mulberry trees planted in the city from the 1950s through the 1990s.

pollen covers everything The return of warm weather in spring means the release of yellowish, dust-like pollen that seems to cover everything. (Photo: Alastair Vance/flickr)

What can you do? Ogren advocates encouraging local growers to sell more female and other low-allergy plants and trees and lobbying your municipality to use them. He calls it "considerate landscaping," and urges all homeowners to do the same in their own yards.

To help, he’s developed the Ogren Plant Allergy Scale (OPALS), which ranks the allergy-potential of thousands of common trees, shrubs, flowers and grasses.

Perhaps we should all send this link to our local garden center and city officials.

Is 'botanical sexism' making allergies worse?
Preference for "cleaner" male trees and plants in urban settings may trigger extra sneezing and wheezing. That's botanical sexism, says Thomas Ogren.