A battle of the sexes is happening in Canada's urban forests and the losers are those with allergies.

A study shows the dominance of male trees is making it worse for people who suffer from allergies.

The study found that because female trees tend to make more of a mess with flowers and fruit, cities and nurseries shy away from planting them, which results in more male, pollen-producing trees.

"The female trees tend to make a little more mess in terms of seed production and fruit production, so they would move to male trees, because they aren’t making a mess," said Bill Roesel, a municipal forester in Windsor, Ont.

Percentage of male trees

Victoria 74%

Halifax 90%

Regina 92%

Edmonton 92%

Vancouver 92%

Ottawa 94%

Saskatoon 95%

Toronto 96%

Montreal 97%

Winnipeg 97%

Source: Canadian Urban Allergy Audit

According to the study, which was commissioned by the makers of the allergey drug Reactine, 74-98 per cent of trees in Canada's urban forests are male, depending on the city.

"Female trees drop seeds and fruits while males are considered litter-free, so they are favoured in urban planning," said horticulturist Thomas Leo Ogren, who was contracted by Reactine to conduct the study.

Roesel said that in a city like Windsor he would opt to plant monoecious trees, ones that produce equal amounts of male pollen and female flowers and fruit, such as maple or oak.

Roesel said the city used to have more male than female trees — most of them of the ash variety — but when the emerald ash borer arrived, it killed off the trees.

Roesel admitted the city refuses to buy ginkgo biloba trees, because the "female fruit is rancid."

And the city no longer plants poplars, because they produce so much pollen and make such a mess.

Ogren recommends that city planners add the allergy potential of a tree to their list of considerations when selecting new trees to plant.

He also said gardeners and allergy sufferers can help reduce irritants by recognizing the sex of a neighbour's tree. If a neighbour has a male tree, plant a female one, Orgen suggests.

Roesel said determining a tree's sex isn't that easy.

"A lot of time, you can't really tell for the first little while," he said. "Trees have to be fairly mature before they begin producing pollen or flowers."