Ragweed’s Rule

in Grass & Ragweed, Outdoor Allergies
Published: July 2, 2010

How did the dastardly king of hay fever become so potent and so widespread?
Allergic Living

The ragweed in my backyard in the Ottawa Valley waves at me smugly every morning. It has formed a towering wall along the back corner of the small meadow we’ve let flourish as an environmental gesture. All the other wildflowers in the meadow co-exist happily, but the ragweed, which characteristically found a toehold in the least promising soil, has now started to elbow its way toward to the front. Unfortunately, its braggadocio is well-founded.

Not only is ragweed an aggressive competitor, it is an able procreator that produces pollen of unrivaled allergic potency. Its pollen triggers reactions in up to 75 per cent of hay fever sufferers, depending on where they live in North America, making this one weed to be reckoned with.

Operation Proliferate

The tens of millions of Americans who now suffer from ragweed allergies can thank Canada for their misery. Common ragweed’s likely point of origin in North America was Ontario, according to Dr. Peter Creticos, clinical director of the Division of Allergy and Clinical Immunology at Johns Hopkins School of Medicine in Baltimore. He jokingly calls ragweed “Canada’s gift to the colonies,” referring to the plant’s ability to dump hundreds of thousand of tons of dust-like pollen across up to 1,000 kilometres of the continent every year.

Now ragweed can be found all across eastern and central Canada and the United States, though there is thankfully little of it west of the Rockies. The crown-shaped seeds are highly mobile and have even deployed overseas, particularly near existing and former Canadian and U.S. military bases in Western Europe.

Creticos suspects the seeds hitched a ride on equipment shipped from North America during and after the wars, then launched a horticultural offensive into Eastern Europe, where it now enjoys a strong presence. For allergy sufferers, misery loves company.

Flourishing on the Farm

Back in North America, common ragweed and its big brother, giant ragweed, are causing havoc in the crop farming sector. Clarence Swanton, a University of Guelph professor and weed scientist, says ragweed grows well along roadsides and train tracks, but really loves the rich, loosely tilled soil of corn, soybean and cereal crop fields.

“Ragweed seedlings emerge early and therefore are very competitive with the crop for light, water and nutrients,” he says. Swanton estimates bean crop losses in Ontario alone range from 8 to 30 per cent, representing millions of dollars annually.

The Ontario government’s chief of weed control, Mike Cowbrough, says uncontrolled ragweed can cause yield losses of up to 80 per cent in some operations – a financial disaster for farmers. He says ragweed is one of the toughest weeds to control: it takes repeated mowings to prevent the plant from blossoming, its seeds can remain dormant for up to 50 years, and several strains have become resistant to traditional herbicides.

In the United States especially, some ragweed has even become immune to glyphosate (known in the retail sector as Roundup), once thought to be the magic bullet for weed control.

Frustrated farmers with glyphosate-resistant ragweed are creating herbicidal blends, incorporating older herbicides like 2,4-D and dicamba in hopes that their ragweed is not resistant to those mixtures. But farmers who market organic produce are in a conundrum, since herbicides are a no-no. Control often has to be done mechanically with a cultivator, which can speed soil erosion, or even manually.

Soybean farmers must also contend with ragweed’s staining quality, which can turn the pristine white soybean – and the tofu it’s used to make – to an unappealing shade of green.

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