Ragweed frequent culprit in hay fever
Many of the streets in our part of town are intersected and backed by an intertwining maze of alleys that provide a bounty of options for my morning rambles with the brown dog.
These alleyways suit both our temperaments; his nose to the ground picking out the alluring scents of urban raccoons, possums and skunks, and I’m just nosy. Homeowners typically put a fair amount of effort into the appearance of their front yards, and it interests me to see what they do with their less public space in back.
Something else I’ve noted on these walks is that the edge of properties bordering an alley can be a sort of an ecological Wild West for plants, especially where fences separate the alley from backyards. No more than a foot or so of raggedy, low-quality soil between fence and graveled road, these commonly untended strips bring to mind a slow-motion shootout between deadly serious competitors played out over the course of a growing season.
From a human perspective, it’s often the bad guys who win.
The 2010 National Health Interview reported 25.7 million Americans diagnosed with allergic rhinitis — hay fever — in the year preceding the survey. That amounted to 7.8 percent of adults and 9.5 percent of children in the population being so diagnosed; adding undiagnosed cases of hay fever would bring those numbers higher.
The term “hay fever” is a misnomer stemming from the early days when people thought their seasonal affliction with sneezy, runny noses and red, itchy eyes were caused by the fall harvesting of hay. The array of unpleasant symptoms are indeed the result of the immune system’s overreaction to something in the air, but the odor of freshly mown grasses isn’t it.
Tiny molecules that trigger an immune response of some sort are referred to as antibodies. When those antibodies lead to an abnormally vigorous response, way out of proportion to what’s needed to defend the body, they’re referred to as allergens (allergy-generating antibodies).
Some of the more common airborne allergens aggravating those of us affected by hay fever include pollen from grass, trees and weeds, pet hair and dander (minute bits of shed skin), mold spores, dust mite wastes, cigarette smoke, diesel exhaust fumes and perfume. And about half of all pollen-related hay fevers in the U.S. are the result of ragweed pollen, the production of which peaks in Ohio around mid-September.
There are some 50 species of ragweeds native to the Americas, 12 of which are found in the U.S. Most of these reside in arid regions of the Southwest, but the common, giant and western ragweeds are broadly distributed throughout most of North America.
It’s the common ragweed I most often see growing in healthy profusion along the sides of alleys during my morning walks. The bushy, much-branched plant with finely divided leaves — the “rag” of ragweed refers to most species’ ragged appearance — seldom grows much over 2 feet tall. Like many ragweeds, it is easily overlooked because its flowers are tiny, green and arranged in tight clusters along a number of spikes topping the plant.
Unattractive to most human eyes, perhaps, they are undeniably tough competitors, succeeding in hard-scrabble combat against other weedy plants specializing in finding and dominating disturbed habitats. The problem for sufferers during hay fever season stems from the ragweed’s reproductive behavior.
Each plant produces upwards of a billion pollen grains per season, each one so tiny that the wind readily disperses them hundreds of miles away. (It is even tinier proteins on the outer surface of the pollen — rather than the whole grain itself — that actually trigger our allergic reactions.) The ragweed’s ease of travel, coupled with its gung-ho competitive growth, has made it a problematic invasive in Europe, India and mainland Asia, Australia and New Zealand, North and South Africa — virtually anywhere in the world besides the arctic, tropics or deserts.
And they are not just a problem to be addressed with antihistamines. It’s been estimated that common ragweed infestations of soybean fields can result in production losses of up to 30 percent. The problem can be even more dramatic with giant ragweeds. As broadly distributed as the common, they are less easy to overlook, typically growing to over 7 feet with large, 3-5 lobed leaves.
Researchers from Purdue University found that competition from just two giant ragweed plants per 110 square feet reduced corn yield by 13 percent and one plant per 110 square feet reduced soybean yields by 50 percent.
If we humans have our problems with ragweeds, they do have their fans. Common ragweeds produce 30,000 to 67,000 tough-shelled, oil-rich seeds per plant, providing a valuable source of winter food for many birds and small mammals.
Ken Baker is a scientist and a retired biology professor. If you have a natural history topic you’d like the author to consider for an upcoming column, email your idea to firstname.lastname@example.org.