Late summer through early fall is glorious in my garden. By this time I’ve let the natives run wild. Goldenrod, boneset, turtlehead, and obedient plant, all big boys at four feet or more, spill over each other, nodding under the weight of bees and butterflies. The insects are at a crescendo of activity. Crickets and cicadas whir and buzz night and day. Birds are everywhere. And the changing light burnishes everything in a golden glow.
I enjoy this sight through red, itchy eyes, between sneezes, while holding a tissue to my nose. I have fall allergies, otherwise known as hay fever. About 50 million Americans share my misery.
I’m not writing to give advice on what to do about the symptoms that can make this beautiful time of year unpleasant for so many. There’s plenty of information on the web about fall allergies and how to treat them. But from a botanical standpoint, I’m curious. Which of the plants I see in the fields and woods around my house are making me sneeze?
It’s plant pollen that triggers seasonal allergies (mold does as well, but that’s a whole other subject). But that’s not the pollen you see bees and other beneficials loading up on; that’s too heavy to become airborne. No, it’s the airborne stuff that’s the offender. In spring and summer airborne pollen comes from trees or grasses. At this time of year the culprits are weeds.
While the plants that cause symptoms may differ depending on your region, here are three of the biggest offenders:
Common ragweed (Ambrosia artemisiifolia). Photograph courtesy of Laval University, CC BY-SA 4.0, via Wikimedia Commons
Giant ragweed (Ambrosia trifida). Photograph by Chris Light, CC BY-SA 4.0, via Wikimedia Commons
Ragweed (Ambrosia spp.): Ragweed is the national champ when it comes to causing seasonal allergies. It’s estimated that it causes about half of all pollen allergies in the U.S. One reason is that it grows everywhere in the country. There are 50 species of ragweed, some native, some introduced. The plant, including its tiny flowers, is a nondescript green, and it grows anywhere from one to five feet tall. A single plant can produce as many as a billion grains of pollen. It likes the same cultural conditions as our native goldenrod, which is why the two are often seen growing together. Ragweed is so unremarkable that it disappears into the background, letting the flashier goldenrod take the blame for the damage it does.
Russian thistle (Salsola tragus). Photograph by Eric Coombs, Oregon Department of Agriculture, Bugwood.org.
Russian Thistle (Salsola spp.): This spiny weed is prevalent in the western U.S., though it’s been found as far east as Iowa. It arrived here in the late 1800s from Russia, mixed in with flaxseed. An annual, it prefers sandy soil. Doesn’t ring a bell? After the plant dies it’s given a more familiar name: tumbleweed. Rolling through the landscape is how a single Russian thistle disperses up to 200,000 seeds. The plant is not just an allergen. It can be harmful to livestock if eaten and the dead plants are a serious fire hazard.
Redroot pigweed (Amaranthus retroflexus). Howard F. Schwartz, Colorado State University, Bugwood.org.
Amaranthus retroflexus flowers.Photograph by Steve Dewey, Utah State University, Bugwood.org.
Pigweed (Amaranthus spp.): There are about 75 amaranth species, some native to North America. Its ornamental relatives are well known: the old-fashioned annual love-lies-bleeding, the upright-growing tassel amaranth, and A. tricolor, grown for its foliage. Other amaranths are grown for their edible leaves or grain. The weed part of the family grows throughout the U.S. and Canada. One, redroot pigweed, can get to six feet in height, with green flowers. Pigweed is now a big problem for farmers, because it has become resistant to herbicides, including glyphosate, the active ingredient in Roundup.
As for controlling ragweed and its brethren, hey, we’re gardeners. Gardeners weed. So can’t we just weed our property and remove the offenders? If it were only that easy. Controlling these weeds is impossible. They grow everywhere, from rural areas, to the suburbs, to cities. They’re found in fields, vacant lots and neglected spaces, along waterways, and on roadsides. And since that pollen is airborne, it can travel hundreds of miles until it reaches our nostrils. May I offer you a tissue?
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