Giant Ragweed: A Weed of Extremes

Friday, September 27th, 2019

Dr. Emilie Regnier with giant ragweed (Photo: OSU/CFAES Ken Chamberlain)

"Formidable."

That’s how Dr. Emilie Regnier describes giant ragweed (Ambrosia trifida). Regnier, an associate professor and researcher in weed ecology, has completed two USDA-funded studies and a survey of certified crop advisors regarding giant ragweed. Capable of reaching a staggering 17 feet tall and responsible for making millions of allergy sufferers miserable, giant ragweed is on the minds of many this time of year. Regnier’s work has shown the ability of giant ragweed to adapt to changes and expand its range both culturally and geographically.

Many of our worst weeds have been introduced from areas where their spread is controlled by natural predators. Giant ragweed, however, is native to North America and has plenty of natural predators, including, at one time, the indigenous people of North American, says Regnier. Just about every part of it is regularly consumed by something. So how is it that giant ragweed remains such a problem?

Most of its advantages center on an uncanny ability to adapt. Giant ragweed shows a great degree of genetic diversity. Anyone can observe this, says Regnier. If you were to collect seed from five different plants and pile them into 5 piles, chances are that the seed from each of those plants would be different in appearance (size, number and size of points, color).

“It’s almost like each plant has a fingerprint,” she says.

This physical variability is something you can see, but it indicates diversity in other characteristics. Former Ohio State graduate student Brian Schutte showed seed dormancy in giant ragweed had wide variation. Most ragweed emerges in early March, but he found some seeds with emergence windows reaching into July and August. This variation makes it easy for giant ragweed to escape weed control methods.

Giant ragweed’s genetic diversity has allowed it to respond and benefit from changes in land use and cropping systems. Initially considered a riparian plant growing along ditches and waterways, giant ragweed is now frequently found in the midst of row crops. Regnier’s survey of certified crop advisors showed that, over the last 30 years, giant ragweed has expanded to fields outside of the corn belt in all directions. It is one of several weeds developing resistance to common herbicides—an issue much in the current spotlight. Ragweed has also benefitted from changes in what Regnier calls the “architecture of modern agriculture,” as soybeans and shorter corn varieties have become more common in rural landscape over the decades.

“The selection for crop species that are shorter, and therefore don’t lodge as much, has lots of benefits; but, on the other hand, it reduces their ability to shade out weeds,” she says, “That might be one of the cultural reasons that giant ragweed has become worse as a weed.”

Don’t despair! Giant ragweed does have some weaknesses, says Regnier. As an annual plant, giant ragweed is completely reliant on seeds to reproduce. Since flowering time is strongly driven by the hours of daylight we know it will begin around mid-August.

“If you can control the plant so that it’s not big enough to flower at that time then you can prevent the reproduction of seeds.”

Giant ragweed typically requires pollen from another plant, so one plant rarely causes an outbreak. And happily, giant ragweed is not a prolific seed-producer, nor a seed that remains viable in the soil for long periods of time. Most giant ragweed seeds lose viability in the first year but can survive longer if buried by deep tillage or stored underground by earthworms and rodents. Even seeds that are produced, are often consumed by mice and voles and sometimes by insects.

What if you already have a giant ragweed problem?

Knowing giant ragweed’s weaknesses can help solve an existing problem.

  • Since most giant ragweed emerges in early spring, having a competitive cover crop or perennial in place in March can reduce its emergence. Alfalfa or mixed hay should be able to control ragweed effectively by providing competition in the spring and with mowing events to set back late emerging weeds. Since the seeds rarely survive longer than one year, including a forage crop in rotation should help prevent ragweed problems.
  • Alternatively, a crop could be be planted later in the spring after most of the ragweed seeds have germinated. The false seed bed technique uses repeated bouts of light tillage to germinate then terminate weed seeds in the field.
  • Taller fast-growing crops densely planted can help shade out later emerging giant ragweed plants.
  • Giant ragweed seeds can float and are often cached by mice and other rodents. Managing the weed in non-crop areas through mowing can help prevent its spread into planted areas.
  • For more on information and resources on organic weed management, visit go.osu.edu/eco-weed-mngt
giant ragweed in flower

Although giant ragweed is less problematic in tall crops like corn, this weed can certainly outgrow field corn varieties, reaching a staggering 17 feet tall.

giant ragweed seeds

Giant ragweed seeds are relatively large (3/16 to 7/16 inch long and 1/8 to 1/4 inch wide) and encased in a woody hull with a distinctive set of points at the top and smaller points or ridges around the middle of the hull. Left on the soil surface, these seeds are a favorite treat of mice and other small rodents. 

young giant ragweed Giant ragweed typically emerges in March. The first (cotyledon) leaves have an indentation at the base and are fairly large -- 3/8 to 5/8 inch wide, 1 to 1 3/4 inches long. The first true leaves are ovate, lobed and directly across the stem from one another. The second pair of true leaves shows the more familiar shape of giant ragweed. Leaves have stiff hairs that point toward the leaf tip and usually form 3 distinct lobes but can have up to 5. 

Photo of young ragweed plants is from the Ohio State weed lab, courtesy of bugwood.org
Other photos by Ken Chamberlain, Ohio State, College of Food Agricultural and Environmental Sciences.

Posted In: weeds, research
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September 27, 2019 - 1:37pm -- brown.1844
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