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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
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 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.
|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.
written by Andrea Leiva Soto, Horticulture and Crop Science
Ohio State researchers compared an organic system to a conventional one, looking at several soil quality indicators such as bulk density, organic matter content, and nematode populations. After four years, the organic system had fewer harmful nematodes, especially during the hay phase of the rotation. Mineral nitrogen was more abundant in the conventional system, while microbial nitrogen prevailed in the organic system. Soil bulk density did not differ between systems, even though intensive tillage was done in the organically managed fields. However, despite the high carbon inputs added to the organic system, organic matter was only slightly higher compared to the conventional system.
Nematodes have a bad reputation for damaging crops and garden plants, but some can be quite important for plant growth. Certain kinds of nematodes eat bacteria and fungi that cause plant diseases. Others decompose organic matter, providing plant nutrients. Studies indicate that nematodes supply 27% of the soil nitrogen that is available to plants. Today, nematodes are increasingly used as an indicator of the status of the soil food web. The soil food web is a complex network with organisms that provide services to the farm ecosystem like regulating pests, nutrient recycling, modifying soil structure, or even breaking down man-made chemicals.
Organic matter additions have been shown to influence nematode populations. Adding green manure cover crops or decomposed animal waste can decrease root-feeding nematodes. Additionally, organic amendments are known to increase soil nitrogen, organic matter and microbial biomass, and reduce soil bulk density, leading to less soil compaction. As a result, roots explore deeper and have more oxygen available leading to more vigorous growth.
However, the intensive tillage practices used to incorporate amendments or control weeds, disrupt the soil ecosystem, affecting the populations of beneficial microbes and nematodes. Synthetic fertilizers, insecticides, and soil compaction can also cause similar undesirable effects.
To better understand these kinds of interactions and develop insights into how best to manage them, a study at the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio, compared conventional and organic farming systems and how soil characteristics, nitrogen cycling, and nematode populations are affected by each system.
The conventional system used chemical fertilizers, herbicides, and reduced tillage in a corn–soybean rotation. The organic system incorporated fresh straw, beef manure, poultry compost, and intensive tillage in a corn–oat–hay rotation. Soil samples were taken in the spring before soil inputs, and in autumn after crop harvest. Samples were taken from between and within the crop rows. Then for each sample, the nematodes were counted and identified, and soil bulk density, organic matter, and nitrogen were measured.
Results: After four years, the organic system had fewer harmful nematodes, especially for the hay phase of the rotation. Mineral nitrogen was more abundant in the conventional system, while microbial nitrogen prevailed in the organic system. Soil bulk density did not differ between systems, even though intensive tillage was done in the organically managed fields. And despite the high carbon inputs added to the organic system, organic matter was only slightly higher compared to the conventional system.
Take Home Messages
- When you are transitioning to organic, it is important to reduce synthetic inputs gradually. The soil system needs time to build different sources of nutrients to be sustainable in the long-term. It is known that after the transition period, organic farms have more nitrogen in the soil compared to conventional farms, mainly due to a build-up of the microbial nitrogen pool, but these benefits will not be available immediately.
- Organic amendments and crop rotations can decrease harmful root-feeding nematodes in the soil. And by including hay in the rotation cycle, you can decrease these nematode populations even more.
- Intensive tillage can reduce the soil-related benefits of organic farming. On the other hand, organic inputs should significantly increase soil organic matter and decrease soil bulk density. In the organic farming system discussed above, the benefits of the large organic inputs were diminished by the intensive tillage routine. Rather than seeing a decrease in compaction level, the soil bulk density remained the same. And there was only a minor boost in soil organic matter. Decreased use of tillage in organic farming would better take advantage of the benefits that an organic system can provide.
Read more about it:
This study was conducted at Ohio State in the early 2000s. Published results are availabe online.
Briar, Shabeg S.; Grewal, Parwinder S.; Somasekhar, Nethi; Stinner, D.; Miller, Sally A. 2007. Soil nematode community, organic matter, microbial biomass and nitrogen dynamics in field plots transitioning from conventional to organic management. Applied Soil Ecology 37: 256-266.
Read more news and information on organic agriculture research at offer.osu.edu.