weeds

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.

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Emerging weeds like these are easier to kill.

Ah spring!  The war against weeds begins anew. The first major skirmish of the growing season should happen before planting. The stale seed bed technique is an often over-looked practice that can be used before planting. It works by first encouraging weeds to sprout and then killing them when they are young and most vulnerable. For organic growers, a stale seed bed can replace the effects of a pre-emergence herbicide. And when used properly, it can contribute to both short-term and long-term weed management.

Weed control can be handled with short-term or long-term approaches. Short-term management focuses on controlling weeds during the first part of crop growth when weeds are more likely to affect crop yields. Long-term weed management, however, works all season-long to deplete weed seeds from the seedbank (the reservoir of viable weed seeds in the soil). Whichever approach you take, using a stale seed bed is a great cultural weed control technique.

To use the stale seed bed most effectively, start several weeks before planting. An initial cultivation kills any emerged weeds that have overwintered. It also brings weed seeds to the surface where exposure to light and oxygen stimulate germination. Depending on the weather and types of seeds present in the soil, weeds may sprout up overnight or over a few weeks. When weeds have germinated and are still small and young, they are easy to kill with a second light cultivation. This process is then repeated as needed and as time allows. As few as three cycles of light/ shallow tillage can reduce the number of subsequent weeds noticeably. For fields and gardens with very heavy weed infestations more cycles of repeated tillage over a few years will be needed. Using a stale seed bed may push back your planting date; but in the absence of weed competition, the crop will have more access to water and sunlight and be able to make up for lost time.

Keys to Success

  • Do not allow emerged seedlings to grow large. It is best to till lightly just as the first seedlings are emerging as this and the earlier ‘white thread’ stage are the most susceptible to desiccation.  The more time new weeds have to develop roots, the harder they become to kill with a shallow cultivation.
  • Keep the cultivation shallow to avoid bringing new weed seeds to the surface. The implement used to stir the soil should not go deeper than 2 inches with most of the stirring in the top inch. 
  • The technique is dependent upon having adequate soil moisture.  Under drought conditions preparation of a stale seedbed may require irrigation to stimulate weed seed germination.
  • Deeper initial tillage can be used to bury an existing weed problem. Tillage, especially when done with a disc or a power tiller, distributes the previous year’s weed seeds throughout the top 6 inches or so of soil. In contrast, an inversion tillage that turns sod upside down will place last year’s seeds 6 inches or so under the surface. From there they are unlikely to emerge unless further discing or lighter tillage moves them closer to the surface. Used skillfully, a deep inversion plowing followed by stale seed bed can put a serious surface weed problem out-of-sight and out-of-mind, at least until the next time the field is plowed deeply. 

Stale Seedbed is most effective when it’s part of a zero weed threshold system.

The common short-term approach to managing weeds(which weed scientists usually call the “critical period approach”) is to control weeds aggressively during the first 4-6 weeks after the crop is planted. This 4-6-week period is the critical period during which crops stands are established and yield is secured. Afterwards weeds are of less threat to production; therefore, many farmers scale back control efforts. However, weeds that grow before and after the critical period are still a problem. If allowed to flower and set seed, they will be planting a future crop of weed problems. A long-term approach to weed management, called zero weed seed threshold, requires constant diligence and removal of all weeds before they produce seeds--even after harvest. Research indicates that 3-4 years of using this approach will result in a field with relatively few weeds, provided weed seeds are not introduced from without the field (in seed, irrigation water, on equipment, etc.). 

Both short-term and long-term approaches have benefits and drawbacks, many of which depend on a farmer’s individual goals, crops, and available resources. A new online tool from Ohio State allows farmers to think through various weed control approaches in the context of their own individual situations. For those looking to make changes to their weed management, the Organic Weed Decision Making Tool, shows pros and cons of various strategies over time and gives steps to implementing new tactics. Learn more at go.osu.edu/eco-weed-mngt.

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Dr. Christine Sprunger, Ohio State, shares data on the influence of management practices on soil health

Recordings are available from the 27th Annual Conservation Tillage and Technology Conference, held in Ada, Ohio, in March 2019. This two-day event brought together speakers in a variety of subject areas – many of which will be of interest to organic farmers. 

Videos are available on the conference’s You Tube Channel.

Here are some of the offerings:

Cover Crop Panel: Addressing Cover Crop Seed Issues
Sarah Noggle, OSU Extension, Paulding Co., Moderator; Jay and Ann Brandt, Walnut Creek Seeds; Don Grimes, Ohio Seed Improvement; Cody Beacom, Bird Hybrids

Protecting Identity Preserved Crops In The Field:
Managing Pollen Drift to minimize contamination of Non-GMO Corn
Dr. Peter Thomison, OSU Extension Corn Specialist

Enhancing Mycorrhizae And Metarhizium Fungus
Jim Hoorman, USDA-NRCS, Soil Health Specialist

What Management Practices Most Influence Soil Health In Corn Production?  
Dr. Christine Sprunger, OSU Assistant Professor, SENR

Enhancing Beneficial Insects With Pollinators
Dr. Stephanie Frischie, Xerces Society Agronomist / Native Plant Materials Specialist, Plymouth, WI

Can Weeds Be Managed With Calcium Amendments?  
Dr. Doug Doohan, OSU Professor, HCS, and Andrea Leiva Soto, OSU PhD Student, HCS

Elephant In The Room: Why Do So Many Farmers Practice 'Soil Balancing' Despite The Lack Of Scientific Evidence?
Dr. Doug Jackson-Smith, OSU Professor, SENR, and Dr. Caroline Brock, OSU Senior Research Associate, SENR

The Effects Of Manipulating Ca:Mg Ratios On Ohio Crop Yields And Soil Health
Dr. Steve Culman, OSU Assistant Professor, SENR, and Will Osterholz, USDA-ARS

Weather Pattern Effects On Conservation Practices
Dr. Aaron Wilson, OSU, Byrd Polar and Climate Research Center
https://cfaes.osu.edu/news/articles/farmers-need-gear-for-more-rain

Return On Investment With Using Gypsum
Dr. Subbu Kumarappan, OSU Associate Professor, ATI

Gypsum Is More Than Calcium: Summary Of Ohio Field Crop Responses To Sulfur
Louceline Fleuridor, OSU MS student, HCS

organic weed decision making tool

Weed control without chemicals means fewer options. In fact, weed control is often cited as a reason farmers decide not to pursue organic certification. Although ecological weed management tactics are available, research reveals that many organic farmers do not take advantage of them.

This may happen for a variety of reasons. All weed control methods come with pros and cons, which vary depending on specific growing conditions, cropping systems, and farmer priorities. For example, in a low-value crop, investing time into mechanical or hand-weeding may not be economically feasible. In poorly draining soils, frequent cultivation may lead to soil compaction and drainage problems, or field conditions may make it difficult to add a cover crop into the rotation. 

To help farmers consider these trade-offs, a team at The Ohio State University and Michigan State University has developed an online tool to present different weed control strategies, along with their historic long and short-term performance.

How it works.

The Organic Weed Manager tool collects specific information on growing conditions and farmer priorities. The program then compares the users’ current strategy with alternative approaches, showing the predicted impacts over time across diverse objectives (weed seedbank, costs, soil health, etc.). For those who want to consider a new approach to weed control, the tool also suggests specific steps to take.

The online tool is easy to use and gives individualized results. The tool is not meant to make management decisions for you, but rather to provide an opportunity to consider and compare alternative strategies, while reflecting on individual priorities and values. In this way, the research team hopes to lower barriers to on-farm experimentation and improve adoption of organic weed management techniques.

Learn more.

The Organic Weed Manager software is free and available at organicweedmanager.com. Completing the tool takes approximately 20 minutes. Users may save their progress and return later to complete it. Users can access the tool from a desktop or laptop computer, tablet, but not from a phone. Farmers and farm advisors are encouraged to use the tool and provide feedback for further development. The tools itself contains an area for evaluation and comments.

For more information about organic weed control options, including case studies, resources, and ongoing research projects, visit go.osu.edu/eco-weed-mngt.

Try out the tool yourself

Watch a video about the tool.

Read about how the tool works.

Read more about organic weed control strategies.

 

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