» Weeds » Weed myths

Some of the common myths about weeds and weed control are debunked. Please forward your contributions to Joyce Lancaster (jlancaster@allenpress.com).

  • Myth: Spraying herbicides eventually results in “super weeds.”
    Reality: Most resistant weeds have no growth or fitness advantage, and weeds resistant to herbicides with certain modes of action are sometimes less fit.
     
    Adapted from myth submitted by Stephen Duke, USDA Agricultural Research Service:
    Charles Darwin could have predicted that the continued use of a single herbicide or herbicide class would eventually result in the evolution of resistant weeds.  There are many real-world examples that show his theory of evolution in action. But, resistant weeds are not “super weeds.”  They usually have only one mutated gene that confers resistance to the herbicide that was the selection agent.  The mutation does not give the weed an advantage in the absence of the herbicide. In fact, in some cases it puts the weed at a slight disadvantage to the non-resistant weeds of the same species.  
  • Myth: New herbicide trait technology is going to result in older, harsher chemicals being used as a substitute for glyphosate.
    Reality: ALL herbicides – old and new – satisfy the same health, safety and environmental criteria.
     
    Adapted from myth submitted by John Jachetta, Dow AgroSciences:
    A lot of people like glyphosate. That’s why it has been used exclusively for so long, and why in some areas we’re now seeing glyphosate-resistant weeds. Clearly, we can’t keep using glyphosate alone season after season without creating a serious problem for food production.
     
    If people like glyphosate, the surest way to preserve its effectiveness long-term is to bring in other herbicide modes of action (the physiological mechanism by which an herbicide acts upon a plant) to support it. Weeds facing several modes of action will have to try to develop resistance to each of these products. In essence it forces the weeds to win the lottery multiple times in order to survive and stacks the odds against them.
     
    These complementary herbicides used with glyphosate are not “harsher.” They have met the same stringent U.S. regulatory standards and have been through the same thorough health, safety and environmental evaluations as glyphosate. In fact, U.S. EPA is prohibited by federal law from authorizing any herbicide for any food-related use unless scientific evidence shows “a reasonably certainty of no harm.” Regulators must also determine based on the best available scientific evidence that use will not pose “unreasonable adverse” environmental effects.
     
    Using glyphosate with other herbicides having different modes of action is clearly the responsible choice.
  • Myth: We’ll always be able to spray our way out of any weed problem.
    Reality: Integrated techniques are essential to successful weed management.
  • Adapted from myth submitted by Adam Davis, USDA Agricultural Research Service:
    Over the past 60 years, weed scientists have been very successful in developing new herbicide chemistries to control weeds in a variety of crops and growing situations. But relying solely upon herbicides for weed management is not a panacea.First, the herbicide discovery “pipeline” is not inexhaustible. Herbicide registrations have declined substantially over the past 20 years due to regulatory constraints, the cost of product development and the genuine scarcity of novel chemistries. Second, less than 20 herbicide modes of action (the physiological mechanism by which an herbicide acts upon a plant) have been found, with the most recent discovery made almost two decades ago. Finally, excessive reliance upon a single mode of action can cause resistance and reduce herbicide effectiveness.

    The widespread adoption of herbicide-resistant crop cultivars has intensified the problem, with cross-resistance and multiple resistance becoming more common. Successful long-term weed management will depend upon the adoption of integrated weed management practices that balance chemical, physical, biological and cultural control methods to preserve the usefulness of herbicides for generations to come.

  • Myth: Organically-grown fruits and vegetables contain no pesticides.
    Reality: You will find pesticides in ALL crops – even those grown organically.
     
    Adapted from myth submitted by Carol Somody, Syngenta Crop Protection:
    All fruits and vegetables contain natural pesticides, which is nature’s way of protecting them from pests. In fact, our daily consumption of natural pesticides is estimated to outweigh the traces of synthetic pesticides in our food by as much as 10,000:1. If these natural pesticides were tested the same way that synthetic pesticides are (at the maximum tolerated dose), many would cause serious effects on test organisms.
     
    Both conventional and organic growers often use pesticides beyond those naturally present in plants. The organic grower, though, has a much more restricted list of choices, including copper, sulfur and other pesticides found naturally in the environment. But that does not mean these naturally occurring products are “safer” than synthetic pesticides. For example, copper is considered a natural pesticide, but it can be dangerous to apply, can buildup in the environment, and can be very toxic to fish and aquatic organisms. Yet, organic growers can use it and still comply with organic growing guidelines. Many synthetic pesticides prohibited in organic growing systems have lesser safety concerns than copper. The words “natural” and “synthetic” do not mean that something is “safe” or “not safe.” Any pesticide, whether synthetic or natural, must be applied exactly as directed on the label. Use all appropriate stewardship practices and don’t be complacent.
  • Myth: It is best to control dandelions in the spring when they flower.
    Reality: If you live in a climate where dandelions go dormant during the winter, they are best controlled in the fall before they flower the following year.
     
    Adapted from myth submitted by Aaron Patton, Purdue University:
    Extension personnel get many questions in the spring from individuals who want to limit the spread of dandelions by spraying, picking or hand pulling the weed from their lawn. Count the millions of dandelions along the roadsides, farm fields, parks and right-of-ways, though, and you will know it’s a futile task. Each plant can produce hundreds of viable seeds that are easily spread. Controlling the few dandelions on your property will not significantly affect the broader seed source. Instead, concentrate on maintaining the thickest turf possible in order to prevent dandelion from germinating and getting established in the first place.
     
    If you decide herbicides are necessary to manage dandelions and other broadleaf weeds that germinate in the fall, you’ll have the best control if you apply them in fall. The period from late September to mid-November is ideal because broadleaf weeds are most susceptible to herbicides at this time. Read and follow all directions on the herbicide label. Apply on a sunny day with moderate temperatures, no wind, ample soil moisture and no rain in the 24-hour forecast. Herbicides containing two or more active ingredients including 2,4-D, MCPP, MCPA, dicamba, or triclopyr will control most broadleaf weeds with one application. Consider spot-spraying the weeds to limit the amount of pesticide you apply. Many “weed and feed” products should be applied in mid- to late-September to receive the full benefit of the fertilizer and the most efficacious weed control.
  • Myth: Transgenes are running “wild.”
    Reality: Transgenes are not threatening our natural ecosystems.

    Adapted from myth submitted by Jonathan Gressel, Weizmann Institute of Science:
    Genetically modified or “transgenic” herbicide-resistant crops have become common. One concern raised by some has been that these crops can escape into the wild and threaten our natural ecosystems. Transgenic oilseed rape was found along field borders and roadsides in the Canadian prairie provinces as early as 2005. A U.S. group scouted North-Dakota highways years later and also found transgenic rape, but, again, the plants were growing in a “ruderal” setting (disturbed by man), not in the wild, and there is a big difference.
     
    A transgenic plant would need a selective advantage to survive in the wild, but instead it has been bred with “wimpy” domesticated genes. The herbicide resistance trait created in the lab may help transgenic plants survive along roadways, railways and other herbicide-treated “ruderal” ecosystems, but it provides no advantage at all in the wild where herbicides are rarely used. In those undisturbed natural settings, the transgenic plants must compete head-to-head with stronger native species. The odds of them “running wild” are slim.

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