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Wild buckwheat growing on wheat plants (Photo courtesy “Biology and Management of Wild Buckwheat”)
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“The way to think about this is that, if you took a group of 100 people and gave them a toxic substance, one or two of them would have a natural genetic tolerance to that toxin,” Johnson says. “The same thing is true with weeds. As you apply selection pressure—the herbicide—you kill off the ones that are susceptible, but the ones that are resistant will complete their life cycle, produce seed and then repopulate the area. That's how the resistance problem grows.”
Herbicide-resistant weeds can show up almost anywhere, including urban areas. “If herbicide resistance can develop in agronomic fields, it can certainly develop around the home, because, historically, homeowners are much less judicious in pesticide use than are farmers,” Johnson says. “Spraying the cracks in the driveway and around the foundation of your house puts selection pressure on weeds.”
Another pressure point is state law. Indiana statutes require property owners to remove certain noxious and invasive weeds from their land, including Canada thistle and Johnsongrass, says Glenn Nice, another Purdue weed scientist.
“It might surprise people to learn that the penalty for allowing detrimental plants to grow and mature is rather severe,” Nice says. “Any person owning or having charge over land, highway or railroad right-of-way who allows detrimental plants to grow and mature is guilty of a Class C infraction, which carries a maximum fine of $500.”
An end run around resistance
So how does a person control weeds without helping herbicide resistance along? While the resistance cycle cannot be stopped, it can be slowed significantly, Johnson says. He suggests not using the same herbicide year after year, eradicating problem weeds before they produce seeds, frequent mowing and rotating herbicide modes of action when crops are rotated.
Future Purdue herbicide resistance research is planned for giant ragweed, a plant infamous for the misery it bestows on hay fever sufferers. “We will be evaluating how well glyphosate works on giant ragweed,” Johnson says.
A couple months in Johnson's interrogation program, and the weeds will be ready to tell him what he wants to know.
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