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Pest Patrol: Dealing with willowherb

#PestPatrol with Mike Cowbrough, OMAFRA

Q: I’ve noticed small patches of a plant that was identified as “northern willowherb” in some of my fields. It seems tolerant to glyphosate. How do I control this before it becomes a bigger problem?

A flowering plant with pinkish purple petals in early July.
photo: Supplied

A: This perennial weed has traditionally been found along roadsides and waste areas. In the last five years, more farmers have noticed patches popping up in no-till fields, where it has not been adequately controlled by their pre-plant glyphosate application.

In 2017, Dr. François Tardif and Peter Smith (University of Guelph) screened several herbicides in the greenhouse so that they could identify possible solutions to test in the field. Fortunately, they have not been able to locate a field site with a large enough amount of willowherb to conduct a trial. Let’s hope field population densities of this weed remain low, but if you have a small amount of willowherb and don’t want to give it an opportunity to spread and increase in numbers, there were two herbicides, 2,4-D Ester and metribuzin (e.g. Sencor), that completely controlled northern willowherb under greenhouse conditions. Both herbicides can be used as a tank mix partner with glyphosate in a pre-plant burndown in soybean or field corn. Deep tillage (e.g. moldboard plow) is also effective at controlling this species.

An unsprayed control plant.
photo: Supplied

Willowherb control at four weeks after an application of glyphosate (900 gai/ha).
photo: Supplied

Willowherb control at four weeks after an application of 2,4-D Ester 700.

Willowherb control at four weeks after an application of Sencor 75 DF.
photo: Supplied

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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