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Kelly Feehan

Homeowners often turn to Nebraska Extension for answers. One weed related question is why didn’t the herbicide I used kill my weeds?

With late September through October being an ideal time to apply herbicides to broadleaf weeds like dandelion, ground ivy, white clover, violet, and bindweed, here are some questions to help determine why an herbicide did not work.

Was the weed correctly identified? Identification is needed to know what type of herbicide to use and at what time of year to apply the product. For example, herbicide applications made to broadleaf weeds during spring and summer are not as effective as late summer or fall applications.

Did the herbicide label list the weed you wanted to control? Herbicides are formulated to control specific weed types, such as grasses or broadleaf weeds, but not both. Within these types, some herbicides are more effective against specific weeds. Products containing triclopyr are more effective against white clover and violets than a straight 2,4-D product.

Was the recommended label rate used? Too low of a rate, as well as too high of rate, may decrease control. Too low of rates could lead to weeds becoming resistant to class herbicide class.

Was the herbicide applied to the weed when it was small or large? Larger, more mature weeds are more difficult to control than smaller weeds.

Was the temperature during application within the label recommendation? Not only could air temperature affect herbicide effectiveness; too high of temperatures can increase the risk of injury to the lawn or nearby ornamental plants.

Was soil moisture adequate for active weed growth and herbicide uptake? Weeds that are not actively growing will not take up as much herbicide.

Was the herbicide mixed with another type of herbicide? Some products can be mixed together and still work fine. With others, mixing can reduce the effectiveness of one or both of the herbicides.

Did the label recommend two or more applications? If so, was more than one made? Difficult to control weeds often require more than one application.

For postemergence herbicides, those applied to weeds while they are growing, did rain or irrigation wash the herbicide off within a few hours of application?

Did the label recommend the use of an adjuvant or surfactant and if so, was one used? These are sometimes called spreader stickers and they help a herbicide adhere to leaves.

For preemergence herbicides, those applied before weed seed germinates and begins growth, was the herbicide watered in within a few days of application? Was it applied too early in the season so it was no longer active when the majority of weed seed germinated? Was it applied too late and weed seed had already germinated?

Positively identifying weeds, selecting the correct herbicide, applying it at the right time of year, and closely reading and following label directions will increase weed control.

Source: Purdue Extension

Kelly Feehan is a community environment educator for Nebraska Extension-Platte County.

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