Yard and Garden: Timing and Applying Broadleaf Lawn Herbicides

September 6, 2017, 3:43 pm | Richard Jauron, Greg Wallace

AMES, Iowa – The air has a bit of a chill in it in the mornings. Fall is almost here, which means it’s time to think about preparing lawns with weed control. Fall is an excellent time to apply broadleaf herbicides for perennial broadleaf control, but take care to do it properly and effectively.

Iowa State University Extension and Outreach horticulturists can help answer questions about the right time and methods to apply broadleaf herbicides.

When is the best time to apply a broadleaf herbicide to the lawn?

Fall (mid-September to early November) is the best time to control perennial broadleaf weeds in the lawn with broadleaf herbicides. In fall, perennial broadleaf weeds are transporting food (carbohydrates) from their foliage to their roots in preparation for winter. Broadleaf herbicides applied in fall will be absorbed by the broadleaf weed’s foliage and transported to the roots along with the carbohydrates, resulting in the destruction of the broadleaf weeds. Spring applications are less effective than fall applications.


Which herbicides will effectively control broadleaf weeds in the lawn?

Effective broadleaf herbicides include 2,4-D, MCPP, dicamba, triclopyr and others. The most effective broadleaf herbicide products contain a mixture of two or three herbicides, as no single compound will control all broadleaf weeds.

What is the proper way to apply a broadleaf herbicide to the lawn?

Broadleaf herbicides can be applied as liquids or granules. Before applying any herbicide, carefully read and follow label directions.  

When applying liquid formulations, avoid potential spray drift problems by following simple precautions. Don’t spray when winds exceed 5 mph. Also, don’t spray when temperatures are forecast to exceed 85 degrees Fahrenheit within 24 hours of the application. Since coarse droplets are less likely to drift than fine sprays, select a nozzle that produces coarse droplets and use low sprayer pressure when applying liquid broadleaf herbicides. 

When spraying, keep the nozzle close to the ground. If only a few areas in the lawn have broadleaf weeds, spot treat these areas rather than spraying the entire lawn. Apply just enough material to wet the leaf surfaces. A single application of a broadleaf herbicide kills many broadleaf weeds.  Difficult-to-control weeds, such as violets, will likely require two applications.  

Granular broadleaf herbicides are usually combined with fertilizers. Apply granular broadleaf herbicides and fertilizer/broadleaf herbicide combinations when the foliage is wet. Broadleaf herbicides are absorbed by the weed’s foliage, not its roots. To be effective, the granules must stick to the weeds and the herbicide absorbed by the weed’s foliage. Apply granular products in the early morning when the foliage is wet with dew or irrigate the lawn prior to the application.  

To ensure adequate leaf surface and herbicide absorption, don’t mow the lawn two to three days before treatment.  After treatment, allow three to four days to pass before mowing. This allows sufficient time for the broadleaf weeds to absorb the herbicide and translocate it to their roots. To prevent the broadleaf herbicide from being washed off the weed’s foliage, apply these materials when no rain is forecast for 24 hours. Also, don’t irrigate treated lawns within 24 hours of the application.  

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