Yard and Garden: Applying Herbicide and Fighting Fall Weeds


September 4, 2014, 3:51 pm | Richard Jauron, Greg Wallace

Fall is the ideal time to prepare a lawn for the months that lie ahead before spring, making sure it emerges healthy and happy.

Here are some tips from Iowa State University Extension and Outreach on how to apply fall herbicide and control fall weeds, with help from ISU Extension horticulturists. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

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

Fall (late September to early November) is the best time to apply broadleaf herbicides in Iowa. 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 generally less effective than fall applications.

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Which herbicides are effective in controlling broadleaf weeds?

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 controls all broadleaf weeds.

What is the proper way to apply broadleaf herbicides 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, apply just enough material to wet the leaf surfaces. If only a few areas in the lawn have broadleaf weed problems, spot treat these areas rather than spraying the entire lawn. Liquid formulations have the potential to drift.  Spray drift problems can be avoided 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.

Granular broadleaf herbicides are often 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 must be 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.

Which lawn care practices enhance the absorption of broadleaf herbicides?

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.

How do I control creeping Charlie in my lawn?

Ground ivy (“creeping Charlie”) in lawns can be controlled with broadleaf herbicides. Products that contain 2,4-D or triclopyr are most effective. 2,4-D is an active ingredient in many broadleaf herbicide products. Triclopyr can be found in Ortho Weed-B-Gon Chickweed, Clover and Oxalis Killer for Lawns and several other products. In Iowa, herbicide applications should be made between late September and early November. Two applications are necessary to effectively control ground ivy. The first application should be made in late September/early October, the second a month later. 

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