If you're fussy about weeds in your lawn, now through early November is a good time to apply broadleaf herbicide for protection next year.
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.
The most effective broadleaf herbicide products contain a mixture of two or three herbicides, as no single compound will control all broadleaf weeds. The most common broadleaf herbicides are 2,4-D, dicamba, triclopyr, mecoprop (MCPP) and quinclorac. A single application of a broadleaf herbicide effectively kills many broadleaf weeds. Difficult-to-control weeds, such as violets, will likely require two or more applications.
(Remember: some people do not consider violets weeds.)
To ensure adequate herbicide absorption, apply broadleaf herbicides when no rain is forecast for at least 24 hours. After treatment, allow four to five days to pass before mowing. This allows sufficient time for the broadleaf weeds to absorb the herbicide and translocate it to their roots.
Here are two other questions about broadleaf herbicides with answers from horticulturists at Iowa State University and Extension.
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Q: What is the proper way to apply broadleaf herbicides to the lawn?
A: They can be applied as liquids or granules. Before applying any herbicide, carefully read and follow label directions.
Apply liquid broadleaf herbicides when the winds are light and temperatures are forecast to remain below 85 degrees for 24 hours. Select a nozzle that produces coarse droplets and use low sprayer pressure. When spraying, keep the nozzle close to the ground. If only a few areas in the lawn have broadleaf weed problems, spot treat these areas rather than spraying the entire lawn. Apply just enough material to wet the leaf surfaces. Liquid herbicides are usually more effective than granular products.
Granular broadleaf herbicides often are 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. Moisture on the weed’s foliage allows the granular herbicide to stick to the foliage for maximum absorption. Apply granular products in the early morning when the foliage is wet with dew or irrigate the lawn prior to application.
Q: Are broadleaf herbicides effective when applied during dry weather?
A: Broadleaf herbicides are most effective when applied to weeds that are actively growing. During prolonged periods of dry weather, some weeds are likely to curl up or wilt. An application of a broadleaf herbicide to drought stressed weeds will likely be less effective, as wilted foliage will absorb less herbicide than healthy foliage.
Broadleaf herbicides can be applied from late September to early November in Iowa and Illinois. In dry fall weather, wait for a good rain or irrigate the lawn before applying a broadleaf herbicide. One-half inch or more of water (either from rainfall or irrigation) will quickly revive most drought stressed weeds.