Have You Had a Problem With Your Weed Killer Not Working? Here Are Some Possible Reasons

— Written By Bill Hanlin and last updated by JoAnne Gryder
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weedkillerOur office has received calls over the years from people that complain that their weed killer application did not do the job. I have noticed in my own yard that, in some years, herbicides may work better than others. There are a number of reasons why an herbicide may fail to kill certain weeds.

This summer has been relatively dry and has stressed not only desirable plants but also weeds. According to university research, there are a couple of reasons why herbicides may be less effective when weeds are under drought stress.

The first reason is that drought stressed weeds can develop a thicker waxy layer that covers the leaves. This waxy layer prevents or slows down the absorption of herbicides that are sprayed on the leaves. This results in very little of the herbicide actually getting into the plant.

Another reason could be that the biological processes of the weed have slowed down dramatically during a drought. Weed killers, like Glyphosate, must be absorbed and moved throughout the plant in order to work. So, if you are thinking about spraying weeds, you may want to water them first or apply your weed killer after we have had a good rain.

Weed killers may also fail if they have been in storage for a long period of time. If herbicides are stored in outside sheds, freezing or extremely hot conditions can cause the active ingredient to become ineffective. Check the label to see under what conditions the herbicide can be safely stored.

Finally, liquid weed killers may not work because they were diluted too much during mixing. All pesticides work best when they are mixed at the recommended rate on the label. Make sure that you do not go above the recommended rate, as this is illegal and may also damage desirable plants.