Answer: Several university's have studied the effectiveness of herbicides in daytime high air temperatures ranging from 47 to 87?F. The results - there is very little difference in their performance based upon outside air temperature. If you've been having trouble with effectiveness of the herbicide you've been applying, it might be that you've chosen the wrong formula for the targeted weeds or have not applied it according to label directions. Make sure that the weeds you are trying to eliminate are listed on the herbicide label and that you're timing is right - some weeds are more susceptible at certain stages of their growth cycle.
Best wishes with your garden!
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