All herbicides are influenced by environmental conditions. Herbicides generally are most effective when applied to vigorously growing plants at 70 to 85 degrees Fahrenheit.
Weeds growing under hot, dry conditions often become more tolerant to herbicides.
Plants may develop a thicker wax layer on the leaf surface, which is a barrier to herbicide absorption into the plant. Herbicide movement within the plant will likely be reduced due to a slowed rate of translocation and metabolism. Consequently, application of herbicides under such conditions often results in reduced weed control. Reduced weed control during hot, dry weather probably is the biggest concern with the application of systemic herbicides such as 2,4-D, Accent, Ally, Assure II, Cheyenne, Dakota, Express, Fusilade DX, Fusion, Harmony Extra, Hoelon, Matrix, MCPA, Option II, Peak, Pinnacle, Poast, Puma, Roundup, Tiller, UpBeet, or Whip. Application of these herbicides early in the morning after the plants have recovered from the heat of the previous day should provide better weed control than afternoon or evening application.