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.
Most contact herbicides become more active as temperatures increase.
Increased activity may provide improved weed control, but can also result in greater crop injury. Postponing application of these herbicides should be considered if temperatures exceed 90 degrees. However, good weed control with contact herbicides is dependent on timely application. The best control generally is achieved with thorough spray coverage and application to small seedling weeds. Although waiting until temperatures subside will lessen the risk of crop injury, the weeds may develop beyond the optimum treatment stage if application is delayed too long.