Summer temperatures have arrived, with high temperatures over 100°F in parts of Kansas. If you are planning herbicide applications, here are some things to consider when applying herbicides during hot weather.
Management: In general, applying systemic herbicides early in the morning, after plants have had a chance to recover from heat stress, will give the best chance for the herbicide to reach the active site and effectively kill weeds.
Figure 1. Velvetleaf usually changes leaf angles at night, but the leaves on these plants are vertical in response to high temperatures. Photo by Sarah Lancaster, K-State Research and Extension.
Management: Using maximum labeled rates of herbicides and surfactants can help get more spray solution into the plant, increasing effectiveness. Spraying during the cooler parts of the day will reduce the impact of altered leaf angle.
Figure 2. Contact herbicides such can cause bronzing of soybean leaves when applied post-emergence. Photo taken one week after an application that included flumiclorac (Resource, Perpetuo, others). Photo by Sarah Lancaster, K-State Research and Extension.
Management: If possible, postpone application of these herbicides if temperatures are over 90°F. If weed size requires immediate herbicide application, reduce the rate of herbicide and adjuvant, and apply later in the day, when the air temperature will decrease after application.
Management: Avoid applying these herbicides when temperatures are over 90°F. This may occur during morning or late afternoon hours, when temperature inversions are likely to occur. Herbicides should not be sprayed during inversions, when small spray droplets can become trapped in a layer of cooler air near the earth’s surface. Use larger spray droplets to reduce evaporation, which can be accomplished by reducing spray pressure or increasing nozzle orifice size.
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.
Sarah Lancaster, Extension Weed Science Specialist