Summer temperatures have arrived, with expected high temperatures near 100°F expected for parts of Kansas in the coming days. If you are planning herbicide applications, here are some things to consider when applying herbicides during hot weather.
Heat or drought stress slows plant growth processes. This is especially important for systemic herbicides such as glyphosate and grass-killing herbicides like clethodim (Select) or quizalofop (Assure). As temperatures increase above 85°F, many plants begin to slow or stop metabolic processes that move herbicides throughout the plant. Notable exceptions to this rule are HPPD-inhibiting herbicides like Callisto or Balance Flexx. Palmer ‘amaranth plants are able to overcome applications of these herbicides when applied at high temperatures (90°F and greater).
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.
Leaf surfaces change in response to heat. In order to prevent water loss, plant cuticles become waxier in response to heat or drought stress. The greater wax content makes it more difficult for water-based spray solutions to penetrate the plant.
Management: Using maximum labeled rates of herbicides and surfactants can help get more spray solution into the plant, increasing effectiveness.
Crop response to foliar applied, non-translocated herbicides is greater in hot temperatures. When applied in hot, humid conditions, contact herbicides, such as Cobra, Liberty, or Reflex will likely result in greater foliar injury to crops, but also greater weed control (Figure 1).
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.
Figure 1. Contact herbicides such can cause bronzing of soybean leaves when applied post-emergence. Photo taken 1 week after an application that included flumiclorac (Resource, Perpetuo, others) on June 30 near Manhattan KS. Photo by Sarah Lancaster, K-State Research and Extension.
Herbicide volatility increases with high temperatures and low humidity. Herbicides in group four, such as dicamba and 2,4-D are prone to volatility, which means the herbicide becomes a vapor and can move long distances with slight breezes. Volatility of these herbicides increases as temperature rise above 60°F and is greatest at temperatures above 90°F.
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. During the first week of July, temperature inversions were recorded beginning as early as 5 pm and breaking up as late as 9 am at the Ashland Bottom Farm. Monitor temperature inversions using the Kansas Mesonet (mesonet.ksu.edu/agriculture/inversion/). Engenia, XtendiMax, FeXapan carry additional time of day restrictions on the labels. The labels for Engenia, XtendiMax, FeXapan also instruct applicators to use larger spray droplets when temperatures are above 91°F, which can be accomplished by reducing spray pressure or increasing nozzle orifice will reduce evaporation.
Sarah Lancaster, Extension Weed Science Specialist