With weather drying out and heating up, the risk of herbicide burn is increasing. In most cases, herbicide burn is similar to fertilizer burn in that the concentration of the herbicide spray residue on the leaf literally sucks the water out of the leaf, desiccating the leaf. This is similar to how urea fertilizer spilled on a concrete floor absorbs water from the surrounding humid air. Unfortunately there are no definite thresholds dictating when and where herbicide burn will occur, but following are general guidelines.
High rates cause more burn – higher concentrations of pesticide will absorb more water from the leaf
Fertilizers or multiple products in a spray tank – increases concentration of the spray
Specific herbicide or formulation – some active ingredients tend to be a little safer than others in the heat, but there are no guarantees
Low spray volumes – creates a higher concentration of herbicide in the spray
Temperatures over 80 to 85F – higher temperature require more water for the plant to cool, so even a little water lost in diffusion to the herbicide residue can lead to leaf desiccation.
Low humidity – only source of diffusible water is from the leaf when air is dry (and humidity is low)
Low soil moisture/drought stress/poor rooting/compacted soil/grub feeding - plant can barely take up enough water to cool itself and then the added moisture demand as the herbicide residue absorbs water adds more stress to the plant leaf
Unfortunately, humidity and soil moisture levels fluctuate excessively from lawn to lawn and even within the same lawn. Sun, shade, wind patterns, soil type, turfgrass species, irrigation or precipitation pattern, drainage, and other variables change across any property and will also affect the risk of herbicide burn. In most cases, hold back on herbicide applications once temperatures reach 80 to 85F and soil moisture is starting to become limited.
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