By Ramdas Kanissery
Managing weeds during the summer is crucial for Florida citrus. However, due to the high temperatures typical of Florida’s summer months, several factors must be considered when using herbicides in citrus groves during hot weather.
INCREASE EFFECTIVENESS
High heat and drought stress can slow plant growth and affect how herbicides move or translocate in weeds. When temperatures exceed 90°F, many weeds reduce or halt their metabolic processes, which impacts herbicide effectiveness (see Figure 1). This is particularly important for systemic herbicides like glyphosate and selective grass-killers such as sethoxydim (e.g., Poast Plus) or fluazifop-butyl (e.g., Fusilade). To maximize effectiveness, apply systemic herbicides early in the morning after weeds have had a chance to recover from the previous day’s heat.
Keep in mind that plants use their cuticles as a defense against heat. Hot weather can cause weed cuticles to become waxier, which makes it more difficult for herbicides to penetrate. Changes in leaf angle due to heat may also reduce the amount of herbicide that contacts the leaf surface. To tackle these issues, use approved surfactants to enhance herbicide penetration and schedule herbicide applications during the cooler parts of the day to ensure the herbicide contacts the leaves effectively when they are in their optimal position.
RISKS OF TREE INJURY
Remember that the heat affecting weeds also impacts how trees respond to herbicides. If a contact or non-translocated herbicide like carfentrazone-ethyl (e.g., Aim) is applied in hot and humid conditions and drifts onto citrus foliage, it can cause more severe damage to the trees.
Additionally, herbicides such as 2,4-D (e.g., Embed Extra) can become volatile in hot conditions, turning into vapor that can be carried long distances by light breezes. This volatility increases as temperatures rise above 85 to 90°F. If possible, postpone applying these herbicides on days when temperatures are expected to exceed 90°F. To reduce evaporation, use larger spray droplets by lowering the spray pressure or increasing the nozzle size.
Ramdas Kanissery is an assistant professor at the University of Florida Institute of Food and Agricultural Sciences Southwest Florida Research and Education Center in Immokalee.
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