Pointers for Pre-emergent Herbicide Use

Jim Rogers Herbicides, Tip of the Week

By Ramdas Kanissery

Weeds emerging in citrus rows are problematic as they compete for resources with the trees, support pests and interfere with grove operations like irrigation (Figure 1). Hence, effective strategies to control weed emergence should be in place.   Pre-emergence (or residual) herbicides are an excellent tool for the long-term suppression of weeds in tree rows. These herbicides persist in the soil and suppress the germination of susceptible weeds. The effectiveness of these herbicides depends on several factors, including the type of herbicide applied, soil properties and weather conditions. Following are some strategies to improve the effectiveness of a pre-emergent herbicide program.

Pre-emergent Herbicide Use
Figure 1. Clean, weed-free citrus tree row (left) and weeds emerging in the tree row (right)

START CLEAN
Pre-emergent herbicides work best when applied to clean tree rows with no or minimal existing weeds. Existing weed growth can act as a barrier and prevent the pre-emergence herbicide from fully incorporating into the topsoil. If weeds have begun to sprout in the tree rows, consider tank-mixing pre-emergent herbicides with a compatible post-emergent like glyphosate or glufosinate-ammonium. If the weed infestation is heavy, consider making a “clean-up” application first with a post-emergent herbicide before the pre-emergent herbicide program.

IRRIGATE BEFORE OR AFTER APPLICATION
Sprinkler irrigating the applied areas within two days of application will help activate most pre-emergent herbicides in the soil. Adequate moisture content in the soil helps the germinating weed seeds or emerging weed seedlings readily uptake the herbicide active ingredient from the soil solution.

CALIBRATE SPRAYERS
Uniform distribution of the herbicide active ingredient in the soil is crucial for maximum weed suppression. So, it’s essential to ensure the proper calibration of the sprayer by testing the flow rate of the pump and nozzle combination and checking the nozzle angles. See this EDIS article for more information on this topic. Also, the product label should be read carefully, and the correct application rates should be used.

USE APPROPRIATE ADJUVANTS
After consulting the product label, appropriate adjuvants/additives should be used to improve the efficacy of pre-emergent herbicides. Some herbicides have optimal performance at a spray solution pH of 4 to 6. If mixed with hard water with a high pH, that may reduce the herbicide’s performance. In this case, using a pH buffer could help enhance the effectiveness. This Citrus Industry article details the use of herbicide adjuvants in citrus weed control.

CONSIDER TANK MIXES
Tank-mixing different pre-emergent herbicides with multiple selectivity and modes of action will broaden the range of weeds controlled in a single application. Check out this Citrus Industry article for more information on utilizing synergy between herbicides for longer-term weed management in citrus.

More information on weed management in Florida citrus production can be found in the weeds chapter of the Florida Citrus Production Guide published by University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Extension.

Ramdas Kanissery is an assistant professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.

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