With the emergence of herbicide resistant weed populations, several spray programs now incorporate the use of pre-emergent herbicides. The use of these types of products provide the ability to control weeds for an extended period of time – sometimes into the late spring or early summer. In order to maximize the efficacy of these products and prevent drift, they must be applied correctly.
Berm Preparation. Leaves, hulls, dead weeds, and other organic debris will reduce the efficacy of pre-emergent herbicide applications. Berms should be clean when the applications are made. This includes removing large weeds that were not controlled with earlier applications. Although some chemistries have the ability to work with some leaf litter, many newer chemistries bind to organic matter and will not be effective if berms are not clean. Similarly, other soil amendments such as lime, compost, biochar etc. can bind up some pre-emergent herbicides and reduce their efficacy if spread before herbicides are applied.
Tank Mixing Order. Proper mixing of herbicides is needed in order to prevent nozzle clogging and good spray uniformity. Unless the herbicide label specifies a different order, herbicides and other components should be mixed in the following order: water conditioners (i.e. ammonium sulfate), wettable powders, flowable concentrates, emulsifiable concentrates, water solubles, and, finally, other adjuvants and surfactants. If compatibility enhancers are used, they should be added first to the water. It is important to remember that any new combination or any other addition (i.e. Solubor) should be tested using a jar test to ensure that no unexpected problems (like a tank full of goo!), arise. Using a pint of water, add each pesticide to the jar in the described order. One half of a teaspoon is equivalent to one pint or one pound of herbicide in 25 gallons water. After adding the herbicides, invert the jar several times. View the jar immediately after mixing and 30 minutes later. If any types of clumps or sludge forms, the mixture is incompatible. If minor separation occurs after 30 minutes, and readily remixes, it should be suitable for application.
When tank mixing multiple herbicides, it is important to not completely fill the tank before loading the herbicides. Instead, fill the tank 2/3rds full, and begin adding the herbicides in the mentioned order with the agitation system running. After all ingredients have been added, fill the tank to the desired level and agitate at full pressure and maintain agitation through the spray application.
Timing. Spray timing is dependent upon product. Post-emergent materials are most effective when weeds are small, roughly the size of a half dollar (1.5″ in diameter) for broadleaf weeds and 3-5 leaves for grasses. Pre-emergents often require 0.5-1.0 inch of rain or irrigation to incorporate the herbicide. Late application and excessive incorporation of pre-emergent herbicides may increase the risk of herbicide uptake, especially on sandy soils. In orchards with these conditions, earlier applications (December-early January) should be considered.
Herbicide Application. Spray rigs need to be maintained and properly calibrated to maximize application efficiency, uniformity, and not waste herbicides. At least annually, nozzles should be checked and replaced if needed to increase spray distribution uniformity and minimize skips. Prior to every application, check for an even application of herbicide across the boom. Make sure the product that is planned to be applied per acre is actually applied. If not, recalibrate the spray rig. Applying too much herbicide/acre will lead to increase costs and potential tree damage, applying too little will lead to product failure and a second application. Both are more expensive than taking the time to maintain the spray rig in the first place.
Spray Nozzles. Nozzle selection will vary by the required coverage, operating pressure, spacing, and ground height. Newer nozzle technologies have been developed to reduce drift and provide more uniform droplet distribution. Calibrate nozzles frequently, and replace any nozzles that are out of the 5-10% target range. Although they may seem expensive, using old, worn out nozzles may cost more in wasted herbicide. More information can be found in this handout by Kurt Hembree (UCCE Fresno) and Brad Hanson’s (UC Weed Specialist) weed blog. Also, Kurt Hembree has put together this presentation on the effects of spray nozzle selection on weed control.