by David Doll, UCCE Pomology Farm Advisor (see also http://thealmonddoctor.com/)
With the emergence of herbicide-resistant weed populations, several perennial nut crop spray programs incorporate the use of pre-emergent herbicides. The correct use of these types of products provide the ability to control weeds for an extended period – sometimes into the late spring or early summer – reducing in-season burndown applications. When applying these products, proper application is critical to maximize weed control and the following points should be considered.
Herbicide selection. Pre-emergent herbicide selection should be based on the weed spectrum present. Check notes from earlier in the year to determine problematic weeds (e.g. fleabane, goosegrass, etc). Survey germinated weeds to make sure the proper burndown material is selected for the mix. If attempting to manage summer emerging weeds such as goosegrass, junglerice or sprangletop, consider a split application program in which preemergent chemistries are applied twice – a broad-spectrum combination once in late November/December (e.g. Goal 2XL, Pindar GT, Chateau, Alion or Prowl H2O) and one targeting grasses in March or April (e.g. Surflan or Prowl H2O). The late season booster shot of one of these good grass herbicides can provide control of these weeds through the season.
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. Rain should be in the forecast prior to application. If the application is delayed past late January due to lack of rain or other reasons, consider changing products or reducing the material applied to the lower end of the label rate to reduce chances of crop injury. This is especially important if the orchard is planted within sandy or gravelly soils. Tree uptake (with injury) has been observed in Merced County with applications of flumioxazin, indaziflam, norflurazon and penoxsulam – as well as many of the other pre-emergent products.
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 as effective if berms are not clean (e.g. indaziflam, pendimethalin). Similarly, other soil amendments such as lime, compost, wood chips, 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 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 (e.g. 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 (e.g. Solubor) should be tested using a jar test to ensure that no unexpected problems 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.
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 and poor performance.