PACIFIC SOUTHWEST: Over the past few years, a trend developed in California where growers of permanent crops are not killing or removing already emerged weeds before applying a pre-emergence herbicide. They add a burndown herbicide into the tank mix, which allows them to take only one trip through the field and thus reduce cost – but at this point, the existing weed pressure can often be very high.
If the weeds are very large, this can affect spray nozzle coverage and soil contact of the spray solution, leading to uneven or missed areas. Also, with some weeds like yellow nutsedge or johnsongrass, the mature stages produce both above-ground seeds and underground reproductive organs leading to a large reservoir of future weeds. Missed or uneven coverage areas can then become troublesome weed patches later in the season, leading to multiple and expensive weed treatments that can have negative impacts on the crop.
If done correctly, a properly timed pre-emergence herbicide application can provide lasting weed control long into the season, which saves money and reduces the potential for negative effects that sometimes go along with certain herbicide applications.
In the photo above at the top of this entry, you can see an orchard under extreme pressure from winter weeds that already escaped one attempt at control. If a pre-emergence herbicide is applied in a situation with weed pressure like this, its impact likely will not be as good as desired.
All photos are the property of Syngenta unless otherwise noted.
Reporting from Hanford, CA