Despite the increasing potential for rain this week, California remains poised to experience exceptional drought conditions in 2014. Under prolonged periods of dry weather, weed control is likely to suffer. This is especially troubling for growers as both crops and weeds will be competing to capture limited soil moisture, which could result in significant yield losses.

Although fewer weed seeds may germinate under dry conditions, weeds that do emerge and become established may be more difficult to manage with herbicides. Drought-stressed weeds are likely to have thicker cuticles (which is the waxy coating on the surface of the leaf), which can inhibit the absorption of post-emergence products. Additionally, plant architecture can be altered when it is hot and dry (e.g. fewer and drooping leaves) meaning that herbicide capture and retention may be reduced. When weeds aren't actively growing, systemic herbicides (e.g. glyphosate) may not be effectively translocated to their target sites. Although contact herbicides (e.g. paraquat, carfentrazone, oxyfluorfen) are less likely to be affected by dry conditions, herbicide efficacy could be reduced if spray droplets dry rapidly (either in the air or on plant surfaces) and sufficient coverage isn't achieved.

Growers can utilize certain spray adjuvants (such as COCs and nitrogen-containing additives) and increase carrier rates to optimize deposition and uptake, but should always consult the product label to minimize/prevent the potential for crop injury. Sprayers should be properly calibrated to prevent 'over-applications' which can be a waste of product and money (and which could result in crop injury). Reduced herbicide rates should never be considered.

 Death Valley NP. 2013.

Without precipitation or irrigation, many soil-applied herbicides cannot be activated (which means moved into solution so that they can be taken up by emerging weed seedlings). Some herbicides can be mechanically incorporated, although product distribution may be uneven in dry soils. Furthermore, herbicides may become tightly bound (adsorbed) to soil particles, which could result in carryover injury on following crops; growers may need to be more cautious with respect to subsequent crop planting dates after a dry season.  Additionally, the potentials for photo-degradation or volatilization can be increased under hot and dry conditions, resulting in reduced herbicide efficacy and/or off-target movement. 

Weed control will, undoubtedly, be made more difficult during a drought. The efficacy of both foliar- and soil-applied herbicides may be reduced. Poor performance may be the result of drought affects on the weeds (e.g. altered growth, development and physiological activity) or on the herbicides (e.g. reduced activation, adsorption, uneven distribution, volatilization). Mechanical weed control (e.g. shallow cultivation) may be an option to eliminate small, annual weeds, but could result in the loss of soil moisture. Needless to say, the timing of control applications will be critical; growers should attempt to make foliar applications when emerged weeds are still small and succulent or soil applications as close to a rainfall or irrigation event as possible in order to maximize performance. Seek the advice of consultants and extension personnel if you are unsure about the use of a specific product, whether it be an herbicide or an adjuvant, in a drought situation. ALWAYS follow instructions/recommendations on the product label and ensure that equipment is properly calibrated so that pesticides (and money) are not being wasted, the environment is being protected, the potential for injury to current and future crops is minimized, and illegal residue levels are prevented.

Bodie SP. 2013.