Despite the promise of rain for the next few days (April 5-7, 2015), California is poised to experience a fourth consecutive year of drought. As of April 1, 2015, the Sierra snowpack is essentially gone. According to the California Department of Water Resources "Electronic readings today found that the statewide snowpack holds only 1.4 inches of water content, just 5 percent of the historical average of 28.3 inches for April 1. The previous low for the date was 25 percent in 2014 and 1977." As a consequence, Governor Jerry Brown has enacted mandatory water restriction across the state; these will impact all of California's 38 million citizens, including it's farmers.
Brad Hanson, John Roncoroni and Cheryl Wilen, Kurt Hembree, and myself have all written, previously, about weeds and weed control under drought conditions. These can be accessed, in full, here, here, here, and here, respectively. They are all worth reviewing as we head into the (historically) hottest and driest months of the calendar year.
Several important points are common to all of the posts, including:
- Weeds AND herbicides (and their interactions) can be affected by drought conditions.
- With respect to weeds:
- Although the total number of emerged weeds is expected to be reduced under dry conditions, community composition shifts in favor of deep-rooted perennials and large seeded weeds that can emerge from deeper in the soil profile could occur. Known drought-tolerant species include: field bindweed (Convolvulus arvensis), knotweed (Polygonum aviculare), and dallisgrass (Paspalum dilatatum), among many others.
- Morphological and physiological changes to the weeds themselves can affect herbicide efficacy. 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; adjuvants (e.g. COC or MSO) can often be used to increase absorption. Plant architecture can also be altered when it is hot and dry (e.g. fewer and drooping leaves), which can reduce herbicide capture and retention. When weeds aren't actively growing, systemic herbicides (e.g. glyphosate) may not be effectively translocated to their target sites.
- With respect to herbicides:
- Both pre-emergence and post-emergence herbicide efficacy can be affected by drought conditions.
- In the absence of sufficient quantities of water, many pre-emergence herbicides may not be incorporated/activated; as a consequence, some herbicides may be more rapidly degraded (e.g. via the sun) or volatilized, or even removed from the application site (e.g. if blowing winds pick up treated soil particles). The residency of some soil-applied may be increased under dry conditions, which can result in carryover injury in succeeding crops. Many herbicides can be mechanically incorporated, although product distribution may be uneven in dry soils.
- Brad Hanson describes the effects of dry conditions on spray droplet performance the best: "Droplets are subject to evaporation between the time they leave the spray nozzle and being absorbed by the leaf surface – this is much more dramatic under hot, low humidity conditions. Drying of the spray droplet in the air can increase the chances for drift as larger droplets become smaller. More importantly, the droplets may dry very quickly once they are on the leaf surface leaving insufficient moisture to facilitate transfer of the herbicide (which is usually a crystalline form) from the droplet into the plant."
I STRONGLY encourage you to read each of the aforementioned blogs/extension pieces; each reinforce the key points put forward in this post, and all include additional information that has not been covered here.
I'd like to end with the summary that I wrote for my previous drought posting: "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 [and] 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."