The term El Niño has been used more often than before in the news and across various media platforms in August. This follows a recent announcement by the National Oceanic and Atmospheric Administration (NOAA) forecasting a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter 2026-27 (NOAA, August 13th, 2026).
Local UC ANR climate scientist Dr. Daniel Swain explained how this El Niño event may affect California. Potential impacts include “warmer, more humid nearshore ocean conditions; late-summer and autumn heatwaves; and increased winter precipitation with flood risk” (Swain, 2026). What does El Niño mean for weeds and weed management?
I have not found any documentation on the impact of El Niño on weed populations in agricultural systems per se; however, past observations of the 1997-1998 El Niño showed increased vegetative growth of invasive plants in coastal wetlands. For example, high precipitation during the wet season was associated with increased growth and reproductive output of common reed (Phragmites australis) (Minchinton, 2002). Or ephemeral (i.e., short-lived) species of the California desert showed increased vegetative growth and ground cover during the 1997-1998 El Niño event ( Holmgren et al., 2001).
Although these observations come from natural systems rather than crop fields, the underlying biology applies to common agricultural weeds. Most problematic weeds are annual and have a large seed bank. A wetter, milder winter means more dormancy break, larger and repeated emergence flushes, and perhaps bigger plants setting more seed.
For growers and PCAs, the more immediate consequences may be operational rather than biological. Saturated fields delay pre-emergence herbicide applications and planting, so weeds could get a head start on the crop. By the time the ground is accessible, weeds may be too large for post-emergence products to work well. This condition could also exacerbate herbicide resistance, as weeds are exposed to low herbicide rates.
Flooded and waterlogged areas could favor species like sedges while thinning crop stands and opening gaps for weeds to fill. Floodwater can also carry weed seeds between fields and from ditches into fields.
So, what can we do now, before the rain starts?
Scout and monitor: walk fields, fly drones, and look for potential weed populations around the canals and irrigation ditches.
Plan for spraying in advance: Assume fewer workable days between rain events. Decide residual herbicide programs and have product on hand early rather than waiting. Think about which fields get priority, and what the pre- and post-emergence backup looks like.
Protect drainage and control water where you can: clean ditches and check drainage for the rain so the fields shed water faster and spray windows reopen sooner. This also protects the crop from waterlogging and stand loss.
And finally, keep an eye on the weeds this winter. Because there is no published data on how weed populations respond to an El Niño year. Such observations are genuinely valuable. Noting what emerges, how the weed species pattern looks, and any observations will help all of us to learn. El Niño forecasts only describe the probabilities, not certainties, but it is better to be prepared.
References:
NOAA Climate Prediction Center (2026). ENSO Diagnostic Discussion, August 13, 2026. (>90% chance of a very strong El Niño during fall–winter 2026–27. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml
NOAA Climate Prediction Center (2026). Seasonal Precipitation Outlook, December–February 2026–27, issued August 20, 2026. https://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php
Swain, D. (2026). El Niño 2026-2027: A Potentially Historic Global Climate Event. https://link.ucanr.edu/elnino2026.
Minchinton, T.E. (2002). Precipitation during El Niño correlates with increasing spread of Phragmites australis in New England, USA, coastal marshes. Marine Ecology Progress Series, 242:305–309. https://www.int-res.com/abstracts/meps/v242/p305-309/
Holmgren, M., Scheffer, M., Ezcurra, E., Gutiérrez, J.R., & Mohren, G.M.J. (2001). El Niño effects on the dynamics of terrestrial ecosystems. Trends in Ecology & Evolution 16(2):89–94. https://www.sciencedirect.com/science/article/abs/pii/S0169534700020528