With the recent dry conditions and highly variable forecasted chances for rainfall for parts of Iowa just as postemergence herbicide applications begin, questions often come up regarding use of residual herbicides and efficacy of postemergence applications during these conditions.
The forecast
The current weather forecast shows highly variable chances of rainfall across the state in the next week, and the National Oceanic and Atmospheric Administration Climate Prediction Center (NOAA CPC) is predicting warmer than normal and near normal rainfall for the next 6-14 days for most of Iowa*. See more of their forecasts on the NOAA CPC website.
The average rainfall over the next two-week period for Ames, IA has been approximately two inches over the last 132 years. Rainfall has exceeded one inch during the May 29-June 11 period in about 75% of years and exceeded 0.5 inch in about 90% of years. While rainfall may not arrive immediately following a herbicide application, it will eventually come to activate herbicides.
Things to consider when making postemergence applications
- Scout fields prior to postemergence applications to prioritize fields for sprays and check that weeds appear healthy and not wilting.
- If warm, dry weather persists, emerged weeds may become more challenging to control, especially on hot, low humidity afternoons.
- Waterhemp continues to grow, albeit at a slower rate, under drought conditions so don’t wait for rain if waterhemp is 2 to 4 inches in size.
- Consult herbicide labels, ISU Extension Field Agronomists, and your agricultural service provider to tailor adjuvant selection appropriately for the weather and weed situation.
- Avoid known “hot” tank-mixtures to reduce crop injury, especially if air temperatures are predicted to exceed 90 degrees Fahrenheit.
- New weeds may continue to emerge in dry weather prior to activation of residual herbicides but herbicides will be activated with approximately 0.5-1 inch of rainfall.
Read more about considerations for herbicide applications in the Encyclopedia article “Herbicides and Hot, Dry Weather.”
Additional resources
- Compendium of Herbicide Adjuvants, 13th Edition (2016), Purdue University
- Adjuvants with Herbicides. When and Why They Are Needed. Crop Protection Network (2024)
- Preemergence Herbicides, Dry Soils, and Rain, Integrated Crop Management News (2021), Iowa State University
- Role of spray adjuvants with postemergence herbicides, Integrated Crop Management Encyclopedia, Iowa State University
*The authors are pleased to report that simply by writing this article, Mother Nature will most likely assure that everyone in the state receives timely rainfall in the next week.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on May 29, 2026. The information contained within may not be the most current and accurate depending on when it is accessed.
- Assistant Professor
- Extension Weed Scientist Specialist
- Field Agronomist in Central Iowa