Target Audience
This publication is part of a three-part series designed as a resource for turfgrass professionals, Extension specialists, and clientele, offering a practical overview of foundational concepts and identifying key factors that contribute to incidents of herbicide underperformance across warm-season turfgrass systems. While examples and management considerations are drawn primarily from Florida turfgrasses, the principles discussed are broadly applicable to other warm-season turf regions. This publication focuses specifically on how environmental conditions influence herbicide efficacy and safety.
Introduction
Herbicides are widely used in turfgrass systems to manage weeds (Figure 1), yet applications do not always deliver the expected level of control. This phenomenon results in economic losses, increased risk of resistance selection, and potential environmental concerns. While some incidents of such herbicide failure may result from a single, easily identifiable cause, they more often arise from a combination of plant biology, environmental conditions, herbicide properties, and application-related factors. Understanding each of these factors independently is essential for interpreting their interactions, diagnosing herbicide failures, and preventing their recurrence. By recognizing these challenges and implementing best management practices, turfgrass managers can improve weed control outcomes, reduce losses from unsuccessful applications, and build more sustainable long-term weed management programs. This publication is the second in a series and focuses on environmental causes. For a deeper understanding of the full range of herbicide performance challenges, see the other publications in this series: Part 1: Plant-Related Causes, and Part 3: Herbicide Limitations and Handling Errors.
Credit: Todd Lowe, Todd Lowe Consulting.
Environmental Conditions
Environmental conditions before, during, and after application have a significant impact on the efficacy of herbicide treatments. These effects can be direct (e.g., herbicide degradation, volatilization, or wash-off) or indirect (e.g., influencing how the plant interacts with and absorbs the herbicide). Weather and site conditions at the time of application can alter both weed susceptibility and turfgrass tolerance, making timing and environmental awareness critical for avoiding herbicide failures.
Temperature Extremes
Sunlight and air temperatures during and after application can influence both plant physiology and herbicide breakdown, ultimately affecting weed control and turf safety. Cold weather slows down plant metabolism and herbicide activity, which can lead to reduced performance, especially in northern parts of Florida during winter when daytime temperatures often range from 45°F to 65°F and nighttime temperatures may fall between the 30s or low 40s (°F). High temperatures, common year-round in Florida and especially from late spring to fall, can increase herbicide volatilization and cause spray droplets to dry rapidly on the leaf surface, reducing absorption. Hot weather can also increase the risk of turf injury and off-target movement. Managers should always refer to label-specified temperature ranges for optimal application timing and avoid spraying outside these limits. When turf injury is a concern, a small-scale test in an area where injury would be acceptable should be conducted before making a full broadcast application.
Rainfall and Humidity
Too little or no rainfall or failure to irrigate after a preemergence (PRE) application within the label-specified window (usually within one week) can prevent activation, drastically reducing or eliminating efficacy. Where irrigation systems are available, this can be avoided by watering in PRE herbicides immediately after application or later the same day to move them into the soil profile (Figure 2). In non-irrigated areas, applications should be timed just ahead of an expected rain event. If drought is expected after rainfall, applying slightly earlier can help prevent escapes, as soil moisture may still be sufficient for weed germination even without continued rainfall.
Credit: Tyler Jones, UF/IFAS Communications.
Excessive rainfall (Figure 3) can wash herbicides below the soil layer where weed seeds are located or create conditions that reduce persistence. Too much rain or irrigation after a PRE application can cause leaching, lateral movement, or accelerated loss through chemical processes such as hydrolysis. Although saturated soil slows down oxygen-dependent microbial degradation, prolonged soil moisture can alter breakdown pathways and increase herbicide dissipation through non-microbial mechanisms. If heavy rainfall is forecasted, PRE applications should be delayed until after the event, when possible. If heavy rain occurs shortly after application, shortened reapplication intervals (to the minimum allowed by the label) or putting additional PRE modes of action into rotation can help maintain coverage. Areas should be scouted for escapes and treated postemergence (POST) to prevent weed establishment. When using irrigation, runoff should be avoided. The total water volume specified on the label can be applied in split events to minimize such risk.
Credit: Marco Schiavon, UF/IFAS.
For POST herbicides, products may be foliar- or root-absorbed. For root-absorbed POST herbicides, rainfall or irrigation is needed to move the product into the soil, so lack of water after application will reduce performance. In situations with too little or too much rainfall, similar practices to the ones mentioned earlier in this section may be considered for PRE treatments. Foliar-absorbed herbicides must remain on the leaf surface for a specified “rainfast period” (often 4 to 12 hours, depending on the product) before rainfall or irrigation to allow sufficient uptake. Unexpected rainfall or incorrect irrigation soon after application can wash the product off and reduce efficacy. Where irrigation systems are present, they should be paused before application to avoid premature wash-off. In Florida, however, rainfall is often unpredictable; even careful weather monitoring may not prevent applications from being interrupted by sudden storms, particularly during hurricane season. To reduce risk, managers should consider using modern chemistries with improved rainfastness and pairing them with appropriate adjuvants or surfactants to improve foliar absorption (if the label allows).
Relative humidity at the time of application can also influence herbicide performance, particularly for foliar-applied POST herbicides. High humidity slows droplet drying on the leaf surface, increasing absorption time and often improving efficacy, especially in hot, dry weather when droplets might otherwise evaporate quickly. Conversely, low humidity speeds up drying, reducing uptake and potentially lowering control. The use of surfactants that improve wetting and spreading can help offset the negative effects of low humidity. For PRE herbicides, humidity plays a less direct role, but prolonged dry air combined with high temperatures can accelerate soil drying, reducing activation and persistence in non-irrigated areas. In such cases, timely irrigation or rainfall is critical to maintain an effective barrier.
Heat and Drought Stress
Systemic POST herbicides, especially those targeting perennial weeds, rely on healthy, actively growing plants for effective translocation. Under dry or heat-stressed conditions (Figure 4), plant physiological processes slow down and weeds may enter dormancy as a protective response. For example, drought stress often increases cuticle thickness to reduce moisture loss, which can limit foliar absorption of POST herbicides. This reduces herbicide uptake and movement within the plant, limiting its ability to reach target sites such as roots or rhizomes. As a result, even susceptible weeds can survive applications during severe stress. Herbicide applications under these conditions should be avoided, or management practices should first be used to alleviate stress where feasible.
Credit: Todd Lowe, Todd Lowe Consulting.
Heat and drought can also degrade soil conditions, creating hydrophobic soils or at minimum reducing soil water volume. Similar issues can result from other site factors such as compaction. Low soil moisture before and during application can severely reduce POST herbicide performance by limiting uptake and translocation. Compacted or saturated soils can also slow root absorption of both POST and PRE herbicides, alter microbial breakdown, and lead to uneven distribution or shortened persistence. Low water content and/or compaction issues can be identified using various measurement tools, including moisture sensors, infiltration meters, impact testers, etc. (Figure 5). However, preventing these issues may require long-term improvements in soil management because changes made immediately before application are unlikely to have a meaningful impact.
Credit: Todd Lowe, Todd Lowe Consulting.
Key Takeaways
Herbicide underperformance in turfgrass systems is frequently influenced by environmental conditions before, during, and after application rather than by herbicide selection or application technique alone. The most common environment-related causes include temperature extremes, improper timing relative to rainfall or irrigation, excessive precipitation leading to herbicide movement or premature loss or degradation, low relative humidity reducing foliar uptake, and heat or drought stress that limits plant growth as well as herbicide absorption and translocation. Soil-related factors such as low moisture availability, compaction, or prolonged saturation can further alter herbicide activation, distribution, and persistence, contributing to inconsistent weed control.
To reduce the risk of environmentally driven herbicide failures, turfgrass managers should closely monitor weather and site conditions and time applications to coincide with favorable temperatures, adequate soil moisture, and active weed growth. Herbicides with preemergence (PRE) activity or root-absorbed postemergence (POST) activity should be applied with careful attention to proper activation and movement into the soil profile. Irrigation on the day of application can aid activation when rainfall is not imminent, but runoff should be avoided. Label-specified water volumes may be applied in split events to reduce runoff risk. Applications may also be timed close to forecasted precipitation, but should be avoided ahead of very heavy rainfall events that could result in leaching or off-target movement. When drought conditions are expected to follow rainfall, slightly earlier application may help prevent escapes, as soil moisture may still be sufficient for weed germination even without continued precipitation. Reapplication intervals, herbicide selection, and additive combinations should be adjusted based on prevailing and anticipated environmental conditions to optimize efficacy and counter unfavorable effects. In postemergence programs, applications should respect label-specified drying periods and avoid periods of severe plant stress. The use of newer chemistries with improved rainfastness, along with labeled adjuvants or surfactants that enhance foliar absorption, can further improve performance when conditions are marginal. Where feasible, irrigation management, appropriate surfactant selection, and long-term soil improvement practices can help mitigate environmental limitations. Recognizing environmental constraints early allows managers to adjust expectations, refine application timing, and integrate cultural practices that support more consistent and sustainable weed control outcomes.
Additional Resources
For a comprehensive summary of professional-grade herbicides for use in turfgrass and information on proper resistance management practices, consult Ask IFAS publication SS-AGR-394, “Turfgrass Herbicides: Mode of Action and Resistance Management.”
For additional information and guidance on proper management practices and control strategies for weeds in turfgrass and landscape, consult Ask IFAS publications ENH884, “Weed Management Guide for Florida Lawns,” ENH1262, “Improving Weed Control in Landscape Planting Beds,” ENH1311, “Florida Homeowner Herbicide Guide: Considerations, Applications, and Selection,” and/or ENH95, “Postemergent Herbicides for Use in Ornamentals.”
References
Gannon, T. W., M. LeCompte, and F. H. Yelverton. 2021. “Effect of Waterlogged Soil on Preemergence Herbicides in Turfgrass Systems.” TurfFiles. NC State Extension. https://www.turffiles.ncsu.edu/2021/02/effect-of-waterlogged-soil-on-preemergence-herbicides-in-turfgrass-systems/
Marble, C. 2019. “Florida Homeowner Herbicide Guide: Considerations, Applications, and Selection: ENH1331/EP575, 8/2019.” EDIS 2019(4). https://doi.org/10.32473/edis-ep575-2019
Marble, C., and A. Koeser. 2022. “Improving Weed Control in Landscape Planting Beds: ENH1262/EP523, 6/2015.” EDIS 2015(6). https://ask.ifas.ufl.edu/publication/EP523
Petelewicz, P., and J. B. Unruh. 2025. “Turfgrass Herbicides: Mode of Action and Resistance Management: SS-AGR-394/AG398, rev. 1/2025.” EDIS 2025(1). https://ask.ifas.ufl.edu/publication/AG398
Shekoofa, A., J. T. Brosnan, J. J. Vargas, D. P. Tuck, and M. T. Elmore. 2020. “Environmental Effects on Efficacy of Herbicides for Postemergence Goosegrass (Eleusine indica) Control.” Scientific Reports 10: 20579. https://doi.org/10.1038/s41598-020-77570-5
Turgeon, A. J., L. B. McCarty, and N. Christians. 2009. Weed Control in Turf and Ornamentals. Prentice Hall.
Unruh, J. B., P. Petelewicz, L. E. Trenholm, E. E. Harlow, and R. Leon. 2024. “Weed Management Guide for Florida Lawns: ENH884/EP14, 3/2003.” EDIS 2003(3). https://ask.ifas.ufl.edu/publication/EP141