Effective Weed Management
The most effective weed management strategy is to maintain a thick and healthy lawn. This minimizes sunlight from reaching the soil surface, as many weeds require sunlight for maximum germination. Weeds are plants of opportunity, and they will utilize any turfgrass weakness to establish themselves in a lawn. These weaknesses may include improper soil fertility or soil pH, soil compaction, excessive thatch, too much or too little water, and mowing too low for the lawn type. If these cultural problems are not addressed, weed management will remain unsatisfactory.
The first step in effective weed management is to evaluate the lawn and determine what conditions may have contributed to the existing weed problems. Begin by soil testing to determine the current nutrient levels and pH of the soil. After soil fertility issues are corrected, use appropriate measures to eliminate any soil compaction and excessive thatch issues. Mowing too low (also known as scalping) stresses the turfgrass, and the mowing height should be adjusted by raising the mower to the correct mowing height for your lawn type. Adjust irrigation as needed to apply 1 inch of water per week in combination with rainfall when the grass is actively growing. Depending on the soil type, the irrigation schedule may be once or twice per week. Finally, correct areas with poor soil drainage. Weak turfgrass stands may also require renovation.
Weed Identification and Herbicide Selection
After these cultural problems have been addressed, the weeds of concern must be accurately identified so that the appropriate herbicide may be selected. Proper weed identification is very important because understanding the weed’s life cycle is the key to weed management. Herbicides vary in their effectiveness in controlling specific weeds, and some may not provide any control against the targeted weed. Choosing the wrong herbicide due to a misidentified weed can easily result in large amounts of money spent with no control of the weed. Common Weeds and Wildflowers is a complete, up-to-date weed identification guide available from the Cooperative Extension Service. The local Cooperative Extension agent or the Home & Garden Information Center can also help with weed identification, and determine which herbicide will be most effective based on research data and herbicide label information. They can also point out possible concerns when using a specific herbicide. These may include wind drift to nearby plants, volatilization problems, spray water pH and water quality issues, potentially harmful application over root systems of nearby woody plants, and requirements to apply the herbicide during a specific growth stage of the turfgrass or the weed to be controlled.
The herbicide selected must be labeled for both the weed to be controlled and the type of lawn. It is against Federal law to use an herbicide on a lawn or a usage area not listed on the label. The herbicide label provides information on weeds controlled, lawn types on which the herbicide may be applied, mixing procedures, application rates, and proper safety apparel required during mixing and application. More information on herbicide handling and safety may be found in HGIC 2751, “Pesticide Safety”. Selecting the wrong herbicide, or over-applying the correct one, is not only illegal, but can also injure or kill the desirable turfgrass. The herbicide label is the law, and it is there to provide directions on proper and safe application and use. Read the label!
Additional weed identification and management information can be found in the Clemson Extension’s Home and Garden Information Center fact sheets. These include HGIC 2310, Broadleaf Weeds, HGIC 2300, Grassy Weeds, HGIC 2310, Managing Weeds in Warm Season Lawns, and HGIC 2309, Managing Weeds in Fescue Lawns, as well as in the fact sheets for many specific lawns weeds.
The Weed Control Timing Chart
Herbicides may be almost completely ineffective if applied at the wrong time of year. There is an optimum time frame to apply an herbicide to each weed. In most cases, this time is early in the weed’s life cycle, but in some cases, herbicides should be applied during a different growth stage. Generally, weeds should be actively growing when an herbicide is applied. In certain instances, only a pre-emergent herbicide (an herbicide applied before the weed emerges to prevent weed seed germination) may provide effective control. Remembering when to apply herbicides for the many different weeds can be a challenge. The herbicide label provides definitive information concerning the turf growth stage, time of day, and weather conditions when the product may be applied, but this simple chart can help in making a plan for the timing of lawn weed control.
The Lawn Weed Control Timing Chart was developed to help remove confusion and guesswork when determining the recommended time of year to control various weeds in lawns. The chart is arranged with a list of common weeds in rows on the left, and months of the year in columns on the top of the chart. The row for each weed listed has a colored bar in the row, indicating the time frame when the weed may be most effectively controlled. Green bars indicate application time ranges for post-emergent herbicides (herbicides applied after the weed has emerged and is actively growing), and goldenrod bars indicate application time ranges for pre-emergent herbicides.
Turfgrass professionals and homeowners with established weed problems may use this chart to plan the timing of herbicide applications. The chart may also be used to determine if a recently discovered weed issue is best addressed immediately, or if an herbicide application should be delayed until later in the season or even the following year to be most effective.
If there are several different weeds to be controlled, the chart may also help in the decision of spray timing. Perhaps a single application of one herbicide will be effective. If not, then two different applications may be needed due to the timing required, or two entirely different herbicides may be required to control the different types of weeds. If the recommended control times for several weeds overlap, an application time may be selected that will offer good control all of those weeds (provided the herbicide selected is effective in controlling them).
Herbicide Application Cautions
Herbicides should not be applied to warm-season turfgrasses, such as centipedegrass, St. Augustinegrass, bermudagrass, and zoysiagrass, during their spring green-up in March and April. Likewise, for turfgrass tall fescue lawns, herbicide applications should generally not be applied during the heat of summer (July/August). Herbicide application during these times may severely injure the turfgrasses.
Herbicides should not be applied to lawns when the temperature exceeds 85 °F or when the lawn is under moisture stress (wilted). The surfactant found in most herbicides can cause severe injury if applied during these conditions. Plan any needed herbicide applications during cooler days, or perhaps in the evening after the temperature has decreased below 85 °F. Do not spray in the early morning on a day when temperatures are forecasted to exceed 85 °F – the hotter temperatures later in the day will cause injury similar to applying the herbicide during the heat of the day.
Irrigate lawns one to two days prior to herbicide application to help prevent herbicide injury to the turfgrass. Irrigation removes drought stress from the turfgrass, making it more resilient to herbicide applications. If irrigation is not an option, apply herbicides one to two days after a rainfall of one-half inch or more. Herbicides applications to stressed turfgrass, regardless of the cause of the stress, can cause unintended injury.
Lawns should not be mowed just before herbicide application. Mowing is a form of stress for the turfgrass, so allow three to four days of recovery before applying herbicides. Additionally, lawns should also not be mowed within two or three days after an herbicide application. The weeds to be controlled need time to absorb and translocate the herbicide applied down to their root system, and the turfgrass should be given time to overcome any stress from the herbicide. The herbicide label may provide more specific instructions about mowing intervals before or after application, so follow those directions.
Proper sprayer calibration is critical to ensure correct application rates. Calibration information for spreaders used for granular herbicides is explained in HGIC1657, Calibrating Spreaders. Calibration information for backpack or pump-up sprayers is presented in the University of Georgia’s “Calibrating Backpack Sprayers” (1).
As mentioned, Extension agents at both local county Extension office and at the Home and Garden Information Center are the best sources of information for weed identification, herbicide selection, and application information. The Extension agent may also be able to offer precautions based on the pesticide label, such as whether care during application should be exercised, or if a different herbicide should be used near sensitive plants, vegetable gardens, or fruit trees.
A listing of Clemson Extension Offices in every county of the State can be found on the web at https://www.clemson.edu/extension/co/index.html.
Turfgrass Weed Spray Dates Chart
This chart is provided as a planning tool. It is not provided as a substitute for herbicide label information, which is the law, nor is it to be used in the absence of the directions on the herbicide label. It is simply provided to help the turfgrass professional or homeowner plan approximate weed control timing as a portion of an effective weed management strategy. This best management strategy includes following all herbicide label directions and observing all warnings and cautions, including setbacks from water bodies, roads, property lines, and wells; warnings concerning volatilization and wind drift; requirements for surfactants, nozzle types and sizes, spray pressures, and water volumes applied; and worker protection apparel required.
- Calibrating Backpack Sprayers, 2019 Georgia Pest management Handbook, Home and Garden Edition, University of Georgia, Athens (GA): c2019 [accessed 2019 September].https://extension.uga.edu/content/dam/extension/programs-and-services/integrated-pest-management/documents/handbooks/2020-pmh-home-chapters/Calibration%20of%20Sprayers.pdf