A pre-emergent herbicide is a type of chemical weed control designed to prevent weed seeds from successfully growing. It works by creating a thin, protective chemical barrier just beneath the soil surface. When a weed seed begins to germinate, it absorbs the herbicide as its root or shoot emerges, stopping the growth process before the weed appears above ground. Determining the correct quantity of pre-emergent for a large area, such as one acre, is highly dependent on the specific product chosen. The application rate is a variable quantity tied directly to the product’s concentration and the target weed species.
Understanding Pre-Emergent Product Formulas
The final application rate per acre is primarily dictated by the specific formulation of the herbicide, which can be broadly classified as granular or liquid concentrate. Granular products are ready-to-use pellets, often blended with fertilizer, that are spread across the turf. These typically have a low concentration of the active ingredient (AI), such as 0.38% or 0.5% by weight.
The concentration of the AI is the most important factor influencing the required volume. A granular product with a low AI percentage means a much greater bulk weight of material is needed to deliver the target amount of the chemical per acre. The label will provide the application rate as pounds of product per 1,000 square feet.
Liquid concentrates, on the other hand, are highly potent and must be mixed with water before application. These can be true liquids or water-dispersible granules (WDG), which are high-concentration powders, sometimes up to 65% AI. Because the concentration is so high, a small volume of liquid product can treat a vast area. The application rate for liquid concentrates is typically expressed in fluid ounces or pounds of product per acre.
Calculating Application Rates Per Acre
The necessary calculations scale the standard, small-area directions from the product label up to the full acre size. Since one acre is equal to 43,560 square feet, the conversion factor from a 1,000 square foot rate to an acre rate is 43.56. This conversion is the foundation for determining the total product needed for larger properties.
For granular pre-emergents, the calculation converts the label’s recommended rate of product per 1,000 square feet into total pounds of product per acre. For example, if a product label recommends applying 5.74 pounds of the granular product per 1,000 square feet, the required rate for an acre is found by multiplying 5.74 pounds by the conversion factor 43.56. This calculation yields a total of approximately 250.05 pounds of the granular product needed to cover one full acre.
The calculation for liquid concentrates is slightly different, often involving working backward from the required amount of active ingredient (AI) per acre. If a product label recommends a target of 0.65 pounds of AI per acre and the liquid concentrate is a 65% WDG formulation, you divide the target AI rate by the concentration percentage (0.65 lbs AI / 0.65 AI concentration). This calculation indicates that exactly 1 pound of the 65% WDG product is needed to treat the entire acre.
In cases where the label provides a direct liquid rate, such as ounces per acre, the calculation is simpler, though the product must still be mixed into a carrier volume of water. For a product with a label rate of 6 fluid ounces per acre, the applicator needs 6 ounces of the concentrate regardless of the tank size, provided the sprayer is calibrated to deliver the mixture uniformly over the entire 43,560 square feet. Precise equipment calibration is necessary, as a mathematically correct rate is meaningless if the spreader or sprayer cannot uniformly deliver the product across the entire treatment area.
Critical Timing and Post-Application Steps
Determining the correct quantity of herbicide is only one part of the successful application; timing is equally important for the product to function as intended. The ideal window for spring application occurs when the soil temperature, measured at a depth of two to four inches, consistently reaches 50 to 55 degrees Fahrenheit. This temperature range signals the beginning of germination for common summer annual weeds like crabgrass.
Applying the pre-emergent too early means the chemical barrier may degrade before the target weeds begin to sprout, while applying it too late means the weeds will have already emerged and will not be affected by the product. To ensure the chemical barrier is active when the seeds are at their most vulnerable, the product must be incorporated into the top layer of soil after application.
This incorporation is achieved by “watering in” the pre-emergent immediately after it is spread or sprayed. Granular products require moisture to dissolve the pellet and release the active ingredient into the soil matrix. Liquid products also require irrigation to move the chemical off the grass blades and into the soil where the weed seeds reside. A quarter to a half-inch of water, from either irrigation or rainfall, is typically required within 24 hours of application to activate the protective barrier.
Safety and handling protocols are also necessary, particularly when dealing with large volumes of concentrated product for an acre application. Applicators should wear the personal protective equipment specified on the product label, which often includes gloves, long sleeves, and eye protection. Proper disposal of containers and rinsing of application equipment are also standard procedures to prevent chemical exposure and environmental contamination.