A pre-emergent herbicide is a specialized weed control product designed to stop weeds before they appear above the soil surface. It establishes a protective barrier in the top layer of the soil where weed seeds reside. When a seed begins germination and its root structure emerges, it absorbs the herbicide, which interrupts cell division or root development. This halts the growth of the young plant, preventing weed emergence without affecting established turfgrass.
Optimal Timing for Effectiveness
Successful pre-emergent application relies on accurate timing, which is determined by soil temperature rather than a calendar date. The product must be in place and activated just before weed seeds, like crabgrass, begin to germinate. This crucial window occurs when the soil temperature consistently reaches 50 to 55 degrees Fahrenheit at a two to four-inch depth for several consecutive days.
A spring application targets summer annual weeds, such as crabgrass and foxtail. Applying the herbicide too early allows the chemical barrier to degrade before germination starts. Applying it too late means the weeds have already begun to sprout, rendering the product ineffective. For winter annuals, like Poa annua, a fall application is necessary when soil temperatures drop to approximately 70 degrees Fahrenheit.
Preparation Before Spreading
Before applying the herbicide, the lawn must be prepared so the product can reach the soil surface and form a continuous barrier. Begin by mowing the lawn to its normal height, as excessive grass length interferes with uniform distribution. After mowing, remove debris like fallen leaves or sticks that block the herbicide from contacting the ground.
The next step involves calculating the exact amount of product needed for the coverage area. Determine your lawn’s square footage by multiplying the length by the width. This measurement is used to calibrate your spreader or sprayer according to the manufacturer’s application rate, typically provided per 1,000 square feet.
Application Techniques and Coverage
Achieving a uniform blanket of coverage is essential, as any skipped area allows weeds to break through the protective barrier. For granular pre-emergents, a calibrated broadcast or drop spreader is required. For liquid applications, carefully measure the concentrated formula to ensure the correct concentration before pouring it into the sprayer.
A highly effective method for ensuring even coverage is the half-rate cross-hatch pattern. This technique involves setting the spreader or sprayer to half the recommended rate and applying the product across the entire lawn in one direction. Then, apply the second half perpendicular to the first pass. This dual-direction application minimizes the risk of streaking or gaps. Always wear appropriate safety gear, including gloves and long-sleeved clothing, during application.
Activating the Barrier
Immediately following application, the pre-emergent herbicide must be activated by water to dissolve the product and move the active ingredients into the top layer of the soil. This establishes the chemical barrier that blocks weed seeds from growing. Most products require approximately one-half inch of water, from irrigation or rainfall, to activate correctly.
This watering-in process should occur within 24 to 48 hours to prevent the material from being degraded by sunlight or wind. Once the barrier is established, avoid any activity that might disturb the soil profile for several weeks. Deep aeration, vertical mowing, or heavy raking can break the continuous chemical layer, creating gaps that allow weed seeds to germinate.