A pre-emergent herbicide is a specialized form of weed control that targets weed seeds before they sprout and become visible. It works by creating a thin, invisible chemical barrier in the top layer of the soil, preventing germinated weed seedlings from establishing roots and shoots. Spring application is necessary to stop the emergence of summer annual weeds, which are the most common invaders of a healthy lawn, significantly reducing the need for intensive weed removal later in the season.
Determining the Ideal Spring Application Window
Timing is the most important factor for successful pre-emergent application, as the product must be in place before weed seeds begin their germination cycle. The primary signal is the soil temperature, not the calendar date, which fluctuates significantly across regions. The target is to apply the herbicide just before the top inch of soil consistently reaches 55 degrees Fahrenheit, the threshold that triggers crabgrass seeds to sprout. Monitoring this temperature with a soil thermometer offers the most reliable data for optimal timing.
A secondary indicator, known as phenology, involves observing specific plants that change at the same time as target weed seeds germinate. A reliable natural cue is when the bright yellow flowers of the forsythia bush finish blooming and begin to drop their petals. This visual signal confirms the soil is warming to the point where crabgrass will soon germinate, providing a practical application window if a soil thermometer is unavailable. Missing this narrow window means the barrier will be applied too late, allowing some seeds to escape the effect.
For season-long control against crabgrass and persistent summer weeds, a “split application” strategy is highly effective. This method involves dividing the total recommended herbicide rate into two separate applications, with the first dose applied at the initial spring timing. The second, smaller dose is typically applied six to ten weeks later, depending on the product’s residual activity. This layering approach ensures the chemical barrier remains effective through the warmer summer months when the first application naturally begins to break down.
Selecting the Best Active Ingredient for Your Needs
Homeowners generally choose between two highly effective active ingredients: Prodiamine and Dithiopyr. Prodiamine, often sold as Barricade, is valued for its extended residual control, remaining active in the soil for up to nine months. This long-lasting nature makes it an excellent choice for the first application of the season, providing a robust barrier against a wide spectrum of annual grasses and broadleaf weeds. Prodiamine functions purely as a pre-emergent, meaning it must be applied before any target weed germinates to be effective.
In contrast, Dithiopyr, commonly marketed as Dimension, offers a unique advantage by combining both pre-emergent and early post-emergent activity. While it establishes a protective soil barrier, it can also kill very young crabgrass seedlings that have not yet developed beyond the one-to-five tiller stage. This dual-action capability provides a wider margin of error for application timing, which is beneficial if the initial spring window is missed. However, Dithiopyr’s residual control is generally shorter, lasting closer to three to four months.
The choice between these two chemicals depends on the timing of application and the need for flexibility. Prodiamine is typically more cost-effective and preferred for those who apply early, maximizing its long residual life well ahead of the 55°F soil temperature threshold. Dithiopyr is a better fit for a slightly later application or for a second application in a split program, where its post-emergent properties can clean up early crabgrass escapes. Both ingredients are available in granular form, which is easier to spread, and in liquid concentrates, which require precise mixing but provide a more uniform application.
Proper Application Techniques for Success
Achieving a uniform, effective barrier requires careful attention to the application process, beginning with the correct measurement and calibration of equipment. Whether using a granular spreader or a liquid sprayer, the device must be calibrated to ensure the exact amount of product recommended on the label is distributed. Applying too little product results in gaps in the barrier, while over-application risks injury to the established lawn or exceeding the maximum annual rate.
To guarantee even coverage, a cross-hatch or perpendicular pattern is the recommended technique. This involves applying half of the required product rate while walking in one direction (e.g., north to south), and then applying the remaining half rate by walking perpendicular to the first pass (e.g., east to west). This method minimizes the chance of streaks, skips, or overlaps, ensuring a dense and consistent chemical layer in the soil. It is also helpful to make a trim pass around the perimeter first to establish a boundary and create a turning area.
The final and most important step for activation is the immediate irrigation of the treated area. The pre-emergent chemical is inert until it is watered into the top layer of the soil to form the protective barrier. A general rule is to apply about one-quarter to one-half inch of water within 24 hours of application, either through irrigation or rainfall. This watering dissolves the granular carrier or washes the liquid concentrate off the grass blades, moving the active ingredient into the soil profile where weed seeds reside. For safety, wear gloves and protective gear, and take care not to apply the product near storm drains or bodies of water to prevent runoff pollution.