A pre-emergent herbicide is a specialized product designed to stop weed seeds from germinating. It works by creating a chemical barrier near the soil surface that disrupts the early stages of a weed seedling’s development, specifically inhibiting root or shoot growth. This barrier is a preventative measure, not a treatment for existing weeds, making correct timing crucial. Fall is a particularly important time for this preventative step, as it targets the next wave of weeds before they become visible problems.
The Purpose of Fall Pre-Emergent Application
Many weeds, known as cool-season annuals, begin germinating in the late summer and fall as soil temperatures drop. These weeds, which include types like annual bluegrass, henbit, and chickweed, will grow slowly through the winter and then flower and produce seeds in the early spring before dying off in the summer heat. Applying a product in autumn creates a barrier that prevents these seeds from taking root, eliminating the weeds before they ever appear.
Many summer annual weeds, such as crabgrass, produce a vast number of seeds that remain dormant in the soil, forming what is called a seed bank. While crabgrass germinates in the spring, applying a long-lasting pre-emergent in the fall can establish a protective layer that persists through the winter. This residual barrier can then contribute to the prevention of the following spring’s crabgrass flush, offering a head start on warm-weather weed control.
The weed seeds that successfully germinate in the fall and survive winter become much harder to control in the spring, often requiring more aggressive post-emergent herbicides. These products kill established weeds but do not offer the same broad, season-long prevention. By stopping the seeds before they sprout, the fall application helps maintain a denser, healthier turf by reducing competition for nutrients, light, and water throughout the cooler months.
Recommended Active Ingredients for Autumn Use
Prodiamine is a highly popular and effective choice for fall use due to its long residual effect, often lasting up to six months. This extended longevity makes it excellent for catching both cool-season annuals that germinate immediately and summer annuals like crabgrass that sprout the following spring. Prodiamine works by inhibiting root growth, effectively starving the germinating seedling of water and nutrients.
Another common ingredient is Dithiopyr, which is often favored for its unique dual-action capability. While it functions as a pre-emergent barrier similar to prodiamine, it also offers limited early post-emergent control. This means Dithiopyr can kill young crabgrass seedlings that have just sprouted and have not yet developed tillers, providing a safety net if the application timing was slightly delayed. However, its residual effect is generally slightly shorter than prodiamine, lasting between three and five months.
For broadleaf weeds specifically, such as henbit and chickweed, Isoxaben is a targeted option that works by inhibiting cell wall production in the emerging seedlings. Isoxaben is highly effective against a wide range of broadleaf species but offers little control over grassy weeds. For comprehensive fall protection, Isoxaben is often combined with a grass-focused product like prodiamine to broaden the spectrum of weeds that are controlled by the application. The final choice of chemical should always be matched to the specific turfgrass species in the lawn, as certain chemicals can harm newly established or sensitive grasses.
Proper Application Timing and Technique
The most important factor for a successful fall pre-emergent application is determining the precise window for weed germination, which is tied directly to soil temperature, not air temperature. The target for fall application is when the soil temperature consistently drops to the range of 70 degrees Fahrenheit and continues to trend downward. This temperature threshold signals the beginning of the cool-season weed germination period.
To determine this application time, homeowners can use a soil thermometer inserted one to two inches into the ground or consult local agricultural extension resources that track soil temperatures. Applying the herbicide before the soil temperature drops below 55 degrees Fahrenheit ensures the barrier is established before the bulk of cool-season weeds begin to sprout. Applying too early risks the product breaking down before germination, while applying too late means some weeds will have already broken through.
Application technique depends on the formulation chosen: granular or liquid. Granular products are spread using a broadcast or drop spreader and must be “watered in” immediately with about a quarter to a half inch of irrigation or rainfall. This watering dissolves the granules, moving the active ingredient into the top layer of the soil to form the chemical barrier.
Liquid pre-emergents are mixed with water and sprayed, bonding to the soil surface almost immediately and often requiring less water to activate. Ensuring uniform coverage is paramount, as any gaps in the barrier will allow weeds to emerge. Before application, clear the lawn of heavy leaf litter or debris so the herbicide can make direct contact with the soil surface.