A pre-emergent herbicide is a specialized product designed to stop weeds before they appear above the soil surface. It is a preventive measure, not a weed killer for existing plants, that targets the weed seed during its earliest stage of growth. The primary purpose of using a pre-emergent is to prevent the germination of annual weeds, particularly common pests like crabgrass, by creating a protective layer in the soil.
The Essential Timing of Mowing
Mowing before applying a pre-emergent is generally recommended. Ideally, the lawn should be cut one to two days before the herbicide application is scheduled. This timing ensures the product reaches the soil surface without obstruction. Tall grass or thick clippings can act like a filter, preventing the granular or liquid product from settling where it needs to be. The herbicide must make direct contact with the ground to form an uninterrupted protective barrier. Set the mower height to the standard setting for your turf type, avoiding a cut that is too short, which can stress the grass.
Understanding Pre-Emergent Action
Pre-emergent herbicides operate by forming a continuous chemical barrier within the top layer of the soil. This barrier is activated once the product is dissolved and integrated into the ground. When a weed seed begins to germinate, its initial root, known as the radicle, attempts to grow through this treated zone. As the germinating seed absorbs the chemical, the herbicide disrupts the biological processes necessary for growth and development. Active ingredients, such as dithiopyr or prodiamine, inhibit cell division within the seed embryo, halting the sprout before it can establish a root system. This action differs from post-emergent herbicides, which are designed to kill established plants. The treatment’s efficacy depends entirely on the integrity of this uniform chemical layer. Crabgrass germinates when soil temperatures consistently reach approximately 55°F, requiring the barrier to be created just prior to that temperature window. The compounds must bind to soil particles to provide protection throughout the germination period, which typically lasts three to five months. Any disruption of this layer can create a gap, allowing weed seedlings to emerge.
What to Do After Applying Pre-Emergent
Proper care immediately following the application is necessary to ensure the pre-emergent product performs as intended. The first step involves activation, which requires sufficient moisture to move the chemical from the surface into the soil. Granular herbicides, in particular, must be watered in to dissolve the pellets and form the protective barrier beneath the turf. A general guideline suggests applying about a quarter to a half-inch of water via irrigation or rainfall within 24 hours of application. This amount of water is usually sufficient to carry the active ingredients into the upper soil profile where weed seeds germinate. If a liquid spray is used, wait 12 to 24 hours before watering to allow the product to be fully absorbed. Maintaining the integrity of the barrier is important during the following weeks. Avoid mowing for three to five days after the application and activation process to ensure the product has fully settled. More importantly, postpone any activity that disturbs the soil for an extended period, often six to eight weeks, to prevent breaking the chemical layer. This includes heavy raking, dethatching, or aeration, as these actions create openings through which weed seeds could sprout unimpeded.