Pre-emergent herbicides prevent weed seeds from successfully growing by creating a protective chemical layer in the uppermost portion of the soil. This layer forms a barrier against new weeds. When a seed sprouts, the emerging root or shoot absorbs the herbicide, which inhibits the cell division necessary for growth, stopping the weed before it breaks the soil surface. Unlike post-emergent products that kill existing weeds, pre-emergents must be applied before germination, making timely application crucial.
Why the Condition of the Grass Matters
The condition of the grass and soil at the time of application directly affects how well the pre-emergent product can reach and bind to the soil particles. Applying pre-emergent to extremely saturated soil, such as after a heavy rain, is generally not recommended. When the soil is saturated, any additional moisture used for activation is likely to run off and carry the chemical with it. This runoff leads to uneven coverage and a weakened protective barrier.
The ideal state for application is typically a dry or lightly damp lawn with soil that is not saturated. A dry surface allows the herbicide to be applied uniformly before it is intentionally watered in. However, a slightly damp surface can be beneficial for specific types of products. The goal remains consistent: the herbicide must settle onto the soil surface to be effective.
Differences Between Liquid and Granular Products
The primary application challenge differs based on whether the product is granular or liquid, and this distinction is where the grass’s moisture level becomes most relevant. Granular pre-emergents are small pellets coated with the active herbicide ingredient, spread using a broadcast spreader. A lightly damp lawn can be helpful for granular products because the slight moisture helps the prills stick to the grass blades. This adhesion ensures the granules are not easily dislodged before the activation watering occurs.
If the lawn is too wet, granular prills can begin to dissolve prematurely on the grass blades. This early dissolution can concentrate the herbicide on the blades, potentially leading to turf burn if the product is not watered in quickly.
Liquid pre-emergents, which are mixed with water and sprayed, require a different approach. For liquid applications, a dry grass surface allows the spray solution to pass through the canopy and contact the soil as directly as possible. If the grass is wet, the liquid herbicide may cling to the moisture on the blades, reducing the concentration that reaches the soil surface.
Activation and Post-Application Watering
The single most important step after application, regardless of whether the product was liquid or granular, is the activation process. The pre-emergent herbicide must be dissolved and moved into the top layer of the soil to form the protective chemical barrier. This is achieved by watering the lawn immediately after the product has been spread.
Most product labels recommend applying between 0.25 and 0.5 inches of water to achieve proper activation. This amount of irrigation or rainfall is necessary to dissolve the active ingredient and carry it down a fraction of an inch into the soil profile. This activation should happen within a specific time window, ideally right after application or at least within 24 to 48 hours. Delaying this watering allows the herbicide to sit on the surface, where it can be degraded by sunlight or scattered by wind and foot traffic, compromising the integrity of the weed prevention blanket.