“Weed and feed” products combine fertilizer to nourish turfgrass with an herbicide to manage unwanted plants. The success of this application depends entirely on the proper timing and amount of water applied afterward. Incorrect irrigation can lead to ineffective weed control, wasted product, or damage to the grass. Specific instructions vary because the fertilizer and weed killer components often have opposing requirements for activation, making the manufacturer’s label the most important guide.
The Immediate Watering Rule for Activation
For granular forms of “weed and feed,” initial watering is crucial to activate the fertilizer component and prevent potential scorching of the turfgrass. Fertilizer granules contain salts that, if left sitting on the grass blades in warm, dry conditions, can draw moisture out of the plant tissue, resulting in damage known as fertilizer burn. The initial light irrigation serves to wash these fertilizer particles off the delicate leaves and into the soil, where they can begin to dissolve.
This initial watering also starts the process of making the nutrients available to the grass roots, allowing the fertilizer to begin its work immediately. The recommended amount for this first watering is typically very light, aiming for about 1/8 to 1/4 inch of moisture. This shallow application is enough to dissolve the granules and carry the fertilizer just beneath the surface without causing runoff.
This immediate watering rule, driven by the fertilizer’s needs, often directly conflicts with the requirements of a post-emergent herbicide within the same product. When the product contains a weed killer that targets existing weeds, a delay in watering is necessary, which means the fertilizer’s activation must be momentarily postponed. The conflict highlights why understanding the herbicide type is necessary before turning on the sprinklers.
Understanding Product Type: Pre-Emergent vs. Post-Emergent Requirements
The timing of post-application watering is entirely dependent on the type of herbicide included in the “weed and feed” formula. Products containing a pre-emergent herbicide must be watered in almost immediately to be effective. These chemicals function by forming a continuous barrier layer within the top layer of the soil, which prevents weed seeds from successfully germinating.
To establish this protective chemical barrier, the granular product must be dissolved and carried down into the soil profile. The ideal window for this activation is within 24 hours of application, with a minimum of 1/4 to 1/2 inch of water needed to properly set the herbicide. Waiting too long allows the pre-emergent chemical to become ineffective as it can be scattered by wind or foot traffic before it bonds with the soil particles.
Conversely, a product that includes a post-emergent herbicide, designed to kill weeds already visible in the lawn, requires a significant delay in watering. These herbicides rely on foliar contact, meaning the chemical must sit on the weed’s leaf surface long enough to be absorbed into the plant. Watering too soon, before the product has been fully taken up, will simply wash the active ingredient off the leaves, rendering the weed control ineffective.
The required delay for post-emergent formulas is typically between 24 and 48 hours, though some product labels may recommend waiting up to 72 hours. This waiting period allows the systemic herbicides to penetrate the weed’s vascular system, ensuring the chemical reaches the roots for a complete kill. Once the waiting period has passed, the first watering can occur, which then focuses on activating the fertilizer and moving it into the soil.
Post-Application Watering Schedule and Depth
After the initial activation or absorption period has been satisfied, the focus shifts to a long-term watering plan that supports a healthy lawn. The objective is to encourage deep root growth, which makes the turfgrass more resilient to stress and drought. This is achieved by moving away from the light, frequent watering used for initial activation toward a deeper, less frequent schedule.
Lawns generally require about 1 inch of water per week, either from irrigation or natural rainfall. Watering should be deep enough to penetrate the soil several inches, which encourages the grass roots to grow downward in search of moisture. Shallow, daily watering should be avoided because it trains the roots to stay near the surface, making the lawn vulnerable to heat stress and disease.
Homeowners should monitor the weather closely and adjust the weekly watering schedule based on recent rainfall amounts. If significant rain occurs, the irrigation schedule should be reduced or postponed to prevent overwatering, which can lead to nutrient leaching and poor soil aeration. A deep watering session every three to four days, rather than daily light sprinkles, is the best maintenance practice after the “weed and feed” application has been fully integrated into the soil.