Weed and feed products simplify lawn maintenance by combining a chemical fertilizer (“feed”) with a selective herbicide (“weed” killer) into a single application. The goal is to nourish the desirable turfgrass while simultaneously eliminating invasive broadleaf plants that compete for light, water, and soil nutrients. By addressing these needs at once, homeowners can achieve a denser, greener lawn. The effectiveness of this combined material depends entirely on understanding how each component works and applying it under specific conditions.
The “Feed” Mechanism
The “feed” component is a complete fertilizer, providing the essential macronutrients necessary for robust grass growth. These nutrients are represented by the NPK ratio: Nitrogen, Phosphorus, and Potassium. Nitrogen promotes vigorous growth above the soil line, contributing to the rich green color of the grass blades and encouraging dense, thick turf.
Phosphorus supports root development and cell division, though its inclusion is sometimes restricted due to environmental concerns over runoff. Potassium enhances the plant’s internal health by regulating water uptake and nutrient transport, improving the grass’s tolerance to stresses like drought, heat, and disease. The fertilizer granules must dissolve and enter the soil to be absorbed by the grass roots, providing a sustained nutritional foundation.
The “Weed” Mechanism
The “weed” component relies on selective herbicides designed to target unwanted plants without harming the narrow-leaf turfgrass. These chemicals are highly specific, exploiting biological differences between common broadleaf weeds, such as dandelions and clover, and the grass itself. Many of the post-emergent herbicides used in these products, like 2,4-D, Dicamba, and MCPP, are classified as synthetic auxins. Auxins are natural plant hormones that regulate cell growth.
When a susceptible broadleaf weed absorbs the synthetic auxin, the herbicide triggers an overwhelming, uncontrolled growth response. This rapid cell division causes the weed’s stems and leaves to twist, curl, and distort. This chaotic growth disrupts the plant’s internal systems, preventing the proper movement of nutrients and water throughout the plant’s vascular tissue.
The systemic action of the herbicide means the chemical is translocated from the point of contact on the leaf down to the root system, ensuring the entire plant is affected. Ultimately, the metabolic disruption leads to the death of the weed over several days to a few weeks. The narrow-leaf structure of turfgrass allows it to metabolize the chemical differently or resist its effects, protecting it from the hormonal overstimulation.
Products may also contain pre-emergent herbicides, which create a chemical barrier in the soil. These formulations prevent weed seeds from germinating or inhibit the initial root development of emerging seedlings. Post-emergent formulations are far more common in typical weed and feed products, as they address existing, actively growing weeds.
Timing and Application Requirements
Precise timing and application are required to ensure both the fertilizer and the herbicide components work as intended. The post-emergent herbicide must be applied when broadleaf weeds are actively growing, usually during spring or early fall when temperatures are consistently between 60° and 90° F. Applying outside this range, such as during intense heat or dormancy, significantly reduces the herbicide’s effectiveness.
For the selective herbicide to be absorbed, the granular product must adhere to the surface of the weed’s leaves. Application is often recommended when the lawn is slightly damp, such as after a morning dew, allowing the granules to stick to the foliage. Mowing the lawn one or two days before application is also helpful, ensuring the weeds have enough leaf surface area to absorb the chemical.
Once the product is applied, it is necessary to delay watering for a specific period, generally 24 to 48 hours, to give the herbicide enough time to be fully absorbed and translocated throughout the weed. Watering too soon will wash the herbicide off the leaves before it can take effect, wasting the treatment. After this required waiting period, the lawn should be thoroughly watered to activate the “feed” component. The water dissolves the fertilizer granules, allowing the nitrogen, phosphorus, and potassium to move into the root zone of the turfgrass to promote healthy growth.