The straightforward answer to whether grass fertilizer kills weeds is no. Fertilizer alone does not possess the chemical properties needed to eliminate unwanted plants; it functions exclusively as a source of nutrition designed to feed and strengthen existing grass. For true weed control, a separate chemical component, known as a herbicide, must be introduced.
Defining the Separate Roles of Fertilizer and Herbicide
Fertilizer is essentially a plant food composed of three primary macronutrients, represented by the NPK ratio: Nitrogen (N), Phosphorus (P), and Potassium (K). Nitrogen promotes vigorous blade growth and deep green color. Phosphorus supports the establishment of a robust root system and aids in the early development of the turf.
Potassium enhances the overall durability of the grass, improving its ability to withstand environmental stressors like drought, heat, and disease. When applied, these nutrients are absorbed by all plants in the area, feeding both the turf and any existing weeds. Fertilizer’s goal is to nourish, and it contains no active ingredients that disrupt a plant’s biological processes to cause death.
Herbicides, by contrast, are active chemical agents engineered specifically to interfere with critical plant growth pathways. These chemicals work by targeting a plant’s biochemistry, such as inhibiting enzyme production or disrupting photosynthesis. Selective herbicides are formulated to target broadleaf weeds, allowing them to kill the weed while leaving the turfgrass unharmed.
The mechanism of action for a herbicide is destructive, causing phytotoxicity that leads to symptoms like wilting, yellowing, and eventual plant death. These agents rely on specific chemical interactions to stop the weed’s development, a function that fertilizer’s simple nutrient compounds cannot perform.
How Combination Products Work to Kill Weeds
The common misconception that fertilizer kills weeds stems from the popularity of “weed-and-feed” combination products. These products are granular fertilizers blended with one or more active herbicide chemicals. The product provides NPK nutrients to the grass while simultaneously applying the weed-killing agent.
These combination products typically use two main types of herbicide action. Pre-emergent herbicides are designed to prevent weeds, such as crabgrass, from establishing themselves by creating a chemical barrier in the soil. They must be applied before weed seeds germinate, working by killing the emerging seedling as it attempts to grow through the treated soil layer.
Post-emergent herbicides are intended for the elimination of visible, existing weeds like dandelions and clover. These chemicals are often systemic, absorbed through the leaves, and travel throughout the entire weed plant, disrupting its growth from within. For best results, post-emergent components often require the active chemical to stick to the weed’s foliage, sometimes necessitating a damp surface for proper absorption.
The Indirect Effect of Healthy Grass on Weed Suppression
While fertilizer does not chemically kill weeds, promoting a dense, healthy lawn offers a strong, non-chemical form of weed prevention. A vigorously growing turf is naturally resistant to weed invasion because it creates an environment where weeds struggle to survive. This effect is known as cultural control, where optimal management practices prevent weed establishment.
The enhanced growth stimulated by fertilizer allows grass roots to quickly consume available resources in the soil, including water and nutrients. This strong competition effectively starves out weed seedlings before they develop into mature plants. A dense lawn claims all available space and resources, leaving little opportunity for weeds to take hold.
Furthermore, the dense canopy of a healthy lawn physically blocks sunlight from reaching the soil surface. Many weed seeds require direct sunlight to trigger germination, and the shade created by thick turf prevents this process. Focusing on proper fertilization to increase grass density reduces the open, resource-rich areas that sparse turf provides for opportunistic weeds.