Mixing a dedicated fertilizer with a “weed and feed” product is strongly discouraged. Weed and feed products are combination treatments, designed to deliver a full application of both fertilizer and herbicide in a single pass. Adding a separate fertilizer dramatically increases the concentration of chemicals applied to the lawn. Homeowners should treat these products as single-dose applications where the chemical load has already been set by the manufacturer.
Understanding Product Concentration and Safety
The fundamental issue with mixing a separate fertilizer into a weed and feed product is the immediate and unsafe doubling of the chemical load. Weed and feed contains a pre-measured amount of plant nutrients and broadleaf herbicides, formulated to cover a specific area at a safe rate. Manufacturer instructions are designed to deliver the maximum safe dose of both components for a given square footage.
Introducing an additional full-rate fertilizer results in applying twice the intended amount of nutrients and herbicide active ingredients. This combined concentration far exceeds the maximum safe dose for turfgrass, which is highly sensitive to chemical imbalances. The risk is particularly high with the potent herbicide component, whose dosage is carefully calibrated to kill weeds without harming the grass species. Exceeding this limit can cause widespread damage to the desirable turf.
The chemical formulations are incompatible with simply adding more of one component. For example, herbicides must adhere to weed foliage to be absorbed, which is why granular weed and feed is often applied to wet grass. Conversely, many fertilizers are meant to be watered into the soil immediately to begin feeding the roots. Combining an extra dose of fertilizer alters the intended delivery mechanism and greatly increases the risk of negative side effects due to chemical overload.
The Hazards of Over-Application
The direct consequence of applying excessive chemical concentration is immediate and visible damage to the lawn. A common issue is nitrogen burn, which occurs when too much salt-based nitrogen draws moisture out of the grass blades, causing them to dry out and appear scorched. This rapid desiccation leaves unsightly patches across the lawn, requiring significant time and effort to repair.
Excessive fertilizer salt buildup can damage the turfgrass root system, weakening the overall health of the lawn. Weakened grass becomes more susceptible to disease, pests, and environmental stress, undoing the benefits the fertilizer was meant to provide. High nitrogen can also spur excessive growth, leading to thinner cell walls and more fragile grass blades.
The herbicide component also poses a significant risk when over-applied. Herbicide overdose symptoms on turfgrass include distorted growth, leaf curling, or the complete death of the desirable grass. While these broadleaf herbicides are selective, a concentration that is too high can overwhelm the grass’s natural defenses, causing widespread failure of the entire application. Applying both products together transforms a lawn care step into a chemical hazard.
Optimal Timing for Separate Applications
Since mixing the two products is hazardous, the best strategy for a healthy lawn involves applying them separately with appropriate waiting periods. The effectiveness of weed control and fertilization often requires different timing and conditions. For example, post-emergent broadleaf herbicides must stay on the weed leaves for a period, typically 24 to 48 hours, to be fully absorbed and work effectively.
Fertilizer, especially quick-release synthetic varieties, is generally watered into the soil shortly after application to begin feeding the roots and to prevent burn. Because the needs for these two processes conflict, a separation of application is required. A typical recommendation is to wait approximately two to four weeks between a herbicide application and a fertilizer application, giving the herbicide time to kill the weeds and the lawn time to recover before stimulating new growth.
For pre-emergent herbicides, which prevent weed seeds from germinating, the timing is based on soil temperature, usually applied in early spring before the grass is fully active. Fertilizers, however, are best applied when the grass is actively growing. Separating these tasks allows the homeowner to optimize the timing and application method for each product, ensuring the grass receives the nutrients it needs without compromising the effectiveness of the weed control or risking chemical damage.