“Weed and feed” products combine a granular fertilizer to nourish turfgrass and a broadleaf herbicide to eliminate common weeds. While popular during active growth periods, applying these combination products in the summer, especially during high heat, is generally discouraged. The dual action that makes them appealing during milder seasons poses significant risks to the lawn’s health when temperatures rise.
Why Extreme Heat Makes Application Dangerous
Turfgrasses, particularly cool-season varieties, employ survival mechanisms when subjected to prolonged summer heat and drought. During these conditions, the grass attempts to conserve energy and moisture by entering a state of semi-dormancy. This natural process involves slowing metabolic functions and reducing water uptake through the roots.
Applying a high-nitrogen fertilizer, common in “weed and feed” products, directly interferes with this protective shutdown. The nitrogen forces the grass out of dormancy and encourages a flush of new, tender growth. This forced acceleration requires increased water and nutrient consumption that the plant cannot sustain in the heat.
The result is rapid dehydration of the leaf blades and increased potential for fertilizer burn. The high concentration of salts in the fertilizer draws moisture out of the plant cells, leading to brown, desiccated patches across the lawn. Furthermore, the roots, already weakened by the heat, become susceptible to chemical injury, which can cause long-term damage and thinning of the turf.
Understanding the Herbicide Component and Volatility
The weed-killing agents in these combination products introduce a separate set of risks related to temperature. Many broadleaf herbicides, such as 2,4-D or dicamba, are formulated as amine salts or esters, which can become volatile under high temperatures. Volatility refers to the chemical’s tendency to transition from a solid or liquid state into a gaseous vapor.
When air temperatures consistently exceed 85 to 90 degrees Fahrenheit, the herbicide component can vaporize into the air. This herbicide gas can then drift away from the treated area, sometimes traveling hundreds of feet. The primary concern with this drift is injury to non-target plants, such as vegetable gardens, flower beds, or ornamental shrubs located nearby.
Granular “weed and feed” products often require the lawn to be wet during application so the granules stick to the weed leaves. If this application is followed by intense sunlight and high heat, the combination exacerbates the risk of both plant burn and herbicide volatility. This chemical instability makes summer application risky for both the lawn and surrounding landscape.
Separating Summer Lawn Care Needs
Given the compounded risks of using a combination product during the summer, separating the functions of feeding and weed control is a safer management strategy. If turfgrass health is generally good, the most responsible approach during peak heat is often to suspend all fertilizer applications entirely. A healthy, established lawn can typically withstand a period of summer dormancy without permanent harm.
If the lawn shows signs of nutrient deficiency, switch to a specialized summer formulation. Select a low-nitrogen product where a significant portion of the nitrogen is in a slow-release form. Slow-release nitrogen feeds the turf gradually, preventing the sudden growth surge that draws excessive moisture and causes burn.
Homeowners should look for fertilizers where the first number in the NPK ratio (Nitrogen-Phosphorus-Potassium) is relatively low, such as a 10-0-10 blend. These formulations often contain higher levels of potassium, which helps regulate water movement and improves heat tolerance. This allows the grass to manage stress more effectively without the shock of a high-nitrogen dose.
Timing is critical for any necessary summer feeding, even with a low-nitrogen product. Fertilizer should only be applied when air temperatures are forecasted to be cooler, ideally below 85°F, and during the coolest parts of the day. Applying product in the early morning or late evening minimizes moisture evaporation and reduces heat stress on the plant roots and leaves.
Alternative Strategies for Weed Management
When foregoing the combination product, weed control in the summer requires a more targeted and specific approach. The safest and most environmentally conscious method for tackling individual weeds is mechanical removal, or simply pulling them by hand. This action removes the entire plant, including the root structure, and avoids introducing any chemicals into the soil or air.
For heavier weed infestations, liquid spot treatments offer a safer alternative to broadcasting a granular product across the entire lawn. Spot treatment involves mixing a liquid broadleaf herbicide and applying it directly to the foliage of the undesirable plant. This limits the chemical exposure solely to the target weeds and greatly reduces the risk of overall turf burn and volatility.
When selecting a liquid herbicide for spot treatment, look for products that specifically state they are safe for your turfgrass species. These applications should be performed on days with low wind and when temperatures are not at their peak, to minimize drift or volatility. The goal is to maximize absorption by the weed and minimize contact with the surrounding healthy grass.
While pre-emergent herbicides prevent the germination of many common weeds, mid-summer is generally not the optimal time for application. These preventative treatments are most effective when applied in early spring to stop crabgrass or in the fall to manage winter annuals. Applying a pre-emergent in the heat of summer is typically only done to target specific late-germinating weed varieties.