The question of the best month to apply a weed and feed product is complex because this single application combines two distinct functions: herbicide and fertilizer. The product is designed to kill existing weeds or prevent new ones from growing while simultaneously supplying nutrients to the grass. However, the ideal timing for weed control often conflicts with the optimal timing for grass fertilization, as each component requires specific temperature and growth conditions for maximum effectiveness. Therefore, the best month is not fixed on the calendar but is determined by the type of grass in the lawn and the local climate’s temperature patterns.
The Fundamental Difference Between Weed and Feed Timing
Weed and feed products contain both fertilizer and herbicide, each with different mechanisms of action. Herbicides are generally categorized as pre-emergent or post-emergent, and their efficacy depends on soil and air temperatures. Pre-emergent herbicides must be applied before weed seeds germinate, typically requiring the soil temperature at a four-inch depth to be consistently around 50–55°F to prevent weeds like crabgrass from sprouting.
Post-emergent herbicides, which target weeds that have already sprouted, require weeds to be actively growing to absorb the chemical effectively. This usually means warmer air temperatures, ideally between 60°F and 90°F. The fertilizer component is absorbed most efficiently when the turfgrass is actively growing, which often corresponds to warmer temperatures than the pre-emergent window. Applying fertilizer when the grass is dormant is ineffective and can lead to nutrient runoff. Timing a single product to satisfy these two different temperature and growth requirements simultaneously is the challenge.
Application Timing for Cool-Season Lawns
Cool-season grasses, such as Kentucky Bluegrass, Fescue, and Perennial Ryegrass, have two main growth periods: spring and fall. Pre-emergent control for annual weeds like crabgrass must be applied in March or early April in most northern regions. This timing ensures the herbicide barrier is in place just before the soil temperature consistently reaches 55°F, which signals the start of crabgrass germination. Applying the product too early means the chemical barrier may break down before the weed season begins.
The fall application, typically in September or October, is considered the most beneficial for cool-season lawns. This period is ideal for post-emergent herbicides, as cooler air temperatures allow the grass to tolerate the weed killer better. Weeds like dandelions and clover are actively storing energy in their roots, making them vulnerable. The fertilizer promotes deep root growth and carbohydrate storage, which improves the lawn’s health and resilience through the winter. This fall application should be completed at least six weeks before the first hard frost to allow the grass to absorb the nutrients.
Application Timing for Warm-Season Lawns
Warm-season grasses, including Bermuda, Zoysia, and St. Augustine, thrive in heat and are dormant during the winter. The first application should occur in late spring or early summer, typically from April through June, once the grass has fully greened up and is actively growing. Applying fertilizer containing nitrogen while these grasses are dormant can cause significant damage or delay their spring transition.
This late spring timing aligns the fertilizer with the grass’s active growth phase and places a pre-emergent barrier for summer annual weeds. The soil temperature should be consistently above 65°F at the four-inch depth for optimal nitrogen uptake. A supplemental application can be made in late summer or early September to provide a boost of nutrition and post-emergent weed control before the grass begins to slow down. This late application helps the turf recover from summer stress and prepare for dormancy, but it must be applied well before the grass enters its winter rest period.
Immediate Care After Application and Common Errors
The success of a weed and feed product depends on the care provided immediately following application. Granular products containing post-emergent herbicides require the granules to stick to the moist leaves for absorption. Therefore, the lawn should be lightly damp before application, and watering must be delayed for 24 to 48 hours to give the herbicide time to work before being washed off.
If the product contains a pre-emergent herbicide, it must be watered in within a few days to activate and establish the protective barrier in the soil. Mowing should also be avoided for 24 to 48 hours after application to ensure the granules remain on the weed leaves.
Common mistakes include applying the product during periods of high heat or drought, which can scorch the grass due to the fertilizer salts, and applying it too heavily, which causes fertilizer burn. Using a weed and feed product too close to seeding is also an error, as the herbicide will prevent grass seed from germinating.