Effective lawn maintenance requires precise timing for applying fertilizer and weed killer, as using these products at the wrong time can reduce their effectiveness or even harm the turf. Fertilizer provides the necessary nutrients for grass growth and recovery, while weed killer targets unwanted plants. These different goals mean their optimal application windows often do not align. Understanding the distinct seasonal needs of the grass and the life cycles of weeds is paramount for developing a successful lawn care strategy.
Optimal Timing for Lawn Fertilization
The ideal time to fertilize depends entirely on the type of grass, which is broadly categorized into cool-season and warm-season varieties. Cool-season grasses, such as Kentucky bluegrass and fescue, experience their most active growth during the cooler temperatures of spring and fall. For these grasses, the single most impactful feeding is generally a late-season application, which should be done in the late fall, just before the ground freezes but after the grass has stopped growing above ground. This “winterizing” application allows the roots to store carbohydrates for a strong, early green-up the following spring.
A secondary application for cool-season grass in early fall helps the turf recover from summer heat stress and promotes robust root development. Applying fertilizer too early in the spring can encourage excessive blade growth at the expense of root strength, making the grass more susceptible to summer drought and disease.
Warm-season grasses, including Bermuda and Zoysia, thrive in warmer temperatures and require fertilization during their peak growth period from late spring to early summer. Their initial feeding should occur when soil temperatures have consistently reached 65 degrees Fahrenheit, typically in mid-to-late spring. Warm-season turf often benefits from additional feedings every six to eight weeks throughout the summer until the growth slows in early fall. During periods of extreme summer heat and stress, “spoon-feeding” involves applying very light, frequent doses of a low-nitrogen fertilizer. This method provides necessary nutrients without pushing excessive top growth. Applying fertilizer to any type of grass once it has gone completely dormant is generally ineffective, as the plant cannot properly absorb the nutrients.
Optimal Timing for Weed Control
Weed control timing is dictated by the specific type of herbicide being used, which falls into two main categories based on when they are applied relative to the weed’s emergence.
Pre-Emergent Herbicides
Pre-emergent herbicides create a chemical barrier in the soil to prevent weed seeds from successfully germinating. The precise timing for this application is based on soil temperature, not the calendar date. To prevent the germination of summer annual weeds like crabgrass, the product must be applied before the soil temperature consistently reaches 55 degrees Fahrenheit at a depth of four inches. This window is typically in early to mid-spring, but tracking local soil temperatures is the most accurate method. A second pre-emergent application may be necessary in the fall to target winter annual weeds, such as annual bluegrass, when soil temperatures drop below 70 degrees Fahrenheit.
Post-Emergent Herbicides
Post-emergent herbicides are designed to kill weeds that are already visible and actively growing. These products are most effective when applied in the late spring or early fall when weeds are vigorously moving nutrients to their roots or preparing for dormancy. Systemic post-emergent herbicides are absorbed and translocated throughout the weed, making them most lethal during periods of active growth. The weeds should be healthy and not stressed by drought or extreme heat for the best result. Applying post-emergent products when daytime temperatures exceed 85 degrees Fahrenheit can cause the chemical to volatilize, reducing its effectiveness and increasing the risk of damaging the surrounding turf. Fall is a particularly effective time for treating perennial broadleaf weeds, such as dandelions, because the plant is actively moving sugars down to its root system to store for winter, simultaneously pulling the herbicide with it.
Combining and Sequencing Applications
Applying fertilizer and weed killer simultaneously is possible with “weed and feed” products, which combine fertilizer with a pre-emergent or post-emergent herbicide. These combination products offer convenience, but they often require a compromise in optimal timing because the best time to feed the grass may not perfectly align with the best time to kill the weeds. For instance, early spring pre-emergent timing for crabgrass often occurs before warm-season grasses are ready for their first substantial nitrogen feeding.
The two products are best applied separately to achieve maximum effectiveness and flexibility for both the turf and weed control. When using a granular fertilizer combined with a post-emergent weed killer, the grass should be wet with dew or light moisture so the herbicide granules stick to the weed leaves, but the soil should be relatively dry for the fertilizer to work properly. This requirement often creates a conflict, making separate applications more reliable.
Sequencing with Overseeding
Sequencing is especially important when considering lawn repair activities like overseeding. Most pre-emergent herbicides prevent any seed from germinating, including desirable grass seed. If a lawn is being overseeded, a waiting period of four to six weeks is necessary after applying pre-emergent herbicides before new seed can be safely introduced. Conversely, post-emergent weed killers can also harm new seedlings, requiring a waiting period of at least two to four weeks after application before overseeding, or until the new grass has established a strong root system.