A “weed and feed” product combines granular lawn fertilizer and herbicide to nourish turfgrass while eliminating broadleaf weeds. The effectiveness of this dual-action product depends entirely on when it is applied. Applying it at the wrong time can result in wasted product, ineffective weed control, or damage to the healthy grass. Timing must align with the weed control mechanism, the grass’s seasonal growth cycle, and environmental conditions.
Understanding Pre-Emergent and Post-Emergent Weed Control
The timing for application is dictated by the type of herbicide it contains, which falls into two main categories. Products containing a pre-emergent herbicide are preventative, creating a chemical barrier in the top layer of the soil to stop weed seeds from germinating. This means the product must be applied before the target weeds are visible, typically when soil temperatures consistently reach 55 degrees Fahrenheit, which triggers the germination of common weeds like crabgrass.
Applying a pre-emergent too late, after weed seeds have already sprouted, renders the product ineffective because the chemical barrier only works to prevent cell division in the emerging seedling. This type of herbicide is essentially a proactive measure against future weed problems rather than a solution for existing ones.
Products formulated with a post-emergent herbicide are reactive, designed to kill weeds that are already actively growing and visible in the lawn. These systemic herbicides are absorbed through the leaves and translocated throughout the plant’s system to the roots. For maximum effectiveness, post-emergent treatments must be applied when the target weeds are young and actively metabolizing, not when they are stressed by heat or drought. Timing depends less on soil temperature and more on the physical presence and health of the weed itself.
Seasonal Timing Based on Grass Type
Application must be timed with the active growth phases of the grass type, determining when the turf can safely absorb the fertilizer. For cool-season grasses (Kentucky bluegrass, fescue, and perennial ryegrass), the primary application window is early spring and early fall. The first spring application, ideally when soil temperatures reach 55 degrees Fahrenheit, typically focuses on pre-emergent control to stop crabgrass from emerging.
The most beneficial application for cool-season turf often occurs in early fall (September or October), six weeks before the first hard frost. The grass is recovering from summer stress and entering its most active period of root development. The fertilizer supports root growth and helps the grass store food reserves for winter, while a post-emergent herbicide targets actively growing broadleaf weeds like henbit and chickweed. Applying a nitrogen-heavy product too late can trigger late-season growth, making the grass vulnerable to winter damage.
Warm-season grasses (Bermuda, Zoysia, and St. Augustine) have a later application schedule because they remain dormant longer in the spring. Applying nitrogen too early, before the grass has fully “greened up,” can severely damage the turf. The ideal time for the first application is late spring or early summer, once the grass is actively growing and after it has been mowed at least twice. This timing ensures the grass is metabolically ready to use the nutrients and tolerate the herbicide.
The final application for warm-season lawns should occur in early fall, around September, but never when the grass is preparing for or entering dormancy, as this can increase the risk of winterkill. Always choose a product formulated specifically for the grass type, as the wrong herbicide can cause irreversible harm.
Immediate Weather and Lawn Conditions for Application
Beyond the seasonal calendar, application success relies heavily on immediate weather and lawn condition. Temperature is a major factor; post-emergent herbicides can cause the turf to “scorch” if applied when air temperatures exceed 85 to 90 degrees Fahrenheit. High heat also stresses weeds, making them less likely to absorb the herbicide efficiently, which reduces effectiveness.
Moisture requirements differ depending on the product’s function. Granular post-emergent formulas require damp grass blades, often achieved by applying in the early morning dew, so the herbicide granules stick to the weed’s foliage. Conversely, pre-emergent herbicides and the fertilizer component need to be watered into the soil shortly after application to activate the weed barrier and make the nutrients available to the grass roots.
The mowing schedule must be managed precisely around the application. For post-emergent control, the leaf surface area of the weed must be maximized for absorption. Avoid mowing the lawn for two to four days before treatment to allow weeds to grow sufficient leaf tissue. Furthermore, do not mow for at least two to four days after application, as removing the leaf material too soon will cut away the absorbed herbicide before it has fully translocated and killed the weed.