“Weed and feed” is a combination product that simplifies lawn care by delivering both fertilizer and herbicide in a single application. The fertilizer provides essential nutrients, like nitrogen, to promote healthy grass growth, while the herbicide targets unwanted plants. Accurate timing is crucial, especially in Illinois’s variable climate. Applying it incorrectly can render the herbicide ineffective, waste the fertilizer, or damage the lawn. Understanding the biological triggers for the grass and weeds is the most important step for success.
The Critical Role of Soil Temperature
The precise timing for weed and feed is dictated by consistent soil temperature, not fluctuating air temperature. Pre-emergent herbicides, designed to prevent weed seeds from sprouting, must be applied before germination. The primary target weed for spring application is crabgrass, an annual weed that quickly colonizes lawns during the summer.
Crabgrass seeds germinate once the soil temperature at a depth of a few inches reaches a sustained range of 55 to 60 degrees Fahrenheit for several consecutive days. This warmth signals the seeds to sprout. If the pre-emergent herbicide is applied after germination starts, it cannot form the necessary chemical barrier to halt the weed’s emergence.
Homeowners should monitor soil temperatures using a soil thermometer or local extension reports. Applying the product too early risks the herbicide breaking down before the germination period begins, leaving the lawn unprotected later. Successful control depends on having the barrier in place precisely when the soil reaches this specific temperature threshold.
Timing the Initial Spring Application
The ideal window for the initial spring application requires balancing the needs of the state’s north-to-south geography. The primary goal of this first treatment is to apply the pre-emergent herbicide for crabgrass control just before the soil consistently hits 55 degrees. In Southern Illinois (e.g., Carbondale), this timing is late March to early April, while Northern Illinois (e.g., Chicago) is typically two to three weeks later, closer to late April or early May.
A key consideration is that the fertilizer component is often applied too early for the grass’s needs. Although the pre-emergent must be applied early to stop crabgrass, the nitrogen-heavy fertilizer can prematurely stimulate top growth. Cool-season grasses should instead be focusing energy on deep root development. This early flush of growth can weaken the lawn’s ability to withstand the stress of a hot summer.
For weeds already visible, such as dandelions and clover, a post-emergent herbicide is necessary. These products work best when weeds are actively growing, typically in mid-to-late spring (April or May) during moderate temperatures. Post-emergent granules often require the weed foliage to be slightly damp so the herbicide can stick to the leaf surface and be absorbed by the plant.
Summer and Fall Maintenance Schedule
The application schedule must change significantly once temperatures rise, shifting from prevention to maintenance and recovery. Most weed and feed products should be entirely avoided during the peak heat of the Illinois summer (late June through August). The combination of quick-release nitrogen fertilizer and herbicide can easily cause turf burn on a lawn already stressed by high heat and drought. If feeding is required during this period, a specialized slow-release or light fertilizer is the only safe option, ideally without herbicide.
The second critical time for a weed and feed application is the fall, typically between September and October. Unlike the spring application focused on prevention, the fall treatment prioritizes the “feed” component for winterizing the lawn. This application supplies nutrients that grass roots absorb, allowing them to grow deep and store energy for winter dormancy.
Late-season feeding strengthens the root system and improves resilience, leading to a quicker green-up the following spring. Many fall weed and feed products contain post-emergent herbicides to target broadleaf weeds like dandelions and clover as temperatures cool. Applying a post-emergent product in early fall, when these weeds are actively shuttling nutrients down to their roots, results in more effective control. The final “winterizer” application, which is primarily fertilizer, is usually applied in late October or November just before the ground freezes solid.