The effectiveness of a “Weed and Feed” product against crabgrass depends entirely on the specific chemical formulation and the weed’s current growth stage. A Weed and Feed product is a convenient combination of lawn fertilizer and a weed control chemical, or herbicide. To determine if it kills crabgrass, one must identify if the herbicide component is designed to prevent the weed before it sprouts (pre-emergent) or to kill it once it is actively growing (post-emergent). Crabgrass is a tenacious annual grass, and applying the wrong product at the wrong time will have virtually no effect.
How Weed and Feed Formulations Work
The “Feed” component is a standard lawn fertilizer, supplying nutrients like nitrogen, phosphorus, and potassium to encourage thick, healthy turf growth. The “Weed” portion contains the herbicide, which targets unwanted plants through one of two primary mechanisms. The critical distinction lies in whether the product contains a pre-emergent or a post-emergent herbicide.
Pre-emergent herbicides are a preventative measure, creating a chemical barrier in the top layer of the soil that stops weed seeds from successfully germinating. Once the seed sprouts, its root system absorbs the chemical, which inhibits cell division and kills the seedling before it emerges. Conversely, post-emergent herbicides are designed to kill visible, actively growing weeds by being absorbed through the leaves. These chemicals are translocated throughout the plant, including the roots, to achieve a complete kill.
Most combination Weed and Feed products contain a pre-emergent herbicide, such as prodiamine or dithiopyr. These formulations are highly effective at preventing the weed but cannot kill crabgrass plants that have already established themselves. The post-emergent herbicides in general Weed and Feed products are typically selective, targeting broadleaf weeds like dandelions and clover while leaving desirable turfgrass (and crabgrass) unharmed.
The Timing of Pre-Emergent Prevention
Crabgrass is a warm-season annual grass; its seeds germinate in the spring, the plant grows throughout the summer, produces new seeds, and dies with the first hard frost of autumn. To successfully prevent its emergence, the pre-emergent Weed and Feed application must occur before the seeds sprout. This timing is not based on a specific calendar date but rather on soil temperature, which varies significantly by region and year.
The benchmark for crabgrass germination is when the soil temperature at a four-inch depth consistently reaches 55 degrees Fahrenheit for several consecutive days. Initial germination may begin around 55°F, but the majority of crabgrass seeds will sprout once the soil warms further, typically between 60°F and 70°F. Applying the pre-emergent too early can cause the chemical barrier to break down prematurely, leaving the lawn unprotected when the soil reaches the peak germination temperature later in the season.
The product must be applied just before or precisely at the start of this 55°F window to ensure the protective barrier is active when the crabgrass seeds begin to germinate. Once the plants have emerged and are visible, the window for pre-emergent control has closed. At that point, the product will no longer be effective, and a different strategy is required.
Eliminating Existing Crabgrass
If crabgrass plants are already visible, the preventative pre-emergent approach is no longer an option; a post-emergent herbicide is necessary to eliminate the established weeds. Since most general Weed and Feed products do not contain the specific chemicals needed to kill crabgrass, a separate, targeted application is almost always required. The most widely used and effective active ingredient for post-emergent crabgrass control is quinclorac.
Quinclorac is a selective herbicide successful at controlling crabgrass that has developed up to three or four tillers (side shoots). Another highly effective option is fenoxaprop-ethyl, which can be used to control crabgrass at nearly any growth stage. These specialized herbicides work systemically, absorbing through the leaves and moving to the root system to kill the entire plant.
For the herbicide to be most effective, it should be applied when the crabgrass is actively growing, typically when daytime temperatures are consistently warm. In some cases, a second application may be necessary to control plants that were missed or that have grown too large for a single treatment. Products containing mesotrione are also an option for targeting newly emerged crabgrass, especially because they can be safely used on certain turfgrass species during the seeding process.
Application Precautions for Combined Products
Applying combined fertilizer and herbicide products requires attention to detail to ensure both safety and effectiveness. Proper application starts with reading the manufacturer’s label for the specific product, as instructions can vary regarding watering and temperature. For granular products that contain post-emergent herbicides, the grass must be damp before application so the granules stick to the foliage of the weeds, allowing for absorption of the chemical.
After application, it is recommended to wait at least 24 hours before watering or rainfall to give the post-emergent herbicide time to move into the weed’s system. Following this waiting period, a light watering is usually required to activate the pre-emergent herbicide component and wash the fertilizer into the soil. It is also important to avoid applying the product when air temperatures exceed 85 degrees Fahrenheit, as the combination of fertilizer salts and herbicide can scorch or burn the desirable turfgrass.
Products containing a pre-emergent component will prevent the germination of all seeds, including desirable grass seed. Therefore, a pre-emergent Weed and Feed should never be applied to a newly seeded lawn or within several weeks of planning to sow new grass seed. Calibrating the spreader is also essential to ensure a uniform application rate across the entire lawn, preventing both chemical burn from over-application and ineffective weed control from under-application.